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dioda

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Diode

From Wikipedia, the free encyclopedia

Jump to: navigation, search

It has been suggested that Peak Inverse Voltage be merged into this article or section. (Discuss)

Closeup of the image below, showing the square shaped semiconductor crystal

various semiconductor diodes, below a bridge rectifier

Structure of a vacuum tube diode

In electronics, the word diode describes 2 classes of device:

a device that passes current in one direction much more readily than in the other
Some other devices with structures related to silicon diodes (eg Diac).

Most diodes have 2 terminals, and most are used for their unidirectional current property, but neither of these applies to all diodes. For example the varicap diode is used as an electrically adjustable capacitor, and the direct heated thermionic diode has 3 terminals.

The directionality of current flow most diodes possess is sometimes generically called the rectifying property). The most common function of a diode is to allow an electric current to flow in one direction (called the forward biased condition) but to block it in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve. Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depend on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Early diodes included “cat’s whisker” crystals and vacuum tube devices (called thermionic valves in British English). Today the most common diodes are made from semiconductor materials such as silicon or germanium.

[edit] History

Although the crystal diode was popularised before the thermionic diode, thermionic and solid state diodes developed in parallel. The principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873.^[1] The principle of operation of crystal diodes was discovered in 1874 by the German scientist, Karl Ferdinand Braun.^[2]

Thermionic diode principles were rediscovered by Thomas Edison on February 13, 1880 and he was awarded a patent in 1883 (U.S. Patent 307,031 ), but developed the idea no further. Braun patented the crystal rectifier in 1899 [1]. Braun’s discovery was further developed by Sir Jagdish Bose into a useful device for radio detection.

The first radio receiver using a crystal diode was built around 1900 by Greenleaf Whittier Pickard. The first thermionic diode was patented in Britain by John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee[2]) on November 16, 1904 (U.S. Patent 803,684 in November 1905). Pickard received a patent for a silicon crystal detector on November 20, 1906 [3] (U.S. Patent 836,531 ).

At the time of their invention such devices were known as rectifiers. In 1919 William Henry Eccles coined the term diode from Greek roots; di means ‘two’, and ode (from odos) means ‘path’.

[edit] Thermionic & gaseous state diodes

The symbol for an indirect heated vacuum tube diode. From top to bottom, the components are the anode, the cathode, and the heater filament.

Thermionic diodes are thermionic valve devices (also known as vacuum tubes), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.

In thermionic valve diodes, a current is passed through the heater filament. This indirectly heats the cathode, another filament treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungten filament acts as both cathode and emitter.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode, called the anode, is positively charged, so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed and hence any reverse flow is a very tiny current.

For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies. Today, valve diodes are only used in niche applications, such as rectifiers in guitar and hi-fi valve amplifiers, and specialized high-voltage equipment.

[edit] Semiconductor diodes

Most modern diodes are based on semiconductor p-n junctions. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but cannot flow in the opposite direction. Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.

A semiconductor diode’s current–voltage, or I–V, characteristic curve is related to the transport of carriers through the so-called depletion layer or depletion region that exists at the p-n junction between differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons “recombine”. When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor on the N-side and negatively charged acceptor on the P-side. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator.

However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a “built-in” potential across the depletion zone.

If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be “turned on” as it has a forward bias.

I–V characteristics of a P-N junction diode (not to scale).

A diode’s I–V characteristic can be approximated by four regions of operation (see the figure at right).

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current that usually damages the device permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the Zener diode, the concept of PIV is not applicable. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, such that the reverse voltage is “clamped” to a known value (called the Zener voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse voltage region.

The second region, at reverse biases more positive than the PIV, only a very small reverse saturation current flows. In the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range).

The third region is forward but small bias, where only a small forward current is conducted.

Finally, as the potential difference is increased above a cut-in voltage or on-voltage, the diode current becomes appreciable (the level of current considered “appreciable” and the value of cut-in voltage depends on the application), at which point it can be thought of as a connection with zero (or at least very low) resistance. More precisely, the current–voltage curve is exponential, and is so sharp that it looks like a corner on a zoomed-out graph (see also signal processing). In a normal silicon diode at rated currents, the cut-in voltage is approximately 0.6 to 0.7 volts. The value is different for other diode types — Schottky diodes can be as low as 0.2 V and light-emitting diodes (LEDs) can be 1.4 V or more (Blue LEDs can be up to 4.0 V). At higher currents the forward voltage drop of the diode increases. A drop of 1v - 1.5v is typical at full rated current for power diodes.

[edit] Shockley diode equation

The Shockley ideal diode equation or the diode law (named after transistor co-inventor William Bradford Shockley, not to be confused with tetrode inventor Walter H. Schottky) is the I–V characteristic of an ideal diode in either forward or reverse bias (or no bias). It is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn’t account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn’t describe the “leveling off” of the I–V curve at high forward bias due to internal resistance, nor does it explain the practical deviation from the ideal at very low forward bias due to R-G current in the depletion region.

where

I is the diode current,

I_S is a scale factor called the saturation current,

V_D is the voltage across the diode,

V_T is the thermal voltage,

and n is the emission coefficient, also known as the ideality factor.

The thermal voltage V_T is approximately 25.85 mV at 300 K, a temperature close to “room temperature” commonly used in device simulation software. At any temperature it is a known constant defined by:

where

q is the magnitude of charge on an electron (the elementary charge),

k is Boltzmann’s constant,

T is the absolute temperature of the p-n junction in Kelvins

For even rather small voltages the exponential is very large because the thermal voltage is very small, so the subtracted ‘1’ in the diode equation is negligible and the diode current is often approximated as

The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation $n$ is omitted).

The use of the diode equation in circuit problems is illustrated in the article on diode modeling.

[edit] Hydrodynamic analogy

Main article: Hydraulic analogy

The diode, in the manner of a valve, allows the passage of the current only in one direction. It is a polarized dipole, the anode and cathode is thus located on the component.


The valve is closed, the current is blocked	The valve is opened, the current passes

[edit] Types of semiconductor diode


Diode	Zener Diode	Schottky Diode	Tunnel Diode

Light-emitting diode	Photodiode	Varicap	SCR

Some diode symbols

There are several types of junction diodes, which either emphasizes a different physical aspects of a diode often by geometric scaling, doping level, choosing the right electrodes, are just an application of a diode in a special circuit, or are really different devices like the Gunn and laser diode and the JFET:

Normal (p-n) diodes: which operate as described above. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4–1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include 2 diodes per pin and many other internal diodes.
Switching diodes: Switching diodes, sometimes also called small signal diodes, are a single p-n diode in a discrete package. A switching diode provides essentially the same function as a switch. Below the specified applied voltage it has high resistance similar to an open switch, while above that voltage it suddenly changes to the low resistance of a closed switch. They are used in devices such as ring modulation.
Schottky diodes: Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than any p-n junction diode. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. They can also be used as low loss rectifiers although their reverse leakage current is generally much higher than non Schottky rectifiers. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down most normal diodes — so they have a faster “reverse recovery” than any p-n junction diode. They also tend to have much lower junction capacitance than PN diodes and this contributes towards their high switching speed and their suitability in high speed circuits and RF devices such as switched-mode power supply, mixers and detectors.
Super Barrier Diodes: Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p-n junction diode.
“Gold-doped” diodes: As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers. This allows the diode to operate at signal frequencies, at the expense of a higher forward voltage drop. Gold doped diodes are faster than other p-n diodes (but not as fast as Schottky diodes). They also have less reverse-current leakage than Schottky diodes (but not as good as other p-n diodes).[4].^[3] A typical example is the 1N914.
Snap-off or Step recovery diodes: The term ‘step recovery’ relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.
Point-contact diodes: These work the same as the junction semiconductor diodes described above, but its construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.
Cat’s whisker or crystal diodes: These are a type of point contact diode. The cat’s whisker diode consists of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal.[5] The wire forms the anode and the crystal forms the cathode. Cat’s whisker diodes were also called crystal diodes and found application in crystal radio receivers. Cat’s whisker diodes are obsolete.
PIN diodes: A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type / intrinsic / n-type structure. They are used as radio frequency switches and attenuators. They are also used as large volume ionizing radiation detectors and as photodetectors. PIN diodes are also used in power electronics, as their central layer can withstand high voltages. Furthermore, the PIN structure can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors.
Varicap or varactor diodes: These are used as voltage-controlled capacitors. These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than a FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal). They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.
Zener diodes: Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark). They are named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.
Avalanche diodes: Diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes, and are often mistakenly called Zener diodes, but break down by a different mechanism, the avalanche effect. This occurs when the reverse electric field across the p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed. The difference between the avalanche diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the channel length of the former exceeds the “mean free path” of the electrons, so there are collisions between them on the way out. The only practical difference is that the two types have temperature coefficients of opposite polarities.
Transient voltage suppression diode (TVS): These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients. Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.
Photodiodes: All semiconductors are subject to optical charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two dimensional array. These arrays should not be confused with charge-coupled devices.
Light-emitting diodes (LEDs): In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs are monochromatic; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.
Laser diodes: When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.

Esaki or tunnel diodes: these have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.
Gunn diodes: These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built.
Peltier diodes: are used as sensors, heat engines for thermoelectric cooling. Charge carriers absorb and emit their band gap energies as heat.

Current-limiting field-effect diodes: These are actually a JFET with the gate shorted to the source, and function like a two-terminal current-limiting analog to the Zener diode; they allow a current through them to rise to a certain value, and then level off at a specific value. Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.[6], [7]

Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms (see Operational amplifier applications#Logarithmic).

[edit] Numbering

A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal), IN914/1N4148 (Silicon signal) and 1N4001-1N4007 (Silicon 1A power rectifier). [8] [9] [10]

[edit] Related devices

Transistor
Thyristor or silicon controlled rectifier (SCR)
TRIAC
Diac

[edit] Applications

Several types of diodes. The scale is centimeters.

[edit] Radio demodulation

The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article. In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or “envelope” is proportional to the original audio signal, but whose average value is zero. The diode (originally a crystal diode) rectifies the AM signal, leaving a signal whose average amplitude is the desired audio signal. The average value is extracted using a simple filter and fed into an audio transducer, which generates sound.

[edit] Power conversion

Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode provides better performance than the commutator of earlier dynamo. Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages.

[edit] Over-voltage protection

Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting). For example, diodes are used in ( stepper motor and H-bridge ) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any diode used in such an application is called a flyback diode). Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).

[edit] Logic gates

Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.

[edit] Ionising radiation detectors

In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation. In electronics, cosmic rays and other sources of ionising radiation cause noise pulses and single and multiple bit errors. This effect is sometimes exploited by particle detectors to detect radiation. A single particle of radiation, with thousands or millions of electron volts of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material. If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle’s energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer or etc. These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre) particles they need a very large depletion depth and large area. For short range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and silicon are common materials. Some of these detectors sense position as well as energy. They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.

Semiconductor detectors for high energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

[edit] Temperature measuring

A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature. From the Shockley ideal diode equation given above, it appears the voltage has a positive temperature coefficient (at a constant current)but depends on doping concentration and operating temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to about 20 degrees Kelvin.

[edit] Current steering

Diodes will prevent currents from flowing in unintended directions. To supply power to an electrical circuit during a power failure, the circuit can draw current from a battery. An Uninterruptible power supply built in this may use diodes to ensure that current is only drawn from the battery when necessary. Similarly, small boats typically have two circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Normally both are charged from a single alternator, and a heavy duty split charge diode is used to prevent the higher charge battery (typically the engine battery) from discharging through the lower charged battery when the alternator is not running [[11]].

[edit] Abbreviations

Diodes are usually referred to as D for diode on PCBs. Sometimes the abbreviation CR for controlled rectifiers is seen.

[edit] See also

[edit] Notes

^ 1928 Nobel Prize article on the diode
^ Historical lecture on Karl Braun
^ S. M. Sze, Modern Semiconductor Device Physics, Wiley Interscience, ISBN 0-471-15237-4

[edit] External links

The Unusual Diode FAQ
Interactive Explanation of Semiconductor Diode
“Device Cross Section” for pn diode, Schottky diode, and silicon controlled rectifier

2007-12-03T17:29:00.001-08:00

Diode

From Wikipedia, the free encyclopedia

Jump to: navigation, search

It has been suggested that Peak Inverse Voltage be merged into this article or section. (Discuss)

Closeup of the image below, showing the square shaped semiconductor crystal

various semiconductor diodes, below a bridge rectifier

Structure of a vacuum tube diode

In electronics, the word diode describes 2 classes of device:

a device that passes current in one direction much more readily than in the other
Some other devices with structures related to silicon diodes (eg Diac).

[edit] History

[edit] Thermionic & gaseous state diodes

The symbol for an indirect heated vacuum tube diode. From top to bottom, the components are the anode, the cathode, and the heater filament.

[edit] Semiconductor diodes

I–V characteristics of a P-N junction diode (not to scale).

A diode’s I–V characteristic can be approximated by four regions of operation (see the figure at right).

The third region is forward but small bias, where only a small forward current is conducted.

[edit] Shockley diode equation

where

I is the diode current,

I_S is a scale factor called the saturation current,

V_D is the voltage across the diode,

V_T is the thermal voltage,

and n is the emission coefficient, also known as the ideality factor.

where

q is the magnitude of charge on an electron (the elementary charge),

k is Boltzmann’s constant,

T is the absolute temperature of the p-n junction in Kelvins

The use of the diode equation in circuit problems is illustrated in the article on diode modeling.

[edit] Hydrodynamic analogy

Main article: Hydraulic analogy

The diode, in the manner of a valve, allows the passage of the current only in one direction. It is a polarized dipole, the anode and cathode is thus located on the component.


The valve is closed, the current is blocked	The valve is opened, the current passes

[edit] Types of semiconductor diode


Diode	Zener Diode	Schottky Diode	Tunnel Diode

Light-emitting diode	Photodiode	Varicap	SCR

Some diode symbols

Normal (p-n) diodes: which operate as described above. Usually made of doped silicon or, more rarely, germanium. Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used; its low efficiency gave it a much higher forward voltage drop (typically 1.4–1.7 V per “cell”, with multiple cells stacked to increase the peak inverse voltage rating in high voltage rectifiers), and required a large heat sink (often an extension of the diode’s metal substrate), much larger than a silicon diode of the same current ratings would require. The vast majority of all diodes are the p-n diodes found in CMOS integrated circuits, which include 2 diodes per pin and many other internal diodes.
Switching diodes: Switching diodes, sometimes also called small signal diodes, are a single p-n diode in a discrete package. A switching diode provides essentially the same function as a switch. Below the specified applied voltage it has high resistance similar to an open switch, while above that voltage it suddenly changes to the low resistance of a closed switch. They are used in devices such as ring modulation.
Schottky diodes: Schottky diodes are constructed from a metal to semiconductor contact. They have a lower forward voltage drop than any p-n junction diode. Their forward voltage drop at forward currents of about 1 mA is in the range 0.15 V to 0.45 V, which makes them useful in voltage clamping applications and prevention of transistor saturation. They can also be used as low loss rectifiers although their reverse leakage current is generally much higher than non Schottky rectifiers. Schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down most normal diodes — so they have a faster “reverse recovery” than any p-n junction diode. They also tend to have much lower junction capacitance than PN diodes and this contributes towards their high switching speed and their suitability in high speed circuits and RF devices such as switched-mode power supply, mixers and detectors.
Super Barrier Diodes: Super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the Schottky diode with the surge-handling capability and low reverse leakage current of a normal p-n junction diode.
“Gold-doped” diodes: As a dopant, gold (or platinum) acts as recombination centers, which help a fast recombination of minority carriers. This allows the diode to operate at signal frequencies, at the expense of a higher forward voltage drop. Gold doped diodes are faster than other p-n diodes (but not as fast as Schottky diodes). They also have less reverse-current leakage than Schottky diodes (but not as good as other p-n diodes).[4].^[3] A typical example is the 1N914.
Snap-off or Step recovery diodes: The term ‘step recovery’ relates to the form of the reverse recovery characteristic of these devices. After a forward current has been passing in an SRD and the current is interrupted or reversed, the reverse conduction will cease very abruptly (as in a step waveform). SRDs can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers.
Point-contact diodes: These work the same as the junction semiconductor diodes described above, but its construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.
Cat’s whisker or crystal diodes: These are a type of point contact diode. The cat’s whisker diode consists of a thin or sharpened metal wire pressed against a semiconducting crystal, typically galena or a piece of coal.[5] The wire forms the anode and the crystal forms the cathode. Cat’s whisker diodes were also called crystal diodes and found application in crystal radio receivers. Cat’s whisker diodes are obsolete.
PIN diodes: A PIN diode has a central un-doped, or intrinsic, layer, forming a p-type / intrinsic / n-type structure. They are used as radio frequency switches and attenuators. They are also used as large volume ionizing radiation detectors and as photodetectors. PIN diodes are also used in power electronics, as their central layer can withstand high voltages. Furthermore, the PIN structure can be found in many power semiconductor devices, such as IGBTs, power MOSFETs, and thyristors.
Varicap or varactor diodes: These are used as voltage-controlled capacitors. These are important in PLL (phase-locked loop) and FLL (frequency-locked loop) circuits, allowing tuning circuits, such as those in television receivers, to lock quickly, replacing older designs that took a long time to warm up and lock. A PLL is faster than a FLL, but prone to integer harmonic locking (if one attempts to lock to a broadband signal). They also enabled tunable oscillators in early discrete tuning of radios, where a cheap and stable, but fixed-frequency, crystal oscillator provided the reference frequency for a voltage-controlled oscillator.
Zener diodes: Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see below). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or Transorb, a registered trademark). They are named for Dr. Clarence Melvin Zener of Southern Illinois University, inventor of the device.
Avalanche diodes: Diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically very similar to Zener diodes, and are often mistakenly called Zener diodes, but break down by a different mechanism, the avalanche effect. This occurs when the reverse electric field across the p-n junction causes a wave of ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are designed to break down at a well-defined reverse voltage without being destroyed. The difference between the avalanche diode (which has a reverse breakdown above about 6.2 V) and the Zener is that the channel length of the former exceeds the “mean free path” of the electrons, so there are collisions between them on the way out. The only practical difference is that the two types have temperature coefficients of opposite polarities.
Transient voltage suppression diode (TVS): These are avalanche diodes designed specifically to protect other semiconductor devices from high-voltage transients. Their p-n junctions have a much larger cross-sectional area than those of a normal diode, allowing them to conduct large currents to ground without sustaining damage.
Photodiodes: All semiconductors are subject to optical charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in solar cells, in photometry, or in optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two dimensional array. These arrays should not be confused with charge-coupled devices.
Light-emitting diodes (LEDs): In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs are monochromatic; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.
Laser diodes: When an LED-like structure is contained in a resonant cavity formed by polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in optical storage devices and for high speed optical communication.

Esaki or tunnel diodes: these have a region of operation showing negative resistance caused by quantum tunneling, thus allowing amplification of signals and very simple bistable circuits. These diodes are also the type most resistant to nuclear radiation.
Gunn diodes: These are similar to tunnel diodes in that they are made of materials such as GaAs or InP that exhibit a region of negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, allowing high frequency microwave oscillators to be built.
Peltier diodes: are used as sensors, heat engines for thermoelectric cooling. Charge carriers absorb and emit their band gap energies as heat.

Current-limiting field-effect diodes: These are actually a JFET with the gate shorted to the source, and function like a two-terminal current-limiting analog to the Zener diode; they allow a current through them to rise to a certain value, and then level off at a specific value. Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.[6], [7]

Other uses for semiconductor diodes include sensing temperature, and computing analog logarithms (see Operational amplifier applications#Logarithmic).

[edit] Numbering

[edit] Related devices

Transistor
Thyristor or silicon controlled rectifier (SCR)
TRIAC
Diac

[edit] Applications

Several types of diodes. The scale is centimeters.

[edit] Radio demodulation

[edit] Power conversion

[edit] Over-voltage protection

[edit] Logic gates

Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.

[edit] Ionising radiation detectors

Semiconductor detectors for high energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use.

[edit] Temperature measuring

[edit] Current steering

[edit] Abbreviations

Diodes are usually referred to as D for diode on PCBs. Sometimes the abbreviation CR for controlled rectifiers is seen.

[edit] See also

[edit] Notes

^ 1928 Nobel Prize article on the diode
^ Historical lecture on Karl Braun
^ S. M. Sze, Modern Semiconductor Device Physics, Wiley Interscience, ISBN 0-471-15237-4

[edit] External links

The Unusual Diode FAQ
Interactive Explanation of Semiconductor Diode
“Device Cross Section” for pn diode, Schottky diode, and silicon controlled rectifier

2007-12-03T17:22:00.000-08:00

Transistor

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Assorted discrete transistors

A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the circuitry in computers, cellular phones, and all other modern electronic devices.

Because of its fast response and accuracy, the transistor is used in a wide variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators. Transistors may be packaged individually or as part of an integrated circuit, some with over a billion transistors in a very small area.

[edit] Introduction

An electrical signal can be amplified by using a device that allows a small current or voltage to control the flow of a much larger current. Transistors are the basic devices providing control of this kind. Modern transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Application of current in BJTs and voltage in FETs between the input and common terminals increases the conductivity between the common and output terminals, thereby controlling current flow between them. The transistor characteristics depend on their type.

The term "transistor" originally referred to the point contact type, which saw very limited commercial application, being replaced by the much more practical bipolar junction types in the early 1950s. Today's most widely used schematic symbol, like the term "transistor", originally referred to these long-obsolete devices.^[1] For a short time in the early 1960s, some manufacturers and publishers of electronics magazines started to replace these with symbols that more accurately depicted the different construction of the bipolar transistor, but this idea was soon abandoned.

In analog circuits, transistors are used in amplifiers, (direct current amplifiers, audio amplifiers, radio frequency amplifiers), and linear regulated power supplies. Transistors are also used in digital circuits where they function as electronic switches, but rarely as discrete devices, almost always being incorporated in monolithic Integrated Circuits. Digital circuits include logic gates, random access memory (RAM), microprocessors, and digital signal processors (DSPs).

[edit] Importance

The transistor is considered by many to be the greatest invention of the twentieth century.^[2] It is the key active component in practically all modern electronics. Its importance in today's society rests on its ability to be mass produced using a highly automated process (fabrication) that achieves vanishingly low per-transistor costs.

Although billions of individual (known as discrete) transistors are still used, the vast majority produced are in integrated circuits (often abbreviated as IC and also called microchips or simply chips) along with diodes, resistors, capacitors and other electronic components to produce complete electronic circuits. A logic gate consists of about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors (MOSFETs) [1].

The transistor's low cost, flexibility and reliability have made it a universal device for non-mechanical tasks, such as digital computing. Transistorized circuits have replaced electromechanical devices for the control of appliances and machinery as well. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.

Because of the low cost of transistors and hence digital computers, there is a trend to digitize information. With digital computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making information digital. As a result, today, much media data is delivered in digital form, finally being converted and presented in analog form to the user. Areas influenced by the Digital Revolution include television, radio, and newspapers.

[edit] Advantages of transistors over vacuum tubes

Prior to the development of transistors, vacuum (electron) tubes (or in the UK thermionic valves or just valves) were the main active components in electronic equipment. The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are:

Small size and minimal weight, allowing the development of miniaturized electronic devices.
Highly automated manufacturing processes, resulting in low per-unit cost.
Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
No warm-up period required after power application.
Lower power dissipation and generally greater energy efficiency.
Higher reliability and greater physical ruggedness.
Extremely long life. Some transistorized devices produced more than 30 years ago are still in service.
Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
Though in most transistors the junctions have different doping levels and geometry, some allow bidirectional current
Ability to control very large currents, as much as several hundred amperes.
Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.
More sensitive than the hot and macroscopic tubes

[edit] Disadvantages

Silicon transistors do not operate at voltages higher than about 1 kV, SiC go to 3 kV.
The electron mobility is higher in a vacuum, so that high power, high frequency operation is easier in tubes.

[edit] History

Main article: Transistor history

A replica of the first transistor

The first three patents for the field-effect transistor principle were registered in Germany in 1928 by physicist Julius Edgar Lilienfeld, but Lilienfeld published no research articles about his devices, and they were ignored by industry. In 1934 German physicist Dr. Oskar Heil patented another field-effect transistor. There is no direct evidence that these devices were built, but later work in the 1990s show that one of Lilienfeld's designs worked as described and gave substantial gain. Legal papers from the Bell Labs patent show that Shockley and Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles. The Other Transistor, R. G. Arns

On 16 December 1947, William Shockley, John Bardeen and Walter Brattain succeeded in building the first practical point-contact transistor at Bell Labs. This work followed from their war-time efforts to produce extremely pure germanium "crystal" mixer diodes, used in radar units as a frequency mixer element in microwave radar receivers. A parallel project on germanium diodes at Purdue University succeeded in producing the good-quality germanium semiconducting crystals that were used at Bell Labs.[2] Early tube-based technology did not switch fast enough for this role, leading the Bell team to use solid state diodes instead. With this knowledge in hand they turned to the design of a triode, but found this was not at all easy. Bardeen eventually developed a new branch of surface physics to account for the "odd" behavior they saw, and Bardeen and Brattain eventually succeeded in building a working device.

At the same time some European scientists were led by the idea of solid-state amplifiers. In August 1948 German physicists Herbert F. Mataré (1912– ) and Heinrich Welker (1912–1981), working at Compagnie des Freins et Signaux Westinghouse in Paris, France applied for a patent on an amplifier based on the minority carrier injection process which they called the "transistron". Since Bell Labs did not make a public announcement of the transistor until June 1948, the transistron was considered to be independently developed. Mataré had first observed transconductance effects during the manufacture of germanium duodiodes for German radar equipment during WWII. Transistrons were commercially manufactured for the French telephone company and military, and in 1953 a solid-state radio receiver with four transistrons was demonstrated at the Düsseldorf Radio Fair.

Bell Telephone Laboratories needed a generic name for the new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode", "Crystal Triode" and "Iotatron" were all considered, but "transistor," coined by John R. Pierce, won an internal ballot. The rationale for the name is described in the following extract from the company's Technical Memorandum calling for votes:

Transistor. This is an abbreviated combination of the words "transconductance" or "transfer", and "varistor". The device logically belongs in the varistor family, and has the transconductance or transfer impedance of a device having gain, so that this combination is descriptive.

– Bell Telephone Laboratories — Technical Memorandum (May 28, 1948)

Pierce recalled the naming somewhat differently:

The way I provided the name, was to think of what the device did. And at that time, it was supposed to be the dual of the vacuum tube. The vacuum tube had transconductance, so the transistor would have 'transresistance.' And the name should fit in with the names of other devices, such as varistor and thermistor. And. . . I suggested the name 'transistor.'

– John R. Pierce, interviewed for PBS show "Transistorized!"

Bell immediately put the point-contact transistor into limited production at Western Electric in Allentown, Pennsylvania. Prototypes of all-transistor AM radio receivers were demonstrated, but were really only laboratory curiosities. However, in 1950 Shockley developed a radically different type of solid-state amplifier which became known as the Bipolar Junction "transistor". Although it works on a completely different principle to the point-contact "transistor", this is the device which is most commonly referred to as a "transistor" today. These were also licensed to a number of other electronics companies, including Texas Instruments, who produced a limited run of transistor radios as a sales tool. Early transistors were chemically unstable and only suitable for low-power, low-frequency applications, but as transistor design developed, these problems were slowly overcome.

There are numerous claimants to the title of the first company to produce practical transistor radios. Texas Instruments had demonstrated all-transistor AM radios as early as 1952, but their performance was well below that of equivalent battery tube models. A workable all-transistor radio was demonstrated in August 1953 at the Düsseldorf Radio Fair by the German firm Intermetall. It was built with four of Intermetall's hand-made transistors, based upon the 1948 invention of Herbert Mataré and Heinrich Welker [3]. However, as with the early Texas units (and others) only prototypes were ever built; it was never put into commercial production.

The production of the first commercially successful transistor radio is often incorrectly attributed to Sony (originally Tokyo Tsushin Kogyo). However the Regency TR-1, made by the Regency Division of I.D.E.A. (Industrial Development Engineering Associates) of Indianapolis, Indiana, was the first practical transistor radio made in any significant numbers. The TR-1 was announced on October 18, 1954 and put on sale in November 1954 for $49.95 (the equivalent of about $361 in year-2005 dollars) and sold about 150,000 units.

The TR-1 used four Texas NPN transistors and had to be powered by a 22.5 volt battery, since the only way to get adequate radio frequency performance out of early transistors was to run them close to their collector-to-emitter breakdown voltage. This made the TR-1 very expensive to run, and it was far more popular for its novelty or status value than its actual performance, rather in the fashion of the first MP3 players.

Still, aside from its indifferent performance, the TR-1 was a very advanced product for its time, using printed circuit boards, and what were then considered micro-miniature components.

Masaru Ibuka, co-founder of the Japanese firm Sony, was visiting the USA when Bell Labs announced the availability of manufacturing licenses, including detailed instructions on how to manufacture junction transistors. Ibuka obtained special permission from the Japanese Ministry of Finance to pay the $50,000 license fee, and in 1955 the company introduced their own five-transistor "pocket" radio, the TR-55, under the new brand name Sony. (The term "pocket" was a matter of some interpretation, as Sony allegedly had special shirts made with oversized pockets for their salesmen) This product was soon followed by more ambitious designs, but it is generally regarded as marking the commencement of Sony's growth into a manufacturing superpower.

The TR-55 was quite similar to the Regency TR-1 in many ways, being powered by the same sort of 22.5 volt battery, and was not much more practical. Very few were sold in the USA. It was not until 1957 that Sony produced their ground-breaking "TR-7" 7-transistor portable, a much more advanced design that ran on three ordinary flashlight cells and could compete favorably with vacuum tube portables. However, by this time similar designs were being produced in most industrialized countries.

Over the next two decades, transistors gradually replaced the earlier vacuum tubes in most applications and later made possible many new devices such as integrated circuits and personal computers.

Shockley, Bardeen and Brattain were honored with the Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect". Bardeen would go on to win a second Nobel in physics, one of only two people to receive more than one in the same discipline, for his work on the exploration of superconductivity.

The commercial uses of germanium transistors were limited by their sensitivity to temperature and humidity. Silicon, a semiconductor with crystal structure identical to germanium, looked promising but attempts over several years to make useful transistors were unsuccessful. In early 1954, M. Tanenbaum et al. (Jl. of Applied Physics, 26, 686 (1955)) at Bell Labs made a high performance silicon transistor using npn junctions produced by growth rate fluctuations during crystal growing. A few months later, working independently at Texas Instruments, G. Teal (unpublished) made similar devices using sequential doping.

While these devices had much superior temperature and environmental properties compared to gemanium transistors, the doping processes were difficult to control. That problem was solved by Tanenbaum and Fuller (Bell Sys. Tech. Jl., 35, 1 (1956)) using gas diffusion techniques to produce npn silicon transistors. The resulting diffused base silicon transistor was the subject of the second Bell Labs symposium. The diffusion process was easy to control, quickly adopted by the semiconductor industry and was the basis for the later invention of the integrated circuit initiating the "silicon age".

The first gallium-arsenide Schottky-gate field-effect transistor (MESFET) was made by Carver Mead and reported in 1966.^[3]

[edit] Types

	PNP		P-channel
	NPN		N-channel
BJT		JFET

BJT and JFET symbols

Transistors are categorized by:

Semiconductor material : germanium, silicon, gallium arsenide, silicon carbide, etc.
Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
Maximum power rating: low, medium, high
Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term $f T$ , an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
Application: switch, general purpose, audio, high voltage, super-beta, matched pair
Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power modules

Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch.

[edit] Bipolar junction transistor

The bipolar junction transistor (BJT) was the first type of transistor to be mass-produced. Bipolar transistors are so named because they conduct by using both majority and minority carriers. The three terminals of the BJT are named emitter, base and collector. Two p-n junctions exist inside a BJT: the base/emitter junction and base/collector junction. "The [BJT] is useful in amplifiers because the currents at the emitter and collector are controllable by the relatively small base current."^[4] In an NPN transistor operating in the active region, the emitter-base junction is forward biased, and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased base-collector junction and be swept into the collector; perhaps one-hundredth of the electrons will recombine in the base, which is the dominant mechanism in the base current. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled.^[4]

Unlike the FET, the BJT is a low–input-impedance device. As the base–emitter voltage ( $V b e$ ) is increased the base–emitter current and hence the collector–emitter current ( $I c e$ ) increase exponentially (, where K is a constant). Because of this exponential relationship, the BJT has a higher transconductance than the FET.

Bipolar transistors can be made to conduct by light, since absorption of photons in the base region generates a photocurrent that acts as a base current; the collector current is approximately beta times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called phototransistors.

[edit] Field-effect transistor

The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons in the case of the N-channel FET, or holes for P-channel FET, for conduction. The four terminals of the FET are named source, gate, drain, and body (substrate). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description.

In FETs, the drain-to-source current flows via a conducting channel that connects the drain region to the source region. The channels conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate/source voltage ( $V g s$ ) is increased, the drain/source current ( $I d s$ ) increases at a roughly exponential rate ().

To turn on a transistor it has to be charged like a capacitor. One polarity of charge is responsible for conduction, the other serves for charge neutrality. In the BJT, both types of charge carriers come close together and so the capacitance is high, therefore only low voltages are needed to produce a given amount of charge. In a FET both types of charges are separated by the dielectric and additionally the Debye length, thus reducing the capacity and increasing the voltage needed for switching. Above zero Kelvin, the exponential curve is convoluted with the hard turn on of the BJT and the parabolic turn on of the FET.

For low noise at narrow bandwidth the higher input resistance of the FET is advantageous.

FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal–oxide–semiconductor FET (MOSFET), from their original construction as a layer of metal (the gate), a layer of oxide (the insulation), and a layer of semiconductor. Unlike IGFETs, the JFET gate forms a PN diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage.

Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased PN junction is replaced by a metal–semiconductor Schottky-junction. These, and the HEMTs (high electron mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (microwave frequencies; several GHz).

Unlike bipolar transistors, FETs do not inherently amplify a photocurrent. Nevertheless, there are ways to use them, especially JFETs, as light-sensitive devices, by exploiting the photocurrents in channel–gate or channel–body junctions.

FETs are further divided into depletion-mode and enhancement-mode types, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can "enhance" the conduction. For depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for N-channel devices and a lower current for P-channel devices. Nearly all JFETs are depletion-mode as the diode junctions would forward bias and conduct if they were enhancement mode devices; most IGFETs are enhancement-mode types.

[edit] Other transistor types

Heterojunction Bipolar Transistor
Alloy junction transistor
Tetrode transistor
Pentode transistor
Spacistor
Surface barrier transistor
Micro alloy transistor
Micro alloy diffused transistor
Drift-field transistor
Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P-type or N-type semiconductor with ohmic contacts at each end (terminals Base1 and Base2). A junction with the opposite semiconductor type is formed at a point along the length of the body for the third terminal (Emitter).
Dual gate FETs have a single channel with two gates in cascode; a configuration that is optimized for high frequency amplifiers, mixers, and oscillators.
Darlington transistors are two BJTs connected together to provide a high current gain equal to the product of the current gains of the two transistors.
Insulated gate bipolar transistors (IGBTs) use a medium power IGFET, similarly connected to a power BJT, to give a high input impedance. Power diodes are often connected between certain terminals depending on specific use. IGBTs are particularly suitable for heavy-duty industrial applications. The Asea Brown Boveri (ABB) 5SNA2400E170100 illustrates just how far power semiconductor technology has advanced. Intended for three-phase power supplies, this device houses three NPN IGBTs in a case measuring 38 by 140 by 190 mm and weighing 1.5 kg. Each IGBT is rated at 1,700 volts and can handle 2,400 amperes.
Single-electron transistors (SET) consist of a gate island between two tunnelling junctions. The tunnelling current is controlled by a voltage applied to the gate through a capacitor. [4][5]
Nanofluidic transistor Control the movement of ions through sub-microscopic, water-filled channels. Nanofluidic transistor, the basis of future chemical processors
Trigate transistors (Prototype by Intel)
Avalanche transistor
Ballistic transistor
Spin transistor Magnetically-sensitive
Thin film transistor Used in LCD display.
Floating-gate transistor Used for non-volatile storage.
Photo transistor React to light
Inverted-T field effect transistor
Ion sensitive field effect transistor To measure ion concentrations in solution.
FinFET The source/drain region forms fins on the silicon surface.
FREDFET Fast-Reverse Epitaxal Diode Field-Effect Transistor
EOSFET Electrolyte-Oxide-Semiconductor Field Effect Transistor (Neurochip)
OFET Organic Field-Effect Transistor, in which the semiconductor is an organic compound
DNAFET Deoxyribonucleic acid field-effect transistor

[edit] Semiconductor material

The first BJTs were made from germanium (Ge) and some high power types still are. Silicon (Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor material gallium arsenide (GaAs) and the semiconductor alloy silicon germanium (SiGe). Single element semiconductor material (Ge and Si) is described as elemental.

Rough parameters for the most common semiconductor materials used to make transistors are given in the table below; it must be noted that these parameters will vary with temperature, electric field, impurity level, strain and various other factors:

Semiconductor material characteristics
Semiconductor material	Junction forward voltage V @ 25 °C	Electron mobility m²/(V·s) @ 25 °C	Hole mobility m²/(V·s) @ 25 °C	Max. junction temp. °C
Ge	0.27	0.39	0.19	70 to 100
Si	0.71	0.14	0.05	150 to 200
GaAs	1.03	0.85	0.05	150 to 200
Al-Si junction	0.3	—	—	150 to 200

The junction forward voltage is the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. The current increases exponentially as the junction forward voltage is increased. The values given in the table are typical for a current of 1 mA (the same values apply to semiconductor diodes). The lower the junction forward voltage the better, as this means that less power is required to "drive" the transistor. The junction forward voltage for a given current decreases with temperature. For a typical silicon junction the change is approximately −2.1 mV/°C.

The density of mobile carriers in the channel of a MOSFET is a function of the electric field forming the channel and of various other phenomena such as the impurity level in the channel. Some impurities, called dopants, are introduced deliberately in making a MOSFET, to control the MOSFET electrical behavior.

The electron mobility and hole mobility columns show the average speed that electrons and holes diffuse through the semiconductor material with an electric field of 1 volt per meter applied across the material. In general, the higher the electron mobility the faster the transistor. The table indicates that Ge is a better material than Si in this respect. However, Ge has four major shortcomings compared to silicon and gallium arsenide:

its maximum temperature is limited
it has relatively high leakage current
it cannot withstand high voltages
it is less suitable for fabricating integrated circuits

Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar NPN transistor tends to be faster than an equivalent PNP transistor type. GaAs has the highest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high frequency applications. A relatively recent FET development, the high electron mobility transistor (HEMT), has a heterostructure (junction between different semiconductor materials) of aluminium gallium arsenide (AlGaAs)-gallium arsenide (GaAs) which has double the electron mobility of a GaAs-metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite receivers working at frequencies around 12 GHz.

Max. junction temperature values represent a cross section taken from various manufacturers' data sheets. This temperature should not be exceeded or the transistor may be damaged.

Al-Si junction refers to the high-speed (aluminum-silicon) semiconductor-metal barrier diode, commonly known as a Schottky diode. This is included in the table because some silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain as part of the fabrication process. This diode can be a nuisance, but sometimes it is used in the circuit.

[edit] Packaging

Through-hole transistors (tape measure marked in centimetres)

Transistors come in many different packages (chip carriers) (see images). The two main categories are through-hole (or leaded), and surface-mount, also known as surface mount device (SMD). The ball grid array (BGA) is the latest surface mount package (currently only for large transistor arrays). It has solder "balls" on the underside in place of leads. Because they are smaller and have shorter interconnections, SMDs have better high frequency characteristics but lower power rating.

Transistor packages are made of glass, metal, ceramic or plastic. The package often dictates the power rating and frequency characteristics. Power transistors have large packages that can be clamped to heat sinks for enhanced cooling. Additionally, most power transistors have the collector or drain physically connected to the metal can/metal plate. At the other extreme, some surface-mount microwave transistors are as small as grains of sand.

Often a given transistor type is available in different packages. Transistor packages are mainly standardized, but the assignment of a transistor's functions to the terminals is not: different transistor types can assign different functions to the package's terminals. Even for the same transistor type the terminal assignment can vary (normally indicated by a suffix letter to the part number- i.e. BC212L and BC212K).

[edit] Usage

For a basic guide to operation of transistors, see How a Transistor Works.

In the early days of transistor circuit design, the bipolar junction transistor, or BJT, was the most commonly used transistor. Even after MOSFETs became available, the BJT remained the transistor of choice for digital and analog circuits because of their ease of manufacture and speed. However, desirable properties of MOSFETs, such as their utility in low-power devices, have made them the ubiquitous choice for use in digital circuits and a very common choice for use in analog circuits.

BJT used as an electronic switch, in grounded-emitter configuration

Amplifier circuit, standard common-emitter configuration

[edit] Switches

Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.

[edit] Amplifiers

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Transistors are commonly used in modern musical instrument amplifiers, in which circuits up to a few hundred watts are common and relatively cheap. Transistors have largely replaced valves (electron tubes) in instrument amplifiers. Some musical instrument amplifier manufacturers mix transistors and vacuum tubes in the same circuit, to utilize the inherent benefits of both devices.

[edit] Computers

The "first generation" of electronic computers used vacuum tubes, which generated large amounts of heat, were bulky, and were unreliable. The development of the transistor was key to computer miniaturization and reliability. The "second generation" of computers, through the late 1950s and 1960s featured boards filled with individual transistors and magnetic memory cores. Subsequently, transistors, other components, and their necessary wiring were integrated into a single, mass-manufactured component: the integrated circuit.

[edit] External links to datasheets

A wide range of transistors has been available since the 1960s and manufacturers continually introduce improved types. A few examples from the main families are noted below. Unless otherwise stated, all types are made from silicon semiconductor. Complementary pairs are shown as NPN/PNP or N/P channel. Links go to manufacturer datasheets, which are in PDF format. (On some datasheets the accuracy of the stated transistor category is a matter of debate.)

2N3904/2N3906, BC182/BC212 and BC546/BC556: Ubiquitous, BJT, general-purpose, low-power, complementary pairs. They have plastic cases and cost roughly ten cents U.S. in small quantities, making them popular with hobbyists.

AF107: Germanium, 0.5 watt, 250 MHz PNP BJT.

BFP183: Low power, 8 GHz microwave NPN BJT.

LM394: "supermatch pair", with two NPN BJTs on a single substrate.

2N2219A/2N2905A: BJT, general purpose, medium power, complementary pair. With metal cases they are rated at about one watt.

2N3055/MJ2955: For years, the venerable NPN 2N3055 has been the "standard" power transistor. Its complement, the PNP MJ2955 arrived later. These 1 MHz, 15 A, 60 V, 115 W BJTs are used in audio power amplifiers, power supplies, and control.

2SC3281/2SA1302: Made by Toshiba, these BJTs have low-distortion characteristics and are used in high-power audio amplifiers. They have been widely counterfeited[6].

BU508: NPN, 1500 V power BJT. Designed for television horizontal deflection, its high voltage capability also makes it suitable for use in ignition systems.

MJ11012/MJ11015: 30 A, 120 V, 200 W, high power Darlington complementary pair BJTs. Used in audio amplifiers, control, and power switching.

2N5457/2N5460: JFET (depletion mode), general purpose, low power, complementary pair.

BSP296/BSP171: IGFET (enhancement mode), medium power, near complementary pair. Used for logic level conversion and driving power transistors in amplifiers.

IRF3710/IRF5210: IGFET (enhancement mode), 40 A, 100 V, 200 W, near complementary pair. For high-power amplifiers and power switches, especially in automobiles.

[edit] See also

Electronics Portal

[edit] Patents

U.S. Patent 1,745,175 — Julius Edgar Lilienfeld 1930
U.S. Patent 2,524,035 — J. Bardeen et al.
U.S. Patent 2,569,347 — W. Shockley

[edit] References

^ Ralph S. Carson, Principles of Applied Electronics, McGraw–Hill 1961.
^ Dennis F. Herrick (2003). Media Management in the Age of Giants: Business Dynamics of Journalism. Blackwell Publishing. ISBN 0813816998.
^ C. A. Mead (Feb. 1966). "Schottky barrier gate field effect transistor". Proceedings of the IEEE 54 (2): 307–308.
^ ^a ^b Streetman, Ben (1992). Solid State Electronic Devices. Englewood Cliffs, NJ: Prentice-Hall, 301–305. ISBN 0-13-822023-9.

[edit] Books

Amos S W & James M R (1999). Principles of Transistor Circuits. Butterworth-Heinemann. ISBN 0-7506-4427-3.
Horowitz, Paul & Hill, Winfield (1989). The Art of Electronics. Cambridge University Press. ISBN 0-521-37095-7.
Riordan, Michael & Hoddeson, Lillian (1998). Crystal Fire. W.W Norton & Company Limited. ISBN 0-393-31851-6. The invention of the transistor & the birth of the information age
Warnes, Lionel (1998). Analogue and Digital Electronics. Macmillan Press Ltd. ISBN 0-333-65820-5.

[edit] Other

Robert G. Arns (October 1998). "The other transistor: early history of the metal-oxide-semiconducor field-effect transistor". Engineering Science and Education Journal 7 (5): 233-240. ISSN 0963-7346.
Armand Van Dormael. "The French Transistor". Proceedings of the 2004 IEEE Conference on the History of Electronics, Bletchley Park, June 2004.
"Herbert F. Mataré, An Inventor of the Transistor has his moment", The New York Times, 24 February 2003.
Michael Riordan (November 2005). "How Europe Missed the Transistor". IEEE Spectrum 42 (11): 52–57. ISSN 0018-9235.
C. D. Renmore (1980). Silicon Chips and You.
Wiley-IEEE Press. Complete Guide to Semiconductor Devices, 2nd Edition.

[edit] External links

Wikibooks has a book on the topic of

Transistors

Wikimedia Commons has media related to:

Transistors

How transistors work
BBC: Building the digital age photo history of transistors
Transistor Flow Control — Scientific American Magazine (October 2005)
The Bell Systems Memorial on Transistors
IEEE Virtual Museum, Let's Get Small: The Shrinking World of Microelectronics. All about the history of transistors and integrated circuits.
Transistorized. Historical and technical information from the Public Broadcasting Service
This Month in Physics History: November 17 to December 23, 1947: Invention of the First Transistor. From the American Physical Society
50 Years of the Transistor. From Science Friday, December 12, 1997
Bob's Virtual Transistor Museum & History. Treasure trove of transistor history
Jerry Russell's Transistor Cross Reference Database.
The DatasheetArchive. Searchable database of transistor specifications and datasheets.
History of the Transistor

Retrieved from "http://en.wikipedia.org/wiki/Transistor"

Categories: Transistors | Electronics | Semiconductor devices

rectifier circuits

2007-11-19T17:54:00.000-08:00

Volume III - Semiconductors » DIODES AND RECTIFIERS »

Rectifier circuits

We need your help! This page requires proofreading - If you notice any errors, please post on our forums

Now we come to the most popular application of the diode: rectification. Simply defined, rectification is the conversion of alternating current (AC) to direct current (DC). This involves a device that only allows one-way flow of electrons. As we have seen, this is exactly what a semiconductor diode does. The simplest kind of rectifier circuit is the half-wave rectifier. It only allows one half of an AC waveform to pass through to the load. (Figure below)

Half-wave rectifier circuit.

For most power applications, half-wave rectification is insufficient for the task. The harmonic content of the rectifier's output waveform is very large and consequently difficult to filter. Furthermore, the AC power source only supplies power to the load once every half-cycle, meaning that much of its capacity is unused. Half-wave rectification is, however, a very simple way to reduce power to a resistive load. Some two-position lamp dimmer switches apply full AC power to the lamp filament for “full” brightness and then half-wave rectify it for a lesser light output. (Figure below)

Half-wave rectifier application: Two level lamp dimmer.

In the “Dim” switch position, the incandescent lamp receives approximately one-half the power it would normally receive operating on full-wave AC. Because the half-wave rectified power pulses far more rapidly than the filament has time to heat up and cool down, the lamp does not blink. Instead, its filament merely operates at a lesser temperature than normal, providing less light output. This principle of “pulsing” power rapidly to a slow-responding load device to control the electrical power sent to it is common in the world of industrial electronics. Since the controlling device (the diode, in this case) is either fully conducting or fully nonconducting at any given time, it dissipates little heat energy while controlling load power, making this method of power control very energy-efficient. This circuit is perhaps the crudest possible method of pulsing power to a load, but it suffices as a proof-of-concept application.

If we need to rectify AC power to obtain the full use of both half-cycles of the sine wave, a different rectifier circuit configuration must be used. Such a circuit is called a full-wave rectifier. One kind of full-wave rectifier, called the center-tap design, uses a transformer with a center-tapped secondary winding and two diodes, as in Figure below.

Full-wave rectifier, center-tapped design.

This circuit's operation is easily understood one half-cycle at a time. Consider the first half-cycle, when the source voltage polarity is positive (+) on top and negative (-) on bottom. At this time, only the top diode is conducting; the bottom diode is blocking current, and the load “sees” the first half of the sine wave, positive on top and negative on bottom. Only the top half of the transformer's secondary winding carries current during this half-cycle as in Figure below.

Full-wave center-tap rectifier: Top half of secondary winding conducts during positive half-cycle of input, delivering positive half-cycle to load..

During the next half-cycle, the AC polarity reverses. Now, the other diode and the other half of the transformer's secondary winding carry current while the portions of the circuit formerly carrying current during the last half-cycle sit idle. The load still “sees” half of a sine wave, of the same polarity as before: positive on top and negative on bottom. (Figure below)

Full-wave center-tap rectifier: During negative input half-cycle, bottom half of secondary winding conducts, delivering a positive half-cycle to the load.

One disadvantage of this full-wave rectifier design is the necessity of a transformer with a center-tapped secondary winding. If the circuit in question is one of high power, the size and expense of a suitable transformer is significant. Consequently, the center-tap rectifier design is only seen in low-power applications.

The full-wave center-tapped rectifier polarity at the load may be reversed by changing the direction of the diodes. Furthermore, the reversed diodes can be paralleled with an existing positive-output rectifier. The result is dual-polarity full-wave center-tapped rectifier in Figure below. Note that the connectivity of the diodes themselves is the same configuration as a bridge.

Dual polarity full-wave center tap rectifier

Another, more popular full-wave rectifier design exists, and it is built around a four-diode bridge configuration. For obvious reasons, this design is called a full-wave bridge. (Figure below)

Full-wave bridge rectifier.

Current directions for the full-wave bridge rectifier circuit are as shown in Figure below for positive half-cycle and Figure below for negative half-cycles of the AC source waveform. Note that regardless of the polarity of the input, the current flows in the same direction through the load. That is, the negative half-cycle of source is a positive half-cycle at the load. The current flow is through two diodes in series for both polarities. Thus, two diode drops of the source voltage are lost (0.7·2=1.4 V for Si) in the diodes. This is a disadvantage compared with a full-wave center-tap design. This disadvantage is only a problem in very low voltage power supplies.

Full-wave bridge rectifier: Electron flow for positive half-cycles.

Full-wave bridge rectifier: Electron flow for negative half=cycles.

Remembering the proper layout of diodes in a full-wave bridge rectifier circuit can often be frustrating to the new student of electronics. I've found that an alternative representation of this circuit is easier both to remember and to comprehend. It's the exact same circuit, except all diodes are drawn in a horizontal attitude, all “pointing” the same direction. (Figure below)

Alternative layout style for Full-wave bridge rectifier.

One advantage of remembering this layout for a bridge rectifier circuit is that it expands easily into a polyphase version in Figure below.

Three-phase full-wave bridge rectifier circuit.

Each three-phase line connects between a pair of diodes: one to route power to the positive (+) side of the load, and the other to route power to the negative (-) side of the load. Polyphase systems with more than three phases are easily accommodated into a bridge rectifier scheme. Take for instance the six-phase bridge rectifier circuit in Figure below.

Six-phase full-wave bridge rectifier circuit.

When polyphase AC is rectified, the phase-shifted pulses overlap each other to produce a DC output that is much “smoother” (has less AC content) than that produced by the rectification of single-phase AC. This is a decided advantage in high-power rectifier circuits, where the sheer physical size of filtering components would be prohibitive but low-noise DC power must be obtained. The diagram in Figure below shows the full-wave rectification of three-phase AC.

Three-phase AC and 3-phase full-wave rectifier output.

In any case of rectification -- single-phase or polyphase -- the amount of AC voltage mixed with the rectifier's DC output is called ripple voltage. In most cases, since “pure” DC is the desired goal, ripple voltage is undesirable. If the power levels are not too great, filtering networks may be employed to reduce the amount of ripple in the output voltage.

Sometimes, the method of rectification is referred to by counting the number of DC “pulses” output for every 360^o of electrical “rotation.” A single-phase, half-wave rectifier circuit, then, would be called a 1-pulse rectifier, because it produces a single pulse during the time of one complete cycle (360^o) of the AC waveform. A single-phase, full-wave rectifier (regardless of design, center-tap or bridge) would be called a 2-pulse rectifier, because it outputs two pulses of DC during one AC cycle's worth of time. A three-phase full-wave rectifier would be called a 6-pulse unit.

Modern electrical engineering convention further describes the function of a rectifier circuit by using a three-field notation of phases, ways, and number of pulses. A single-phase, half-wave rectifier circuit is given the somewhat cryptic designation of 1Ph1W1P (1 phase, 1 way, 1 pulse), meaning that the AC supply voltage is single-phase, that current on each phase of the AC supply lines moves in only one direction (way), and that there is a single pulse of DC produced for every 360^o of electrical rotation. A single-phase, full-wave, center-tap rectifier circuit would be designated as 1Ph1W2P in this notational system: 1 phase, 1 way or direction of current in each winding half, and 2 pulses or output voltage per cycle. A single-phase, full-wave, bridge rectifier would be designated as 1Ph2W2P: the same as for the center-tap design, except current can go both ways through the AC lines instead of just one way. The three-phase bridge rectifier circuit shown earlier would be called a 3Ph2W6P rectifier.

Is it possible to obtain more pulses than twice the number of phases in a rectifier circuit? The answer to this question is yes: especially in polyphase circuits. Through the creative use of transformers, sets of full-wave rectifiers may be paralleled in such a way that more than six pulses of DC are produced for three phases of AC. A 30^o phase shift is introduced from primary to secondary of a three-phase transformer when the winding configurations are not of the same type. In other words, a transformer connected either Y-Δ or Δ-Y will exhibit this 30^o phase shift, while a transformer connected Y-Y or Δ-Δ will not. This phenomenon may be exploited by having one transformer connected Y-Y feed a bridge rectifier, and have another transformer connected Y-Δ feed a second bridge rectifier, then parallel the DC outputs of both rectifiers. (Figure below) Since the ripple voltage waveforms of the two rectifiers' outputs are phase-shifted 30^o from one another, their superposition results in less ripple than either rectifier output considered separately: 12 pulses per 360^o instead of just six:

Polyphase rectifier circuit: 3-phase 2-way 12-pulse (3Ph2W12P)

REVIEW:
Rectification is the conversion of alternating current (AC) to direct current (DC).
A half-wave rectifier is a circuit that allows only one half-cycle of the AC voltage waveform to be applied to the load, resulting in one non-alternating polarity across it. The resulting DC delivered to the load “pulsates” significantly.
A full-wave rectifier is a circuit that converts both half-cycles of the AC voltage waveform to an unbroken series of voltage pulses of the same polarity. The resulting DC delivered to the load doesn't “pulsate” as much.
Polyphase alternating current, when rectified, gives a much “smoother” DC waveform (less ripple voltage) than rectified single-phase AC.

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rectifier circuits

2007-11-19T17:19:00.000-08:00

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Volume III - Semiconductors » DIODES AND RECTIFIERS »
Rectifier circuits
We need your help! This page requires proofreading - If you notice any errors, please post on our forums

Now we come to the most popular application of the diode: rectification. Simply defined, rectification is the conversion of alternating current (AC) to direct current (DC). This involves a device that only allows one-way flow of electrons. As we have seen, this is exactly what a semiconductor diode does. The simplest kind of rectifier circuit is the half-wave rectifier. It only allows one half of an AC waveform to pass through to the load. (Figure below)

Half-wave rectifier circuit.

For most power applications, half-wave rectification is insufficient for the task. The harmonic content of the rectifier's output waveform is very large and consequently difficult to filter. Furthermore, the AC power source only supplies power to the load once every half-cycle, meaning that much of its capacity is unused. Half-wave rectification is, however, a very simple way to reduce power to a resistive load. Some two-position lamp dimmer switches apply full AC power to the lamp filament for “full” brightness and then half-wave rectify it for a lesser light output. (Figure below)

Half-wave rectifier application: Two level lamp dimmer.

In the “Dim” switch position, the incandescent lamp receives approximately one-half the power it would normally receive operating on full-wave AC. Because the half-wave rectified power pulses far more rapidly than the filament has time to heat up and cool down, the lamp does not blink. Instead, its filament merely operates at a lesser temperature than normal, providing less light output. This principle of “pulsing” power rapidly to a slow-responding load device to control the electrical power sent to it is common in the world of industrial electronics. Since the controlling device (the diode, in this case) is either fully conducting or fully nonconducting at any given time, it dissipates little heat energy while controlling load power, making this method of power control very energy-efficient. This circuit is perhaps the crudest possible method of pulsing power to a load, but it suffices as a proof-of-concept application.

If we need to rectify AC power to obtain the full use of both half-cycles of the sine wave, a different rectifier circuit configuration must be used. Such a circuit is called a full-wave rectifier. One kind of full-wave rectifier, called the center-tap design, uses a transformer with a center-tapped secondary winding and two diodes, as in Figure below.

Full-wave rectifier, center-tapped design.

This circuit's operation is easily understood one half-cycle at a time. Consider the first half-cycle, when the source voltage polarity is positive (+) on top and negative (-) on bottom. At this time, only the top diode is conducting; the bottom diode is blocking current, and the load “sees” the first half of the sine wave, positive on top and negative on bottom. Only the top half of the transformer's secondary winding carries current during this half-cycle as in Figure below.

Full-wave center-tap rectifier: Top half of secondary winding conducts during positive half-cycle of input, delivering positive half-cycle to load..

During the next half-cycle, the AC polarity reverses. Now, the other diode and the other half of the transformer's secondary winding carry current while the portions of the circuit formerly carrying current during the last half-cycle sit idle. The load still “sees” half of a sine wave, of the same polarity as before: positive on top and negative on bottom. (Figure below)

Full-wave center-tap rectifier: During negative input half-cycle, bottom half of secondary winding conducts, delivering a positive half-cycle to the load.

One disadvantage of this full-wave rectifier design is the necessity of a transformer with a center-tapped secondary winding. If the circuit in question is one of high power, the size and expense of a suitable transformer is significant. Consequently, the center-tap rectifier design is only seen in low-power applications.

The full-wave center-tapped rectifier polarity at the load may be reversed by changing the direction of the diodes. Furthermore, the reversed diodes can be paralleled with an existing positive-output rectifier. The result is dual-polarity full-wave center-tapped rectifier in Figure below. Note that the connectivity of the diodes themselves is the same configuration as a bridge.

Dual polarity full-wave center tap rectifier

Another, more popular full-wave rectifier design exists, and it is built around a four-diode bridge configuration. For obvious reasons, this design is called a full-wave bridge. (Figure below)

Full-wave bridge rectifier.

Current directions for the full-wave bridge rectifier circuit are as shown in Figure below for positive half-cycle and Figure below for negative half-cycles of the AC source waveform. Note that regardless of the polarity of the input, the current flows in the same direction through the load. That is, the negative half-cycle of source is a positive half-cycle at the load. The current flow is through two diodes in series for both polarities. Thus, two diode drops of the source voltage are lost (0.7·2=1.4 V for Si) in the diodes. This is a disadvantage compared with a full-wave center-tap design. This disadvantage is only a problem in very low voltage power supplies.

Full-wave bridge rectifier: Electron flow for positive half-cycles.

Full-wave bridge rectifier: Electron flow for negative half=cycles.

Remembering the proper layout of diodes in a full-wave bridge rectifier circuit can often be frustrating to the new student of electronics. I've found that an alternative representation of this circuit is easier both to remember and to comprehend. It's the exact same circuit, except all diodes are drawn in a horizontal attitude, all “pointing” the same direction. (Figure below)

Alternative layout style for Full-wave bridge rectifier.

One advantage of remembering this layout for a bridge rectifier circuit is that it expands easily into a polyphase version in Figure below.

Three-phase full-wave bridge rectifier circuit.

Each three-phase line connects between a pair of diodes: one to route power to the positive (+) side of the load, and the other to route power to the negative (-) side of the load. Polyphase systems with more than three phases are easily accommodated into a bridge rectifier scheme. Take for instance the six-phase bridge rectifier circuit in Figure below.

Six-phase full-wave bridge rectifier circuit.

When polyphase AC is rectified, the phase-shifted pulses overlap each other to produce a DC output that is much “smoother” (has less AC content) than that produced by the rectification of single-phase AC. This is a decided advantage in high-power rectifier circuits, where the sheer physical size of filtering components would be prohibitive but low-noise DC power must be obtained. The diagram in Figure below shows the full-wave rectification of three-phase AC.

Three-phase AC and 3-phase full-wave rectifier output.

In any case of rectification -- single-phase or polyphase -- the amount of AC voltage mixed with the rectifier's DC output is called ripple voltage. In most cases, since “pure” DC is the desired goal, ripple voltage is undesirable. If the power levels are not too great, filtering networks may be employed to reduce the amount of ripple in the output voltage.

Sometimes, the method of rectification is referred to by counting the number of DC “pulses” output for every 360o of electrical “rotation.” A single-phase, half-wave rectifier circuit, then, would be called a 1-pulse rectifier, because it produces a single pulse during the time of one complete cycle (360o) of the AC waveform. A single-phase, full-wave rectifier (regardless of design, center-tap or bridge) would be called a 2-pulse rectifier, because it outputs two pulses of DC during one AC cycle's worth of time. A three-phase full-wave rectifier would be called a 6-pulse unit.

Modern electrical engineering convention further describes the function of a rectifier circuit by using a three-field notation of phases, ways, and number of pulses. A single-phase, half-wave rectifier circuit is given the somewhat cryptic designation of 1Ph1W1P (1 phase, 1 way, 1 pulse), meaning that the AC supply voltage is single-phase, that current on each phase of the AC supply lines moves in only one direction (way), and that there is a single pulse of DC produced for every 360o of electrical rotation. A single-phase, full-wave, center-tap rectifier circuit would be designated as 1Ph1W2P in this notational system: 1 phase, 1 way or direction of current in each winding half, and 2 pulses or output voltage per cycle. A single-phase, full-wave, bridge rectifier would be designated as 1Ph2W2P: the same as for the center-tap design, except current can go both ways through the AC lines instead of just one way. The three-phase bridge rectifier circuit shown earlier would be called a 3Ph2W6P rectifier.

Is it possible to obtain more pulses than twice the number of phases in a rectifier circuit? The answer to this question is yes: especially in polyphase circuits. Through the creative use of transformers, sets of full-wave rectifiers may be paralleled in such a way that more than six pulses of DC are produced for three phases of AC. A 30o phase shift is introduced from primary to secondary of a three-phase transformer when the winding configurations are not of the same type. In other words, a transformer connected either Y-Δ or Δ-Y will exhibit this 30o phase shift, while a transformer connected Y-Y or Δ-Δ will not. This phenomenon may be exploited by having one transformer connected Y-Y feed a bridge rectifier, and have another transformer connected Y-Δ feed a second bridge rectifier, then parallel the DC outputs of both rectifiers. (Figure below) Since the ripple voltage waveforms of the two rectifiers' outputs are phase-shifted 30o from one another, their superposition results in less ripple than either rectifier output considered separately: 12 pulses per 360o instead of just six:

Polyphase rectifier circuit: 3-phase 2-way 12-pulse (3Ph2W12P)

* REVIEW:
* Rectification is the conversion of alternating current (AC) to direct current (DC).
* A half-wave rectifier is a circuit that allows only one half-cycle of the AC voltage waveform to be applied to the load, resulting in one non-alternating polarity across it. The resulting DC delivered to the load “pulsates” significantly.
* A full-wave rectifier is a circuit that converts both half-cycles of the AC voltage waveform to an unbroken series of voltage pulses of the same polarity. The resulting DC delivered to the load doesn't “pulsate” as much.
* Polyphase alternating current, when rectified, gives a much “smoother” DC waveform (less ripple voltage) than rectified single-phase AC.

Discuss this topic | Feedback «Previous Page | Next Page»

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