TYPES OF DIODES - OVERVIEW The different diode types explained

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Whether the operation is through absorbing light or emitting photons, there are many diode types in the world of semiconductors. These diode types are useful in various applications like rectification, power conversion, microwaves, optics, sensing, protection circuits, and many more. The article explores diode types and symbols of the number of unidirectional semiconducting devices. It also states the types of diodes and their applications.

Types of diode - and what you need to know about them

Different diode types are available from different vendors in the electronics market. There are multiple functions of all types of diodes and their applications are many. The usage of a single type of diode is not limited to one but multiple categories. It is very difficult to classify diodes on the basis of application. However, this article lists 30 important diode types and symbols with the classification of the same diode types based on their popular applications and similar operations.

Rectification diodes

1. P-N junction diode

PN junction diodes are the most basic diode types used in rectification, power conversion, clipping, and voltage level shifters. The term diode or semiconductor diode and PN junction diode are used often interchangeably. PN junction is among the most common types of diodes because of its simple structure. However, the term “diode” applies to a broader range of devices.

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Just as the name suggests, the PN junction diode is a combination of the pentavalent impurity doped P-type and trivalent impurity doped N-type semiconducting materials that create a junction known as the “PN junction”. When the PN junction diode is forward-biased, it acts as a closed switch and conducts. But in reverse bias, the PN junction diode acts as an open switch to block the current flow. Hence, PN junction diodes allow the unidirectional flow of current.

2. Zener diode

Zener diodes are among the types of diodes similar to the basic PN junction diode but are operated in reverse bias as a voltage regulator, clipper circuits, shift registers, etc. In forward bias, the zener diode behaves like a normal PN junction diode type. Upon the application of input voltage in the reverse bias, the narrow depletion layer allows electrons from the valence band of the P-side to “tunnel” into the conduction band of the N-side.

The principle in which charge carriers tunnel through a narrow depletion region in a heavily doped PN junction is known as the “Zener effect”. Electrons can easily tunnel to the other side because of the extremely small depletion layer width and cause reverse current to flow.

3. Power diode

A Power diode is a part of high-power electronics diode types. Just like most power semiconductor devices, power diodes have an additional N- layer, also known as the drift region. A heavily doped P+ region and the N- drift region form a junction. These two layers are grown epitaxially over a heavily-doped N+ layer. Such doping concentration and three-layer arrangement increase current and voltage ratings for high-power applications.

Features include high breakdown voltage and current handling capabilities. The power diode type must dissipate excessive heat due to increased ohmic resistance, making the heatsink a suitable solution. The applications of a power diode include DC power supplies, snubber circuits, power rectification, voltage regulation, inverters, etc.

4. Rectifier diode

A rectifier is among the types of diode that perform the function of conversion from AC to DC in a circuit. Applications include a half-wave rectifier and a full-wave rectifier. Additionally, rectifier diodes are used in groups together to perform the AC to DC power conversion. One such application is a bridge rectifier that uses 4 or 6 diodes to rectify an input signal.

The working principle of a rectifier diode is to conduct and disconnect in consecutive half cycles. The rectifier diode is forward bias in the half cycle to conduct and reverse bias in another to block the current flow. It cuts off some positions of the input waveform and passes the desired output. Hence, DC output is easily obtainable from rectifier diode operation.

5. Schottky diode

In a Schottky diode, there is no PN junction but a metal bonded to either an N-type or P-type semiconductor. An N-type Schottky diode type contains an N-type material with the metal and a P-type Schottky diode consists of a P-type material with the metal. In Schottky types of diode, the bond between metal and N-type semiconductors creates a junction inside the diode.

The Schottky barrier potential is the voltage required for the electrons to cross the junction. The benefits of using such a diode allow a lesser threshold voltage than 0.7 V of a Silicon diode, resulting in a fast switching speed. Schottky diodes are used in rectification and conversion but are applicable in digital electronics. Examples include TTL and CMOS logic families applications.

6. Super barrier diode

A Super Barrier Rectifier (SBR) is a proprietary device belonging to Diodes Incorporated. Contrary to popular belief, SBR is not a Schottky diode but an enhanced diode type that follows a MOS manufacturing process to obtain low forward voltage, less leakage current, and optimal switching performance. The diode type supports a lower barrier potential voltage for a quicker operation with thermal stability compared to Schottky diodes.

Super barrier rectifier “SBR” is another power semiconductor device by Diodes Incorporated that converts AC power to DC power. It offers high power efficiency at low operating temperatures. There are wide SBRs with different voltage ratings, packing, and applications. SBR diode types are used in buck/boost converters, solar panels, automotive LED lighting, and many more applications.

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Emitting diodes

7. LED diode

A light-emitting diode “LED” is one of the most common diode types that converts electrical energy into radiation. The popular display of screens is made of LEDs that work on the principle of electroluminescence, in which an electrical energy source produces photons. In semiconductor physics, electron-hole recombination typically means going from a higher-energy state to a low-energy state. The change of energy state releases heat or light.

Common semiconductors like silicon and germanium release mostly heat but compound semiconductors release photons. Semiconductors such as gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP) release photons upon recombination of electrons and holes. The color of an LED type of diode depends upon the wavelength emitted by the semiconductor used for manufacturing. In electrical and electrical engineering, turning “ON” the LED is the basis of verifying the current flow in a circuit.

8. Laser diode

A laser diode works on the principle of electroluminescence, similar to an LED diode. The change from a higher energy state to a lower state generates radiation in either the infrared, visible, or ultraviolet spectrum. The difference between a laser diode and LED is that a laser diode uses an external voltage source to support lasing conditions internally. Another difference is that the LED diode type produces monochromatic incoherent light but laser diodes produce coherent light. The electric current flow promotes electron-hole recombination to release photons.

Some of the features of laser diode types are injection technology and narrow beam. One of the most popular laser diode types is the Quantum Well diode which works on the principle of “quantum wells”- based on discrete energy level values. Laser diodes are used in laser printing, fiber optics, bar codes, laser scanning, and various industrial applications.

Switching diodes

9. Step recovery diode

A step recovery diode “SRD” is a type of diode used for fast-switching operations. The doping concentration is kept extremely low near the junction on P and N sides. The parts of semiconductors away from the junction are heavily doped. It reduces the number of charge carriers in the junction to support quick turn-on and off. Also known by the name of a Snap-off diode, an SRD diode type works on the principle to prevent electron-hole recombination in the junction to store the charge.

The prevention of recombination is possible through the generation of high minority charge carriers. The majority charge carrier lifetime determines the frequency of operation of the diode. SRDs are used in pulse generators and frequency multiplies for microwave frequencies.

10. PIN diode

The P-I-N diode stands for P-type, Intrinsic, and N-type semiconductors. These diode types consist of an intrinsic semiconducting layer placed between two heavily doped P-type and N-type materials. The idea behind such placement is to allow for maximum reverse voltage. There is a low carrier concentration in the intrinsic layer, making the diode suitable for high-frequency operations.

During reverse bias, the depletion region width increases further to block reverse current and offer insulation. In forward bias, the charge carriers move from P to N for current flow. The increased space between P and N-type semiconductors due to the intrinsic layer in the PIN diodes lowers the junction capacitance compared to the other type of diodes. PIN diodes are used in switching, rectification microwaves, optic networks, and many more applications.

11. Tunnel diode

A tunnel diode is based on the principle of “Quantum Tunneling”. Heavy doping of the PN junction decreases the width of the depletion region. Under forward bias, the current slowly increases as electrons from the valence band tunnel to the empty conduction band states over the small barrier. However, the energy states on the N-side and P-side become misaligned with time, leading to a decreased current flow.

The effect makes the diode exhibit negative resistance where current decreases with an increase in voltage. On the other hand, the energy states continue to align more with time in reverse bias. The operation renamed tunnel diode types to “backward diodes”. Applications of tunnel diode types are switching, fast rectification, oscillation, and amplification.

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Photodetectors

12. Photodiode

Opposite to an LED diode, a photodiode converts light energy into electrical energy. Also known as a photodetector, a photodiode is a PIN diode that operates upon the incident light. Applications of a photodiode include light detection, remote controls, lenses, optical filters, etc. The photons of an intensity greater than the band gap of the semiconductor must hit the surface to generate electron-hole pairs.

The electric field of the junction separates the electron-hole pairs before recombining. The electrons and holes move in opposite directions toward the semiconducting materials. The increase in electrons and hole numbers generates an electromotive force whose magnitude depends upon the intensity of the incident light. Photodiodes operate in two modes- photovoltaic mode- zero bias and photoconductive mode- reverse bias. In the reverse operation, a leakage current known as the dark current flows through the photodiode.

Protective diodes

13. Flyback diode

The Flyback diode type, also known as a Flywheel diode or Suppression diode or Kickback diode or Snubber diode performs a similar function to snubber circuits. A flyback diode is a PN junction diode that is used with an inductor to reduce the voltage spike in a circuit. The sudden removal of the power supply may produce a voltage spike that could lead to a spark. The Flyback diode operates with the inductor to divert the current towards itself.

An important aspect of Flyback diode types is that they do not change the operation of the circuit. The Flyback diode is reverse-biased or turned off when the switch is closed. But when the switch is open, the Flyback diode is forward-biased to conduct the faulty current while the switch is disconnected. It eliminates the arc generation and protects the switch.

14. Transient voltage suppressor (TVS) diode

A Transient Voltage Suppressor (TVS) diode is a semiconductor device that dissipates high transient power surges. It means that the TVS diode type is capable of suppressing high voltage spikes. These diodes are avalanche diodes that are used to clamp overvoltages. TVS diode types operate in two modes- normal mode and surge suppression mode. In the surge suppression mode, TVS cuts overvoltage at the clamping voltage value.

Also known as transil or thyrector, TVS diodes suppress extra current when voltage exceeds the avalanche breakdown point. Technically, it suppresses all voltages above the breakdown point or the specified clamping point. TVS diode types are available in two configurations- unidirectional and bidirectional. Unidirectional TVS shows asymmetrical VI characteristics for protecting unidirectional signals above the reference level. Bidirectional TVS show symmetrical VI characteristics for bidirectional signals above and below the reference level.

15. Small signal diode

Just as the name suggests, a “small signal” diode signifies an AC signal. Small signal types of diodes are applicable in waveform clipping, and clamping. The small signal response of semiconducting diodes specifies the junction and related internal capacitances. The small signal diode is manufactured to reduce the junction capacitance, perform high-speed switching and offer a reverse recovery time in the order of nanoseconds.

The small signal diodes have a smaller junction area to reduce the junction capacitance, making them perform well in higher frequencies. The area of the junction is reduced through the chemical process of etching that removes a significant portion of the PN junction. This portion is called a Mesa diode. Technically, the small signal types of diodes contain N- lightly doped, N+ heavily doped, P- lightly doped, and P+ heavily doped layers.

16. Constant current diode

A constant current diode or current limiting diode (CLD) or current regulating diode (CRD) performs the function of limiting the current to a specified value determined by the manufacturer. The constant current diode types and symbols are a combination of circular anode instead of triangular PN junction diode type anode but the cathode is similar.

The constant current diode is structurally an n-JFET with a shorted gate-to-source terminal that allows unidirectional current flow. Silicon Carbide (SiC) is used in constructing constant current diode types for high-voltage operation. The constant current diode provides a stable value for current for longer periods. The current regulation point is the maximum value of current the device can regulate. Constant current diodes are used in LED drivers, laser diodes, and related circuits.

Microwave or high power diodes

17. Varactor diode

A Varactor diode or Varicap or Variable Capacitance diode type is a device that provides electrically controllable capacitance. The Varactor diode type is used in the place of capacitance for storing the charge in microwave applications. When any type of diode is reverse-biased, the depletion layer thickness increases with an increase in the input voltage. The area of the junction, doping concentration, and various other factors determine the capacitance.

Capacitance is inversely proportional to the width of the depletion region and the square root of the applied voltage. The varactor diode type is constructed in such a way that it provides a variation of capacitance with a low input reverse voltage. A Varactor diode is a small inexpensive device used in tuned circuits, VCOs, RF filters, and other microwave applications.

18. Gunn diode

A Gunn diode consists of three layers of N-type material where a lightly doped N- buffer layer is placed between the two heavily doped N+ layers. The working principle of a Gunn diode is the “Gunn effect” observed in 1962. When voltage is applied to the Gunn diode, it initially conducts like a normal diode. A high electric field develops around the middle lightly doped layer. The electrons gain sufficient kinetic energy to reach higher valleys of the conduction band.

The mass of electrons is observed to be higher in the higher valley, reducing the mobility of the electrons. The conductivity of the diode decreases with increasing voltage up to the peak point, making the device exhibit negative resistance. The diode conducts again beyond a specific point known as the “valley point” because electrons reach the lower level of the conductivity band, gaining high mobility. Gunn diode types are used in microwave oscillators.

Avalanche transit time (ATT) diodes

19. Avalanche diode

An Avalanche diode is a semiconductor device that operates in reverse bias mode to support the avalanche breakdown. Unlike a Zener diode, an avalanche diode is lightly doped and exhibits a positive temperature coefficient. The difference that light-doping creates is that the depletion layer width is thicker compared to that of the Zener diode types. A thicker depletion region allows the generation of a weaker electric field. It further causes breakdown to occur at higher voltages.

Avalanche diode types are manufactured to withstand avalanche breakdown without any physical damage. These diodes show a lesser voltage drop under the breakdown operation. Avalanche diodes are used as voltage reference diodes, protection circuits, SPAD (single photon avalanche diode) receivers for digital signal processing, and as the base for Avalanche Transit Time (ATT) devices in microwaves. However, the Avalanche diode is based on the avalanche effect but is not an ATT.

20. IMPATT diode

IMPATT diode stands for “Impact Ionization Avalanche Transit Time” diode. These types of diodes are capable of withstanding high-power and microwave applications. It is available usually in microwave packaging with a heat sink. The operating frequencies of the IMPATT diode range from 3 GHz to 100 GHz and output power between 1 w CW and above 400 W pulsed. IMPATT types of diodes and their applications are largely microwave oscillators and low-power radar systems.

The IMPATT diode is structurally a PIN diode consisting of four semiconducting layers: a P+ heavily doped layer, a lightly doped N layer, an intrinsic layer, and an N+ heavily doped layer. The IMPATT diode works on the principle of avalanche effect carrier multiplication and transit time delay. In the reverse bias operation, the IMPATT diode exhibits negative resistance at microwave frequencies.

21. TRAPATT diode

TRAPATT diode stands for “Trapped Plasma Avalanche Triggered Transit Time” diode. It belongs to the family of avalanche transit time types of diodes and is widely used in microwave generators and oscillators. TRAPATT diode is more efficient compared to IMPATT diode types but is unstable in terms of noise generation. The operating frequencies of the TRAPATT diode range from 1 GHz to 10 GHz.

TRAPATT diodes consist of either of the structural layers: P+, N, and N+ doped layers. Similar to the IMPATT diode types, the TRAPATT diode operates in reverse bias in the avalanche region. The difference lies in the triggering mechanism of the TRAPATT diode in which the electric field reaches the critical value. The electric field reduces due to charge carriers drifting at lower velocity- in short, “trapped” at a lower velocity. The avalanche breakdown spreads across the whole active region to increase the current flow.

22. BARITT diode

BARITT diode “Barrier Injection Transit Time” diode. It is often compared with IMPATT diode types because it offers less noise, about 15 dB that may or may not be applicable in every system. BARITT diode types are used widely in microwave applications and burglar alarm systems. The operating frequencies of the BARITT diode range from 4 GHz to 8 GHz and low output power in the order of milliwatts.

BARITT diode types work on the principle of Thermionic emission for low noise rather than Avalanche multiplication and plasma generation like IMPATT and TRAPATT types of diodes. Two PN junction diodes are placed back to back for BARITT operation. The N-type material of one of the diode types is larger in area to act as the injection region for the device.

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Historical diodes

23. Vacuum diode

Vacuum diodes were historically used before the introduction of semiconductor diodes. Sir John Ambrose Fleming invented the very first vacuum diode which gained popularity over the years. Vacuum diode types were used in amplification, oscillation, radio transmission, etc. The function of a diode is to allow the unidirectional flow of current- vacuum diodes worked the same.

A vacuum diode is not actually a type of diode engineers use in today’s times but structurally a “vacuum tube” with two electrodes. One of the electrodes is a positive “Anode” and another is a negative “Cathode”. The vacuum diode could be forward or reverse-biased, depending on the application. The cathode must be heated directly or indirectly to enable the current flow. Also known as thermionic diodes, vacuum diode types allow the current to flow from cathode to anode but block in the other.

24. Double diode triode

A double diode is a historical device that used to perform similar operations as today’s semiconductor diode. Contrary to other types of diodes, a double diode is not exactly a diode but a vacuum tube. However, the double diode symbol is a combination of two diodes. In the 20th century, electronic vacuum tubes were extensively used before the rise of semiconductors such that multiple electronic tubes were used in groups to serve various applications.

The “double diode triode” is a special vacuum tube that has a single triode and two so-called diodes. These two diodes are not actually semiconductor diodes but electrodes, then referred to as diodes. The double diodes were used in amplification, rectification, and detection applications. The main use of double diode triode was in American radio receivers extensively.

25. Point contact diode

The point contact diode is an old device that used to perform a similar function to diodes. The device is actually not a PN junction diode type but an N-type semiconductor with a metal wire placed in an envelope or glass tube. Metal wire made up of phosphorus, bronze or silver directly touches the N-type region for the diode to function.

The metal wire or the cat’s whisker acts like a P-type semiconductor upon heating. The direct contact between the metal and the N-type semiconductor creates a small junction that possesses less capacitance. These diode types are no longer in mainstream applications due to the introduction of better technologies. Point contact diodes were used in radio receivers, video detectors, and TVs.

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26. Crystal diode

Crystal diodes were the initial diode types used in radio communication and detection. The name crystal directly suggests the manufacturing “crystal material” of the diode. The solid-state material could either be lead sulfide “Galena” or silicon. Crystal diode types were just a type of point contact diodes with crystal material. These diodes were known by the name of “Cat’s Whisker” because of the use of the thin metallic wire.

The thin metallic wire made up of copper or brass was made to directly touch the semiconducting crystal material for the formation of the junction. The device enabled current flow in one direction and blocking in the other. However, crystal diodes were less efficient and sensitive due to repeated readjustment of direct contact between the wire and semiconductor.

27. Shockley diode

The Shockley Diode was one of the oldest and first semiconductor diode types. Dr. William Shockley, the Nobel Prize winner, invented the Shockley diode in the late 1950s. Shockley Transistor Corporation marketed the device as “Shockley Four-Layer Transistor Diode”. Shockley diode was structurally a four-layer three-terminal semiconductor device “Thyristor” with no controlling gate terminal in the device.

Shockley diodes were called diodes because of their rectification properties in certain operating conditions. These four layers are made from alternating P-type and N-type semiconductor materials. The device exhibits a unique character of negative resistance where current flow decreases with increasing voltage. Applications include switching, sawtooth oscillators, ring counters, control circuitry, computer circuits, telephony, and many more. The developing technologies like DIAC and Dynistors replaced Shockley diode types gradually.

28. Gold doped diode

A gold-doped diode is a Silicon PN junction diode type with gold (AU, 79) dopant for enhancing the switching time in computer diodes. Uniform gold concentrations are doped in the silicon PN junction diode using solid-state diffusion techniques. The doping of gold enables the diode to control the electron-hole recombination without affecting other electrical properties. The resulting gold-doped diode provides a reverse recovery time of about 1 milli microsecond.

The reverse recovery time is a function of the minority carrier lifetime, which in turn is inversely proportional to the golf atom concentration. The gold dopant also reduces leakage current and charge storage capabilities. Gold-doped diode types were historically proposed to be used in computers for high switching speed. However, gold has always been too expensive for doping and fabrication.

Latest diode technology

29. Thermal diode

A thermal diode is a device that allows heat to flow in one direction and blocks in the other. Non-electrical devices are used to allow one-way heat flow but thermal diodes are recently studied, experimented and developed to be used in thermoelectric coolers and heat pumps. One terminal of the diode is hotter than the other. According to the second law of thermodynamics, heat will transfer from the hotter terminal to the low-temperature terminal.

In simple words, a thermal diode effectively manages heat in a system. However, thermal diodes have not been commercialized much like the semiconductor diode types. Thermal diodes could be applicable in various applications including newer technologies like thermal computing, caloric refrigeration, consumer electronics, and renewable energies.

30. Superconducting diode

A superconducting diode type is a new technology under research and development with a few experimental models. The superconducting diode is similar to an ideal diode showcasing ideal characteristics. Superconducting types of diodes exhibit zero voltage drop in the conducting path and infinite resistance in the non-conducting state. The proposed superconducting diode is a magnetically controlled device placed in an artificial superlattice.

Superconducting types of diodes and their applications are predicted to work well with superconductors. Another example could be the Josephson junction where a non-superconducting material is sandwiched between two superconductors for effective current flow.

References

• Boylestad, R., & Nashelesky, L. (2012). Electronic Devices and Circuits Theory (11th ed.). Pearson Education.

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