Types of diodes are very important to learn especially for power electronics circuit. Diode is a two-terminal electrical component which is able to pass a flowing current in a direction and block the flowing current in the opposite direction. Our guide here is mainly about the types of diodes, pictures, symbols, and functions.
What is Diode
We won’t spend much time learning this matter since our main topic will be quite long. Diode is known as a semiconductor component because it is made from semiconductor materials such as silicon or germanium (both give different results of the diodes).
Like mentioned above, diodes can pass a current in our desired direction and block the current if it flows in the opposite direction. Just imagine it as a one way street. This is why we can convert AC voltage into DC voltage because it blocks the current when its value is in the negative area.
What is Diode Symbol
Diode symbol illustrates its function in a smart way. It contains an arrow with a “wall” at its end just as drawn below.
Diode is a two terminal component and its terminals named with:
Anode represents the direction we desire to flow or also known as forward biased. Cathode represents the direction we desire to block or also known as reverse biased.
What is Diode Structure
After looking at the explanation above, we need to know how diodes can behave like that. Its unique work principle comes from its semiconductor materials such as silicon or germanium.
As long as the anode voltage is higher than cathode voltage, the current is forward biased, current is flowing through in the circuit without any problem.
As long as the cathode voltage is higher than anode voltage, the current is reverse biased, current is not able to flow past the diode and no current flows in the circuit.
Types of Diodes
This is where we will spend most of our time, learning what are the types of diodes used in every circuit application. The types of diodes are:
- Light Emitting Diode (LED)
- Laser diode
- Avalanche diode
- Zener diode
- Schottky diode
- PN junction diode
- Small Signal Diode
- Large Signal Diode
- Constant Current Diodes
- Shockley Diode
- Step Recovery Diodes
- Tunnel Diode
- Varactor Diode
- Transient Voltage Suppression Diode
- Gold Doped Diodes
- Super Barrier Diodes
- Peltier Diode
- Crystal Diode
- Silicon Controlled Rectifier
- Vacuum Diodes
- PIN Diode
- Point Contact Devices
- Gunn Diode
Knowing what types of diodes won’t get us anything if we don’t learn what they are and what are their functions. Then let us move to the detailed explanation below.
What is Light Emitting Diode (LED)
Implies from its name, this diode emits light when the diode is forward biased or in other word, the electrodes pass through the diode. When the current is sufficient, the diode is forward biased and a light is emitted.
Its light can be visible or invisible to naked eye depending on its function. In our TV remote, it uses an infrared LED to send signals and it is invisible to our eye. Another example is our room lamp, it often uses white color and it is visible to our naked eye.
What is Laser Diode
This diode works just like LED but it produces coherent light such as laser light. It is often used for reading CD or DVD drives and other laser devices. It costs more than LED obviously but still cheaper than other types of laser generator. This laser diode does not have a long lifetime so keep in mind before using this.
There are some laser diode types we can use:
- Double Heterostructure Laser
- Quantum Well Lasers
- Quantum Cascade Lasers
- Separate Confinement Heterostructure Lasers
- Distributed Bragg Reflector Lasers
What is Avalanche Diode
Avalanche diode is one type of reverse biased diode and implements an avalanche characteristic. This diode is made exclusively to detect or experience avalanche breakdown at one specific point of reverse biased voltage.
Avalanche diode has a special junction to prevent casualty (current concentration, hot spot), hence the diode is safe from breakdown.
Avalanche diodes are mainly used for voltage reference, protection, radio-frequency noise generator, microwave frequency generator, and photon avalanche detector.
Avalanche diodes are mainly used as:
- Microwave frequency generator
- RF noise generator
- Single photon avalanche detector
- Radio equipment
- Hardware RNG (Random Number Generator).
What is Zener Diode
Zener diodes are very popular for electrical circuit designers. It can be used for a stable reference voltage. Zener diode implements the reverse biased principle and reaches breakdown condition when a certain voltage is achieved.
As long as current is limited to a certain degree, the stable voltage is produced. This diode is mainly used in power supply circuits to provide a reference voltage.
The Zener diode works differently from the usual diode. It implements the “Zener Breakdown”. Its forward bias is still similar to normal diodes but the reverse biased is different.
Zener diode’s reverse biased only happens when the applied voltage is equal to the breakdown voltage (Zener Breakdown). It is well designed as a voltage regulator to prevent momentary voltage pulses on other semiconductor devices.
What is Schottky Diode
Schottky diodes have lower forward biased voltage than PN junction diodes. While silicon PN junction has voltage drop 0.7 V, this Schottky diode only has between 0.15 – 0.4 V. This diode is popular for building rectifier circuits.
Schottky Diode is made from its semiconductor material connected with metal. This way results in reducing voltage drop to the minimum value. Its semiconductor type is N silicon as the anode and the metal as the cathode (Tungsten, Platinum, Chromium, etc).
From this construction, this diode has reduced switching time and high current carrying capability. This diode is superior in switching purpose than other types of diodes.
Because there is metal in the diode, the voltage drop is low, hence the diode performance is increased and power loss is lower.
What is Photodiode
This diode’s work principle is the opposite of the Light Emitting Diode. If LEDs are able to emit light when the current is flowing, the photodiode can make the current flowing when it receives some light intensity.
Photodiodes work very well for light detectors and are commonly used for solar cells and photometers, which are able to generate electricity.
Read also : active filter types
What is PN Junction Diode
PN junction diodes are popular to use for rectifier circuits. This diode is made from semiconductor material such as silicon. PN junction diode is built from two semiconductor layers with P-material and N-material.
This is where PN junction name comes from. This diode works just like a regular diode, allows current in the same direction and blocks current in the opposite direction.
What is Small Signal Diode
This diode is mainly used for high frequency but low current circuits (radio, television, and more). This diode is also known as Glass Passivated Diode since it is covered with glass to prevent contamination. One of its examples is 1N4148.
Its size is very small when compared with a power diode. Just like the cathode terminal of a power diode is represented by a marker at one side, the small signal diode also has black or red marker at one side to represent the cathode terminal.
Related to its small size, the small signal diode is only capable of carrying a small current of about 150mA and power dissipation of about 500mW. Its material is made from semiconductor material such as silicon or germanium. Its performance depends on its material.
It is very effective to use small signal diodes when designing a high frequency circuit. This diode is commonly used for high frequency switching circuit, amplifier, any general diode purposes, and many more.
What is Large Signal Diode
Unlike small signal diodes, this diode has a large PN junction layer and is often used for rectifier circuits to convert AC to DC. The large signal diode is capable of carrying higher forward current and reverse voltage threshold.
Unlike small signal diodes, this diode is not effective for high frequency circuits. Large signal diode is able to work in high voltage and current circuits, hence it is used to suppress high peak voltage.
Large signal diode is efficient for power supply circuits such as inverter, converter, rectifier, charging circuit, and so on. Its reverse blocking resistance is almost infinite at Mega Ohms while the forward resistance is low.
What is Constant Current Diode
Constant current diode is also known as:
- Current Regulating Diode
- Current Limiting Diode
- Diode Connected Transistor
This diode is used as a voltage regulator at specific current. It is a two terminal current limiter and this JFET works as a current limiter to get high output impedance.
What is Shockley Diode
Even though its name is similar to Schottky diode, it is completely different. Shockley diode is constructed with four PN layers so it is also known as PNPN diode.
From its construction, it is similar to a thyristor but without the Gate terminal. With the Gate terminal removed, we don’t need to trigger it to make it forward biased. The only thing we need to do is apply forward voltage.
It will stay in “ON” state once it is turned ON and it will stay in “OFF” once it is turned OFF. With these characteristics, Shockley diodes are used as switches to trigger an SCR and can be used for relaxation oscillators.
What is Step Recovery Diode
This diode is also called:
- Snap-off diode
- Charge-storage diode
This is one of the special diodes from other types of diodes. It will store the charge it gets from forward voltage or positive pulse. Later, it will use this charge on the negative pulse of a sinusoidal voltage.
Its rise time is equal to the “snap time” hence its pulse recovery time is fast.
Since it has speed recovery time, it is suitable for high order circuits since its cutoff frequency is very high. One of its applications is as a multiplier with a cut-off frequency in GHz.
What is Tunnel Diode
Among the types of diodes; this is another high-speed switching diode, even in nano-seconds timer. It has stray wiring capacitance and capacitance junction to limit its transient response.
Since its speed is in nano-seconds, it has a high switching frequency and it is used in high frequency applications such as amplifiers and microwave oscillators.
Tunnel diode is mostly used for negative conductance purposes in some devices and it is adjustable mechanically or electrically.
A few of its applications are:
What is Varactor Diode
This type of diode is also known as:
- Varicap diodes
- Variable capacitance diode
- Variable reactance diode
- Tuning diode
As its name has “variable”, it means it has variable value in it. A Varactor diode is used as a variable capacitor and its working region is mostly reverse biased.
Even though it is a diode, it is able to change the capacitance in the circuit with a high value threshold. Its depletion layer can be increased or decreased when we change the reverse biased voltage.
Its popular applications are:
- Voltage-controlled oscillators (cellphone, pre-filter satellite, etc)
- Voltage-controlled capacitors
- Frequency multipliers
- Parametric amplifiers
What is Transient Voltage Suppression Diode
Semiconductor components and devices will experience a transient period when there is a sudden change of voltage. It will cause the device to be damaged by the output response. This is where Voltage Suppression Diodes kick in.
How the voltage suppression diode works is similar to the Zener diode.
It is formed by a normal p-n junction so it works similar to normal diodes. The only difference is its state changes at time when transient voltage appears.
At start (normal condition), its impedance will be high and it will go low when transient voltage appears and the diode goes to avalanche breakdown condition which has low impedance.
What is Gold Doped Diode
The construction of this diode uses Gold as its dopant. From this gold material, it boosts the speed of this diode. Not only that, its leakage current is also less than other types of diode when reverse biased.
What is Super Barrier Diode
Its forward biased voltage drop is as low as Schottky Diode. This diode is considered as a rectifier diode along with the ability of handling electrical surge.
Super barrier diode has reverse biased current leakage as low as PN junction diode.
It is considered a rectifier because it is compatible with high power circuits, fast switching ability, and low power losses. Not only that, the super barrier diode has lower forward voltage drop than Schottky diode.
What is Peltier Diode
Just as the name implies, this diode also acts like a peltier. This diode produces heat at the junction of two-material semiconductors. This is done in a direction following the current’s flow direction.
Not only as a heater, peltier diodes can also act as a cooler, heat engine and sensor for thermoelectric cooling.
What is Crystal Diode
This diode is constructed from a metal wire pressing the semiconductor crystal. From this pressure mechanism, a crystal diode is a contact type diode. How it behaves depends on the pressure to its crystal and the point.
The metal wire acts as the anode while the semiconductor crystal acts as the cathode. This is one of a special types of diodes which is not to be found in nature.
This diode is also known as Cat’s whisker. You will find this type of diode most often in microwave receivers and detectors.
There are several uses of a crystal diode such as:
- Crystal diode receiver
- Crystal diode detector
- Radio receiver
What is Silicon Controlled Rectifier
Silicon Controlled Rectifier or SCR is a popular among the types of diodes used in power electronics. Unlike the common diode, SCR has three terminals: a cathode, an anode, and a gate. It is almost similar to Shockley diodes.
Its gate terminal is made for a control purpose. Even if it is in forward biased condition, we still need to trigger the gate to make the current pass through.
From the extra gate terminal, SCR has extra operation modes:
- Forward Blocking Mode (Off state) = positive voltage (+) is applied to the anode, negative voltage (-) is applied to the cathode, and the gate terminal is open circuit. SCR is in forward biased condition but still blocks the current through it.
- Forward Conducting Mode (On state) = positive voltage (+) is applied to anode, negative voltage (-) is applied to cathode, positive voltage (+) is applied to gate terminal. SCR is in forward biased condition and the current is able to flow through it. Another method to achieve this state without applying voltage to the gate terminal is applying more voltage to anode until it reaches breakdown condition.
- Reverse Blocking Mode (Off state) = positive voltage (+) is applied to the cathode, negative voltage (-) is applied to the anode, and the gate terminal is open circuit. SCR is in reverse biased condition and blocks the current through it.
What is Vacuum Diode
This type of diode is constructed quite differently than other conventional diodes. It is made from two electrodes instead of semiconductor material as the cathode and anode.
Tungsten is used as the cathode and able to emit electrons to the anode. The direction of the electrons is always from cathode to anode, just like switches.
Oxide material is used as a coat for the cathode to make its capability of electrons emission is high. Anode’s surface is rough to lessen the heat temperature in the diode when operating.
Its conducting state is only achieved if the anode is positive with respect to the cathode terminal.
What is PIN Diode
This diode is the improved version of the PN junction diode we have read above. This improved PN junction diode eliminates the need of doping.
The natural materials (intrinsic materials) such as no charge carrier materials, are inserted between PN layers in between. Hence, the depletion layer area is increased.
When forward voltage is applied, both electrons and holes are pushed into its intrinsic material layer. From this behavior, electric fields will be produced through the intrinsic material due to high injection levels.
It is quite complicated to understand and use this type of diode, but it is used in some applications:
- RF switches
- Photo detector
What is Point Contact Devices
Among the types of diode, this one doesn’t share the name “diode” but it still works well as a diode. Here we use Tungsten or gold as the wire acts as a contact point to generate PN layer and junction regions. This is achieved by passing high electric current through them.
After passing the high current, a small PN junction region is generated around the edge of the wire connected to the metal plate. This is the forward biased state, quite similar to other diodes.
For the reverse biased, the wire will act as an insulator between the plates while the diode acts as a capacitor. Since it acts like a capacitor, it naturally blocks the DC current while the AC current can flow at a higher frequency.
Looking at this characteristic, this device is often used for high frequency signal detectors.
What is Gunn Diode
Unlike other diodes which are constructed from P and N types of semiconductor materials, this diode is made only from n-type semiconductor material. The depletion region of these two N-type materials is very thin.
When we increase the voltage, the current will be increased at the same time. After the voltage reaches a specific point, the current will drop exponentially. From this behaviour, this diode implements negative differential resistance.
There are two electrodes made from Gallium Arsenide and Indium Phosphide which are the cause of the negative differential resistance. This diode is often used for microwave RF devices or as an amplifier.
Frequently Asked Questions
How many types of diodes are there?
What are the 5 general types of diodes?
-PN junction diode
-Light Emitting Diode
-Silicon Controlled Rectifier
-Transient Voltage Suppression diode
What is diode and its types?
What are the 3 main uses of diodes?
How are diodes classified?