Different Types of Diodes and Their Applications & Symbols

Diodes are essential components in electronics, each designed for unique purposes. They allow current to flow in a single direction, making them crucial for rectification, signal modulation, and voltage regulation. This guide explores various types of diodes, including p-n junction diode, zener diode, point-contact diode, varactor diode, gunn diode, tunnel diode, PIN diode, schottky diode, impatt diode, trapatt diode, baritt diode, step recovery diode, Light emitting diode, laser diode, photodiode etc. explaining their symbols, functionalities and applications in electronics.

What is a diode ?

Introduction:
• The diode is a electronic device which consists of only two electrodes i.e. anode and cathode.
• As shown in diode symbol, it is a two terminal device which allows current to flow in only one direction e.g. forward direction. In this mode anode has higher potential than cathode. This state of diode is known as forward biased state.

• If the polarity of voltage is reversed , the diode is known to be in reverse biased state. In this state diode will attempt to block the current flow within its rated limits.
• It is used for numerous applications such as switch, rectifier, regulator, voltage multiplier, clipping, clamping etc.

As mentioned diode has two leads viz. anode and cathode. The cathode is often marked by a band at one end. This is the lead from which conventional current flows out of the diode in forward biased mode.

There are two identification codes for the diode viz. american system and conventional system.
• In american system, code starts with 1N followed by serial number of the diode.
• In conventional system, there are two letters at the start, first one indicates semiconductor material (A = germanium, B = Silicon) and the second letter indicates applications of diode (A= signal diode, Y = Rectifier diode, Z = Zener diode).
For example, AA119 refers to germanium signal diode.

What are the types of diode

The commonly used diodes in electronics industry include following and more. Let us understand functions, characteristics, applications and symbols of these diodes.

p-n junction diode

The p-n diode consists of p-n junction with one connection to p-side (e.g. anode) and another connection to n-side (e.g. cathode). The structure of such diode is shown in the figure. The two useful electrical specifications of the diode are forward current and max. reverse voltage.

Function: The p-n junction diodes are used as rectifiers. This diode converts AC to DC in power supply modules. In this diode type, silicon is preferred over germanium due to its lower reverse current and higher breakdown voltage.

Zener diode

When reverse biase voltage is increased to ordinary junction diodes until depletion region breaks down, the diode suffers permanent damage. Zener diode is used in the breakdown region so long as resistor limits the current.

It functions as rectifier diode.

Point-contact diode

The figure depicts structure of germanium point contact diode. As shown tip of gold or tungsten wire is pressed on pellet of n-type germanium. During manufacturing, brief current is passed through the diode which produces tiny p-type region in pellet around the tip. This process forms p-n junction in very small area.

They are used as signal diodes in order to detect radio frequency signals due to their very low capacitance. In reverse biased mode, depletion layer acts as insulator between the two plates and it functions as capacitor. It is suitable for high frequency signal detection due to its tiny junction area. Germanium is used for signal diodes due to its lower "turn-on" voltage compare to silicon.

Varactor diode

Most diodes are designed to have minimum capacitance but varactor diode is designed to have certain range of capacitance e.g. 2 to 10 pF. The value of capacitance is changed by varying reverse voltage which changes width of the depletion layer.

The varactor diode is used to tune TV and VHF radio receiver sets to desired stations. This process is known as AFC (Automatic Frequency Control).

Gunn diode

It is made from n-type gallium arsenide sandwiched between metal electrodes. It has negative resistance property by which gunn diode act as oscillator. To achieve this capacitance and shunt load resistance need to be tuned but not greater than negative resistance. The figure describes GUNN diode equivalent circuit. The GUNN diode is basically a TED i.e. Transferred Electron Device capable of oscillating based on different modes.

It is used for microwave oscillators. They are used as pump sources in parametric amplifiers. They are used in police radars and CW doppler radars.

Tunnel diode

• It is heavily doped p-n junction diode. Impurity concentration is 1 part in 10³ compare to 1 part in 10⁸ in p-n junction diode.
• They are fabricated from germanium, GaAs (Gallium Arsenide) and gallium Antimonide.
• It is also known as Esaki diode.
• Width of depletion layer is very small (about 100A).

This diode exhibits special characteristic known as negative resistance. This feature makes it useful in oscillator and microwave amplifiers.

PIN diode

In a PIN diode, there is a insulating layer between P and N type of materials. Hence PIN diode structure is different than the normal PN junction diode. I region is lightly doped N type region. When forward bias is applied charge carried are injected into Insulating layer from both the P and N regions/layers. Due to lightly doped region in the I layer, small amount of charge carriers are left without combining. Due to this resistivity of the Insulating region is very low.

PIN diode is used as RF delay line/phase shifter, used as amplitude modulator, used as switch and so on.

schottky diode

The diode is constructed on a thin silicon (n+ type) substrate by growing epitaxially on n-type active layer of about 2 micron thickness. A thin SiO2 layer is grown thermally over this ative layer. Metal semiconductor junction is formed by depositing metal over SiO2. Schottky diodes exhibit square law characteristics. They have high burnout ratings.

It is low voltage diode. The schottky diode is widely used in different applications such as RF mixer, as rectifier in power electronics circuits.

Impatt diode

Full name : Impact Ionisation Avalanche Transit Time
Working operation : Avalanche Multiplication
Frequency range : 4 GHz to 200 GHz

Impatt diodes are used as amplifier and oscillator.

Trapatt diode

Full name : Trapped Plasma Avalanche Triggered Transit
Working operation : Plasma Avalanche
Frequency range : 1 to 10 GHz

It is used as Oscillator.

Baritt diode

Full name : Barrier Injection Transit Time
Working operation : Thermonic emission
Frequency range : 4 GHz to 8 GHz

Baritt diodes are used as Local Oscillators in electronics circuits.

Step recovery diode

In this diode type, p and n type materials are very much dopped heavily at the end of the component then at the junction of the device. It is fabricated with doping level gradually decreasing as the junction is approached or as direct PIN structure. This reduces switching time due to fewer charge carriers in the region of the junction. Hence less charge is stored in this region.

Following are applications of step recovery diodes.
• It is also known as snap off diode or memory varactor or charge-storage diode.
• Used as parametric amplifier or pulse generator in microwave circuits.
• It is used to generate extremely fast rise time pulses.
• It is used in frequency comb generation, harmonic frequency multipliers and samplers.
• It is also used as charge controlled switch.

Light emitting diode (LED)

An LED is a junction diode made from semiconductor compound gallium arsenide phosphide. LEDs used as optical fiber transmitters emit infrared radiation at a wavelength of about 850 nm (0.85 �m). Pulse code modulated signals from the coder supply input current to the LED. This will produce equivalent stream of infrared pulses for transmission along the optical fiber system. The spectral spread of wavelengths in the output is about 30-40 nm.

LEDs are very cheap and convenient source of light.

Laser diode

Laser is derived from Light Amplification by the Stimulated Emission of Radiation. It produces a very intense beam of light or infrared radiation which is having following properties. • Monochromatic ( meaning consists of one wavelength)
• Coherent (meaning all parts are in phase)
• Collimated (meaning all parts travel in one and same direction)

Laser diode used in optical fiber systems are made of gallium arsenide phosphide.

Photodiode

A photodiode is a type of photodetector capable of converting light into either current or voltage. The device operates in reverse bias and electric field developed across the p-n junction sweeps the mobile charge carriers to their respective majority sides. Hence a depletion region is formed. This barrier stops the flow of majority carrier and supports the flow of only minority carriers and hence leakage current flows.

They are very useful for various applications such as fire alarms, counting systems and automatic control systems. Photodiodes are used as fast counters and used in light meters to measure the light energy.

Conclusion

Understanding different types of diodes and their specific applications provides a strong foundation in electronics. Whether you’re working with rectification, voltage regulation or signal modulation, choosing the right diode is essential. With these insights, you can make informed decisions on diodes in your projects, enhancing circuit performance.