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# GATE : Different types of Diodes questions

There will be many “Match the following” questions related to diodes in GATE and IES examinations every year. So, it is very important to get these marks.

In this post we will see different types of diodes and their applications and a small quiz at the end.

Tunnel Diode :

Tunnel Diode is a high doped p-n junction diode

Applications

• In high speed switches, switching speed is in the order of nano or pico seconds due to a phenomenon called “Tunneling”
• Microwave oscillator
• Amplifier
• High Speed/Low power

Detailed analysis can be found in the video lecture.

PIN Diode :

PIntrinsic-N diode is a diode with a wide, lightly doped near intrinsic semiconductor region between P and N type semiconductor region. P and N type are heavily doped because they are used as Ohmic contacts. Mainly used in Radio frequency applications.

• Used as a Variable attenuator/resistor in RF and microwave frequency range
• High speed switches
• Used in RF Modulator circuit and in phase shifter circuit.
• It is a current controlled device.

Photo Diode

Under reverse biased conditions, the current flowing through a P-N junction diode is very small and it is called reverse saturation current. It is in the order of microamperes and it is due to flow of minority carriers. If light is focused on the junction, the light photons generate electron-hole pairs on both sides of the junction. The minority charge carriers will increase and so does the saturation current. Increase in the current is directly proportional to intensity of light. This phenomenon is used in photo diode.

• Dark current in photo diode refers to reverse saturation current without any “LIGHT”.
• Photo diodes find application in high speed counting and solar energy systems

LIGHT EMITTING DIODES (LED)

By far, the most popular diode in the world. Every one who has studied electronics has used LED’s in one of their digital electronics project for sure.

Working : In a forward biased P-N junction diode, recombination of electrons and holes takes place at the junction, after recombination, energy is released in the form of heat.

In case of Gallium Arsenide the energy of electron is converted into light energy in the form of photons. This process of light emission in p-n junction is called electroluminescence. This is the principle behind light emitting diodes.

A LED is a P-N junction diode, which when sufficiently forward biased causes electrons to recombine with holes which then releases  energy in the form of photons . The color of the light depends on the band gap of the semiconductor.

Depending on the energy, different colors are emitted.

$\Delta E$=hf= $\frac {hc}{\lambda}$

For example

A LED made up of gallium phospide doped with nitrogen emits green light and A LED made up of gallium arsenide phospide produces red light.

IES Question :

Q) Match List -1 (Diode) with List-2 (common application)

List -1                                                                                         List-2

A. Tunnel Diode                                                  1. Reading of film sound track,

B. PIN Diode                                                       2. High frequency oscillator circuits.

C. Zener Diode                                                    3. Very high frequency switching  circuits

D. Photo Diode                                                   4. Reference voltage