# Tunnel diode – Definition, Construction, Working.

## What is tunnel diode?

A tunnel diode is a PN junction diode having a very small depletion region and a very high concentration of impurity atoms in both p and n regions. The tunnel diode was invented by Dr. Leo Esaki.

## Construction and Working of Tunnel Diode

### Tunnel Diode Construction

The tunnel diode is made by doping the semiconductor material ( Germanium or gallium arsenide) with a large number of impurities. This will result in a greatly reduced depletion region of the order of 10-8m.

Because of this thin depletion region, the carriers can tunnel through rather than tending to surmount the depletion region at low forward bias potentials.

The operation of a tunnel diode depends upon the tunneling effect, a quantum-mechanical phenomenon, and hence this diode is named a tunnel diode.

### Working of Tunnel Diode

The energy bands of an unbiased tunnel diode are shown in the figure. The energy bands of an unbiased tunnel diode differ from those of an unbiased ordinary PN junction diode.

In the case of the tunnel diode, the depletion layer is narrow because of the higher impurity concentration in both n and p regions. Moreover, the acceptor and donor levels in p-type and n-type semiconductors are not as sharp as in the case of an ordinary PN junction diode but become broadband.

As a result of this, valence and conduction bands overlap. Therefore, the Fermi energy level moves up into the conduction band in n region and down into the valence band in p reason as shown in the figure.

Since the depletion player is narrow, therefore, in the electrons penetrate through the forbidden energy band. At equilibrium, the rate of electron tunneling through the depletion layer is the same in both directions.

When tunnel diode is forward biased, the Fermi energy level in n reason rises up as shown in the figure. The electron tunneling from n region to the p-region increases because there are empty allowed energy states in the valence band of the p-region.

On the other hand, electron tunneling from the p-region to the n-region decreases considerably. Hence, the current flows through the junction.

The increase in current is represented by the OA part of the curve. As the forward bias across the junction increases, the current through the junction decreases because the number of empty allowed energy states in the valence band of the p region decreases. The decrease in current is represented by the AB part of the curve.

When the forward bias is increased considerably, common the difference in the energies of band and decreases.

Now the electrons from the conduction band of n region diffuse through the junction into the conduction band of a p region and the current begin to increase as in case of an ordinary PN junction when forward bias. The increase in current is represented by the BC part of the curve.

In the region between A and B, the current through the tunnel diode decreases with the applied voltage. It means the Tunnel diode has negative resistance in this region the resistance of the tunnel diode decreases with applied voltage. Due to this fact, tunnel diode has many applications in electronics.

## Applications of tunnel diode

• Tunnel diodes are used as logic memory storage devices.
• Tunnel diodes are used in relaxation oscillator circuits.
•  In high-speed switching circuits of a computer.
• As an oscillator at frequencies greater than 1011Hz.
• Tunnel diodes are used in FM receivers.

## Tunnel diode characteristics

• Long life
• High-speed operation
• Low noise
• Low power consumption