Question

Sketch the construction of tunnel diode and write its operation.

Answer 1

Operation: We can explain the diode theory with the help of three biasing levels:

i. When the bias voltage is zero energy state in the CB and VB are at the same height as shown in fig below

Electrons can now tunnel from one side of junction to the other because of its thickness. But the tunneling currents in the two directions are the same. Hence overall current flow is zero.

 ii.When the small forward bias is applied across the junction, the energy level of the p side is lowered as compared with the n side as shown in fig below

Thus the electrons are able to tunnel through from the n side as the electrons in the conduction band find themselves opposite vacant states on the p side. Tunneling in the other direction is not possible. 

iii.When the forward bias is increased beyond this point, tunneling will decrease as shown in the fig below

The energy level on the p side is now depressed further, with the result that fewer n side free electrons are opposite unoccupied p side energy levels. As the bias is raised, the forward current drops and this corresponds to negative resistance region of the diode characteristic.

iv.As a forward bias is reached at which there are no conduction band electrons opposite valence band states and tunneling stops altogether. When the forward is increased even further normal forward current flows and tunnel diode acts as the normal diode. 

Answer 2

Construction:

Applications:

• In digital networks.

• As a high speed switch.

• As a high frequency oscillator.

• As a logic memory storage device.

• In relaxation oscillator circuits.