What is a junction diode ? Explain the formation of depletion region at the junction. Explain the variation of depletion region in forward and reverse-biased condition.

Explanation

p-n junction diode `:` When a p-type semiconductor is suitably joined to n-type semiconductor, a p-n junction diode is formed.
The circuit symbol of p-n junction diode is shown in figure.
Formation of depletion layer at the junction `:` When p-n junction is formed, the free electrons on n-side diffuse over to p-side , they combine with holes and become neutral. Similarly holeson p-side diffuse over to n-side and combine with electrons become neutral.

This results in a narrow region formed on either side of the junction. This region is called depletion layer. Depletion layer is free from charge carriers.
The n-type material near the junction becomes positively charged due to immobile donor ions andp-type material becomes negatively charged due to immobile acceptor ions. This creates an electric field near the junction directed from n-region to p-region and cause a potential barrier.
The potential barrier stops further diffusion of holes and electrons across the junction.The value of the potential barrier depends upon the nature of the crystal, its temperature and the amount of doping.

Forward bias `:`
"When a positive terminal of a battery is connected to p-side and negative terminal is connected to n-side, then p-n junction diode is said to be forward bias ".
The holesin the p-region are repelled by the positive polarity and
move towards the junction. Similarly electron in the n-region are repelled by the negative polarity and move towards the junction.
As a result, the width of the depletion layer decreases.The charge carriers cross the junction and electric current flows in the circuit.
Hence in forward bias resistance of diode is low. This position is called switch on position.
Reverse bias `:`

"When the negative terminal of the battery is connected to p-side and positive terminal of the battery is connected n-side of the p-n junction, then the diode is said to be reverse bias".
The holes in the p-region are attracted towards negative polarity and move away from the junction . Similarly the electron in the n-region are attracted towards positive polarity and move away, from the junction.
So, which of the depletion layer and potential barrier increases. Hence resistance of p-n junction diode increases. Thus the reverse biased diode is called switch off position.