What is the depletion region in pn junction?

In a PN junction, the depletion region refers to the region near the junction where there is a depletion or absence of majority charge carriers (electrons in the P-region and holes in the N-region). This region is formed due to the diffusion of charge carriers across the junction.

When a P-type semiconductor (with an excess of holes) is brought in contact with an N-type semiconductor (with an excess of electrons), the electrons in the N-side and the holes in the P-side begin to diffuse across the junction. This diffusion process continues until equilibrium is reached.

As electrons diffuse from the N-side to the P-side, they combine with the holes present in the P-side, resulting in a neutral region near the junction. Similarly, the holes from the P-side diffuse to the N-side and combine with the electrons present there, also creating a neutral region.

The result is a region near the junction with a depletion of charge carriers, hence called the depletion region. In this region, the immobile ions (fixed charged atoms) of the donor and acceptor impurities dominate the electric field. The depletion region acts as a barrier to further diffusion of charge carriers across the junction.

This depletion region creates a potential barrier, known as the built-in potential, that opposes the further movement of charge carriers. It forms the basis of various electronic devices such as diodes and transistors, enabling them to control the flow of current.