Explain the roles of diffusion current and drift current in the formation of the depletion layer in a p-n junction diode.

During the formation of p-n junction ,and due to the concentration gradient across p-,and n-sides, holes diffuse from p-side to n-side \(\displaystyle (\mathrm{p} \rightarrow \mathrm{n})\) and electrons diffuse from n-side to p-side(n→p). When an electron diffuses from (n→p), it leaves behind an ionized donor(positive charge) on n-side which is immobile. Similarly, when a hole diffuses from (p→n) due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) which is immobile. This space-charge region on either side of the junction together is known as depletion region. As a result, an electric field is developed across the junction. Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side. The motion of charge carriers due to the electric field is called drift .Initially, diffusion current is large and drift current is small. As the diffusion process continues, the electric field strength & hence drift current increases .This process continues till diffusion & drift current becomes equal.