What is p-n junction diode? Define depletion layer.

A p-n junction can be formed by adding a precisely small quantity of donor impurity to a p-type semiconductor. The semiconductor wafer now contains a p-region and n-region and a metallurgical junction between p- region and n-region. This junction is a p-n junction. Metallic contacts are provided at the two ends for the application of external voltage, and a p-n junction diode is formed. It is a two-terminal device as shown below: During the formation of a p-n junction, the diffusion of electrons and holes takes place due to the concentration gradient. They leave behind an ionised donor(positive charge) and ionised acceptor(negative charge) respectively which is immobile. Hence, there is a space-charge region on either side of the junction together which is called the depletion layer.