Two important processes which occur during formation of p−n junction are diffusion and drift. Due to concentration gradient in p and n sides holes diffuse from p to n and electrons diffuse from n to p side. Due to this there is a layer of ionised donor atoms on n side having positive charge and similarly a layer of ionised acceptor ions on p side having negative charge. This region is known as space charges region.

This in turn, produces potential barrier VB across the junction which opposes the further diffusion through the junction. Thus, small region forms in the vicinity of the junction which is depleted of free charge carrier and has only immobile ions is called the depletion region.
The potential distribution near the pn junction is known as potential barrier.