Formation of barrier potential: A p−n junction is formed by bringing p-type and n-type semiconductors together in a very close proximity.At the instant of p - n-junction, movement of free electrons from n-side and free holes from p-side diffuse across the junction and combine and thus leave -ve ions in p-side and +ve ions in n-side.These two layers of +ve and -ve ions form the depletion region and the potential difference thus sets up is called potential barrier.