Question

Explain the formation of potential barrier across the unbiased p-n junction region

Answer 1

The p-n junction is formed at the initial stage , the fermi level on the p and n sides are at different energy states. This is because of the fact that p side has excess holes and n side has excess electrons as free carriers.

The diffusion through the junction start to equalize the number of charge carrier on both sides. As the conduction band of the n side has high density of carrier, electrons of the conduction band diffuse through the junction from the n side to the p side. Similarly holes of the valence band diffuse through the junction from p side to n side.
As more electrons occupy band of p side the fermi level E_FP starts moving upwards along with the entire p-type energy band structure. Similarly more holes entering the valence band of n-side make the fermi level E_Fn  moves downwards. The energy band structure also move downwards with E_Fn  . Finally a stage comes when the charge density on both sides of the same band is uniform. In this equilibrium condition the fermi levels E_Fn
and E_Fp  align with each other.
The conduction band and the valence band of both sides are positioned in such a manner that a conduction hill of height -eV_O  and valence hill of height eV_O  are formed. These are the potential energies of the electrons and holes respectively due to the potential barrier V_O