question_answer

An n-type semiconductor has excess of free electrons while a p-type semiconductor has a deficiency of these. But when a p-n junction is formed, all the electrons do not flow from n-region to the p-region. Why?

Answer:

                    Though the p-type and n-type semiconductors have excess free electrons and holes respectively, yet they have equal number of fixed positive donor ions and negative acceptor ions respectively. When a p-n junction is formed, electrons diffuse from p-region to p-region while holes diffuse from p-region to n-region. As a result, the n-region near the junction becomes increasingly positive and the p-region becomes increasingly negative. This sets up a potential barrier across the junction which opposes the further diffusion of electrons and holes across the junction. That is why all the electrons do not flow from n-region to p-region.