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.
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