In the p-type semiconductor the Fermi level lies close to the valence band.In the n-type semiconductor the Fermi level lies close to the conduction band.At a p-n junction the Fermi levels match up across the junction in the dark at equilibrium, creating a barrier that prevents additional net current flow across the junction.Under forward bias (bottom, middle picture) the barrier is reduced.Under reverse bias (bottom, left picture) the barrier is increased.In forward bias, current flows easily, whereas in reverse bias, little current flows.
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