Physics of Semiconductor Devices

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Schottky diode / p+n diode

Consider a Schottky diode and a p+n diode where the p+ region is doped so heavily that the semiconductor is degenerate. Since the Fermi energy of a degenerately doped p+ semiconductor is in the valence band, it behaves like a metal. Why then is the current in forward bias of a Schottky diode dominated by thermionic emission while the current in forward bias of a p+n diode dominated by diffusion?