Degenerate semiconductor

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Degenerate semiconductors are extremely heavily doped, so they act more like metals than ordinary semiconductors. They do not follow the usual relation between carrier density, temperature, and bandgap. At moderate doping, dopant atoms create localized states that can donate electrons or holes to the conduction or valence bands when heated or illuminated. If the doping is very high, these impurity states merge into an impurity band, and the material stops showing typical semiconductor behavior; for example, its conductivity may not rise with temperature as a normal semiconductor would. Still, degenerate semiconductors have far fewer charge carriers than true metals, so they sit between semiconductors and metals in their behavior.

Examples include many copper chalcogenides, which are degenerate p-type semiconductors with many holes in the valence band. LaCuOS1−xSex with Mg doping is a wide-gap, p-type degenerate semiconductor, and its hole concentration stays nearly constant with temperature. Another well-known example is indium tin oxide (ITO). Its plasma frequency lies in the infrared, so it conducts like a metal but remains transparent in the visible range.