Impurity semiconductors contain atoms of foreign elements embedded in the crystal lattice. Impurities are specially introduced into a semiconductor to change its electrophysical properties (this process is called doping). In impurity semiconductors, the concentrations of electrons and holes can differ by many orders of magnitude.For four valence elementary semiconductors, such as germanium and silicon, the donor impurities are atoms of pentavalent elements, such as phosphorus P, arsenic As, antimony Sb, acceptor atoms of trivalent elements: boron In indium In, gallium Ga, aluminum AI.
Embedding in the lattice, the atoms of the five valence elements form four bonds with the nearest neighbors, the fifth electron turns out to be superfluous.
It does not participate in the formation of a chemical bond and is weakly bound to the impurity atom, easily detaches from it and becomes free.
With the introduction of a three valence atom, it lacks one electron to form four bonds. The missing electron can be captured from a neighboring atom, in which a hole is formed.
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