Doping can create additional mobile "electrons" and "holes" in the silicon crystal by substitution of the silicon atoms with Group 15 (VA) atoms or Group 13 (IIIA) atoms, respectively.Doping with P creates a donor level just below the conduction band in silicon and enhances conductivity; a P atom has one more electron than needed to form four bonds. Its extra electron is readily ionized, adding a mobile electron to the conduction band.Doping with Al creates an acceptor level just above the valence band in silicon and enhances conductivity; an Al atom has one less valence electron than needed to form four bonds, adding a mobile hole to the valence band.
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