Doping is the process
of adding impurities in semiconductor with the intent of modulating (changing
or controlling ) its electrical properties. The
impurities are dependent upon the type of semiconductor. Lightly and moderately
doped semiconductors are referred to as extrinsic. A semiconductor doped to
such high levels that it acts more like a conductor than a semiconductor is
referred to as degenerate.Two of the most important materials silicon can be doped with, are boron (3 valence electrons = 3-valent) and phosphorus (5 valence electrons = 5-valent). Other materials are aluminum, indium (3-valent) and arsenic, antimony (5-valent).
The dopant is integrated into the lattice structure of the semiconductor crystal, the number of outer electrons define the type of doping. Elements with 3 valence electrons are used for p-type doping, 5-valued elements for n-doping. The conductivity of a deliberately contaminated silicon crystal can be increased by a factor of 10^6.
Through the introduction of a dopant with five outer electrons, in n-doped semiconductors there is an electron in the crystal which is not bound and therefore can be moved with relatively little energy into the conduction band. Thus in n-doped semiconductors one finds a donator energy level near the conduction band edge, the band gap to overcome is very small.
Analog, through introduction of a 3-valent dopant in a semiconductor, a hole is available, which may be already occupied at low-energy by an electron from the valence band of the silicon. For p-doped semiconductors one finds an acceptor energy level near the valence band.