1. Doping
Doping means introducing foreign atoms into a semiconductor crystal in order to specifically alter its conductivity. Two of the most important substances used to dope silicon are boron (3 valence electrons = trivalent) and phosphorus (5 valence electrons = pentavalent). Others include aluminium and indium (trivalent), as well as arsenic and antimony (pentavalent).
The dopant elements are incorporated into the lattice structure of the semiconductor crystal; the number of outer electrons determines the type of doping. Elements with 3 valence electrons are used for p-type doping, while pentavalent elements are used for n-type doping. The conductivity of a deliberately impurified silicon crystal can be increased in this way by a factor of 106.