any impurity deliberately added to a semiconductor for the purpose of modifying its electrical conductivity. The most commonly used elemental semiconductors are silicon and germanium, which form crystalline lattices in which each atom shares one electron with each of its four nearest neighbours. If a small proportion of the atoms in such a lattice is replaced by atoms such as phosphorus or arsenic, which have five electrons available for bond formation, the extra electron of each such dopant atom becomes available for electrical conduction. The semiconductor is then said to be doped with phosphorus or arsenic, which are called donor atoms, and the semiconductor is classed as n-type (n for negative, because the charge carriers are electrons, which are negatively charged particles). Doping with atoms such as boron or indium, which have only three electrons available, creates a positively charged site, or “ hole,” in the bonding arrangement. Conduction can occur by migration of the positively charged site through the crystal lattice, and a semiconductor doped with an atom of this type, an acceptor atom, is called p-type.