p-type silicon

Consider silicon (or germanium), it is four-valent. In a crystal lattice each silicon atom will be surrounded by four other silicon atoms. If one of these atoms is replaced by an impurity atom of a valency of three (e.g. gallium or boron), there will be one electron too few to maintain the electronic configuration. In effect we have a hole at the site of the impurity atom. Such impurities are referred to as acceptor impurities and when distributed throughout the semiconductor material give rise to extra energy states just above the valence band, called acceptor states. Silicon with such impurities is called p-type silicon.