n-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 five (arsenic or phosphorus), there will be one electron in excess of that required to maintain the electronic configuration. The impurity atom will be a donor atom sitting in a donor site, and will introduce donor states just below the conduction band. Silicon with such impurities is called n-type silicon.