Foreign atoms in a semiconductor that are either naturally occurring or deliberately introduced. Impurities have a fundamental effect on the type and amount of conductivity of the semiconductor. Impurity diffusion is the deliberate diffusion of impurities into selected regions of a semiconductor in order to produce the desired characteristics. The energy levels due to the impurities are impurity levels. The presence of impurities affects the mobilities (both Hall and drift mobilities) of charge carriers due to impurity scattering between the carriers and the impurity atoms. See also diffusion; semiconductor.