A chemical element added to silicon or germanium to give it specific electrical characteristics. Arsenic, phosphorus, and antimony are pentavalent elements, elements having five electrons in their valence shell. When these elements are used as impurities, they form N-type material because there are more electrons than are needed to form covalent bonds within the material. Atoms of N-type material are called donor atoms.

Aluminum, boron, gallium, and indium are elements having only three electrons in their valence shell, and when used as impurities with silicon or germanium, produce P-type materials. P-type materials have fewer electrons than are needed to form covalent bonds, and they accept electrons from other materials. Atoms of P-type materials are called acceptor atoms.