Consider a pentavalent impurity such as phosphorus (P, Z = 15) is doped in a pure semiconductor germanium (Ge, Z = 320. Each phosphorus atom forms four covalent bonds with four germanium atoms as shown in the figure. Thus fifth electron does not take part in the formation of covalent bonds. Hence it is free. This means the phosphorus atom has an excess of one electron and is ready to donate. Therefore all such pentavalent impurities are called ‘donors'. Thus, a small quantity of phosphorus provides millions of electrons in a n-type semiconductor which are excess in number compared to number of holes available for conduction. The resulting semiconductor is called a n-type semiconductor.
A n-type semiconductor is formed when a small quantity of donor impurity is added to a pure semiconductor. In a n-type semiconductor, conduction is mainly by means of electrons in the conduction band. Electrons are majority carriers and holes are minority carriers in a n-type semiconductor.