To make a semi-conductor conductive, a small amount of suitable impurity is added. It is then called extrinsic semi-conductor. Depending upon the type of impurity added, extrinsic semi-conductor might be classified as n-type and p-type semi-conductor.
n-type semi-conductor: The addition of pentavalent impurity such as arsenic and antimony provide a large number of free electrons in the semi-conductor crystal. Such impurities, which produce n-type semi-conductors, are known as donor impurities, because each atom of them donates one free electron to the semi-conductor crystal. When an electric field is applied to a crystal of n-type material having sufficient donor impurity the effect of the donor electrons is much more predominant than the effect of electron hole pairs achieved by the breaking of the covalent bonds. The Fermi level of an n-type semi-conductor occurs in the forbidden energy gap but near to the bottom of the conduction band.
p-type semi-conductor: When a trivalent impurity like indium, gallium are added to a semi-conductor, a large number of holes are created and the semi-conductor formed is known as p-type semi-conductor. Such impurities, which produce p-type semi-conductor, are known as acceptor impurities. Conduction is now by means of positive holes. The Fermi level in this case occurs near the top of the valence bonds in the forbidden energy gap.