Explain the effect of impurity on the conductivity of a semiconductor.
To form a semi-conductor conductive, a small amount of appropriate impurity is added. This is then termed as extrinsic semi-conductor. Depending on the kind of impurity added, extrinsic semi-conductor might be categorized n-type and p-type semi-conductor.
n-type semi-conductor: The addition of pentavalent impurity as antimony and arsenic give many of free electrons in the semi-conductor crystal. These types of impurities, that produce n-type semi-conductors, are called donor impurities, as each atom of them donates one free electron to semi-conductor crystal. While an electric field is applied to a crystal of n-type material consisting of sufficient donor impurity the effect of the donor electrons is extremely predominant than the effect of electron hole pairs achieved through the breaking of the covalent bonds. The Fermi level of an n-type semi-conductor arises in the forbidden energy gap although near to the bottom of the conduction band.
p-type semi-conductor: While a trivalent impurity such as indium, gallium are added to a semi- conductor, many of holes are created and the semi-conductor formed is called p- type semi-conductor. These impurities that produce p-type semi-conductor, are termed as acceptor impurities. Now conduction is using positive holes. The Fermi level in that case arises near the top of the valence bonds into the forbidden energy gap.