Intrinsic semiconductors have high resistivity. Also, their conductivity shows little flexibility. For these reasons, intrinsic (pure) semiconductors are of little use; at best these can be used as a heat or light sensitive resistance. These limitations are overcome by adding a small and measured quantity of another material to intrinsic (pure) semiconductor, which either increases the number of holes or electrons.
The process of addition of impurities to a pure or intrinsic semiconductor is called doping and the impurity atom that is added is called dopant. Such doped semiconductors are called extrinsic semiconductors.
The dopants are generally taken from either group III (having three valence electrons) or group V (having five valence electrons) of the Periodic Table. Normally, a very small amount of impurity atoms is added to the pure semiconductor. It is of the order of one atom per 108 atoms of intrinsic semiconductor. These atoms change the balance of charge carriers; either they add free electrons or create holes. Either of these additions makes the material more conducting.