We can however, change the electrical properties of the pure semiconductors by adding certain impurities to their structure, a process called doping.

When doping semiconductors of groups 3 & 4, these impurities are usually elements of group 3(acceptors) or 5(donors).

This gives rise to two kinds of extrinsic semiconductors : ones having free electrons as their majority charge carriers(called n-type) & those which have holes as their majority charge carriers(called p-type).

Extrinsic semiconductors are used in many electrical devices. A more useful version of doped semiconductors is the p-n junction.