An extrinsic semiconductor is formed by adding impurities, called dopants to an intrinsic semiconductor to modify the former's electrical properties. There are two types of such impurities - those which provide electrons as majority carriers are known as n-type and those which provide holes as majority carriers are known as p-type .

Consider a matrix of silicon where the atoms are covalently bonded.

If we add a pentavalent atom (As, P etc.) as an impurity, the dopant atom replaces a silicon atom substitutionally. As the dopant has five electrons, only four of these can be used in forming covalent bonds while the fifth electron is loosely bonded to the parent atom. This electron can become detached from the dopant atom by absorbing thermal energy.