Doping in Semiconductors refers to the addition of donor atoms into the crystal structure of intrinsic semiconductors material to modify their conducting property. In doping, an impurity or foreign atom is introduced into the lattice structure of a semiconductor material. This makes free electrons or holes available in the lattice structure of the material.
A semiconductor with an excess of free electrons is called an n-type semi-conductor while a semiconductor with an excess of holes is called a p-type semi-conductor. N type semiconductors are made when the semi-conductor material is doped with a pentavalent impurity. A pentavalent impurity is an impurity whose atom has 5 electrons in its outer most shell. Examples of pentavalent impurity are Antimony, Arsenic and Phosphorus.
A p-type semiconductor is obtained when the semiconductor material is doped with a trivalent impurity i.e. an element whose atom has three electrons in its outermost shell. Examples of trivalent impurity are Boron, Aluminium and Gallium.