Majority and minority charge carriers 

A pure semiconductor has equal number of holes and electrons. But an extrinsic semiconductor has majority charge carriers.

When a trivalent impurity is introduced into a semiconductor substance, every impurity atom leaves an electron deficiency called ‘hole' along with thermally generated holes and electrons. Some of the electrons recombine with holes, so the number of electrons get reduced. Hence holes become majority charge carriers as they are large in number and electrons become minority charge carriers. When a pentavalent impurity is added, every impurity atom leaves an electron in addition to the existing number. 

Some of the electrons and holes recombine. This results in number of holes to reduce and number of electrons increase. Hence electrons become majority charge carriers and holes become minority charge carriers.