When a small amount of trivalent metallic impurity is added to a pure semiconductor it is called p-type semiconductor. The addition of trivalent impurity provides a large number of holes in the semi conductor crystal. Typical examples of trivalent impurities are indium and gallium.
Such impurity which produces p-type semiconductors are known as acceptors impurities because they provide or create hole in the crystal.
Also read : P-type Semiconductors conductivity
We know that germanium has four valence electrons .If a trivalent impurity is added to the germanium crystal , a large number of holes are available in the crystal.The gallium is trivalent and its atom has three valence electrons. The gallium atom fits in the germanium crystal in such a way that its three valence electrons form covalent bonds with three germanium atoms.In the fourth covalent bond only one germanium atom contributes one valence electron while gallium has no valence electron. Therefore for each gallium atom added , one hole will be available in the crystal.
1 ) Majority of holes
2 ) Minority of free electrons