From the electronic configuration of Si - 1s2, 2s2, 2p6, 3s2, 3p2, ten electrons are tightly bound to the nucleus and four electrons revolve around the nucleus in the outermost orbit. In an intrinsic silicon semiconductor, the Si atom attains stability by sharing one electron each with four neighbouring Si atoms. This is called covalent bonding.
The same holds true for germanium. Its electronic configuration is 1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s2, 4p2. When silicon (or germanium) is doped with a pentavalent (five electrons in the outermost orbit) atom like phosphorus, arsenic or antimony, four electrons form covalent bonds with the four neighbouring silicon atoms, but the fifth (valence) electron remains unbound and is available for conduction.
Thus, when a silicon (or germanium) crystal is doped with a pentavalent element, it develops excess free electrons and is said to be an n-type semiconductor. Such impurities are known as donor impurities. In n-type semiconductors, the number of free electrons is far greater than the number of holes.
If silicon (or germanium) is doped with a trivalent (three electrons in the outermost shell) atom like boron, aluminium, gallium or indium, three valence electrons form covalent bonds with three silicon atoms and deficiency of one electron is created. This deficiency of electron is referred to as hole. Such a semiconductor is said to be a p-type semiconductor and the impurities are known as acceptor impurities.
In a p-type semiconductor, more holes are created due to addition of acceptor impurity than by breaking covalent bonds due to thermal energy at room temperature. Hence, the net concentration of holes is significantly greater than that of electrons.