p - type Semiconductor

p-TYPE SEMICONDUCTOR

When a small amount of trivalent impurity is doped pure semiconductor, it becomes p-type semi-conductor.

The addition of trivalent impurity provides a large number of holes in semiconductor.

Typical examples of trivalent impurities are gallium (Atomic No: 31) and indium (Atomic No. 49). Such impurities are known as acceptor impurities because holes created can accept electrons.

In a pure semiconductor (germanium) having 4 valence electrons, if a trivalent impurity (boron) having. '3' valence electrons is added, then 3 valence electrons of trivalent impurity form a covalent bond with three valence electrons of germanium.

The fourth valence electron of Ge atom is unable to form a covalent bond. The incomplete covalent bond is being short of one electron. This missing electron is called a hole. (Fig. 3.9(a))

Every trivalent impurity atom contributes one hole in addition to thermally generated electron - hole pairs. Therefore, number of holes is more than number of electrons.

The addition of trivalent impurity creates large number of holes (positive charge carriers) in semiconductor and hence it is called p-type semiconductor where p stands for positive type.

Hence in this type of semiconductor, holes are majority charge carriers and electrons are minority charge carriers.

In this case, allowable energy level (acceptor energy level) is created just above valence band (fig. 3.9(b)).

A very small amount of energy is needed for an electron to enter acceptor energy level from valence band. Thus, a hole is generated in the valence band corresponding to each ionised acceptor.

In other words, a large number of positive charge carriers are created.