P-type semiconductor

From ArticleWorld

A p-type semiconductor can be obtained by adding a small amount of a group III element in pure Germanium or Silicon. The impurity atom or dopant is surrounded by the semiconductor atoms as the concentration of the impurity is very low. The semiconducting element becomes stable by forming four covalent bonds. The impurity or dopant has only three electrons, so one bond is left incomplete.

Consider a small amount of group III element such as aluminium which has three valence electrons to be doped with pure silicon. It forms the covalent bonds with three neighbouring atoms of silicon. The fourth electron in the neighbouring silicon does not find an electron in the impurity to share. Thus the fourth bond is incomplete, as there is shortage of one electron. This absence of electron in the covalent bond is called a hole. The number of holes depends on the atoms of dopant added.

How conduction takes place

Under the influence of electric field across such a crystal, an electron from the neighbouring covalent bond jumps into the hole and fills it up. Thus, absence of this electron creates a new hole in the previous position. This hole appears to move in the direction of the field, as it gets filled successively by electrons from neighbouring atom. Conduction in p-type semiconductors is mainly due to holes, which are positively charged carriers. This is why this type of semiconductor is called a p-type semiconductor.

Examples of p-type semiconductor dopants

Examples of the elements used to dope p-type semiconductors are from the group III elements such as indium, gallium, aluminium, boron, etc. They are called ‘acceptors’ because they create holes which accept electrons.