Intrinsic and extrinsic semiconductor

Intrinsic semiconductor

Intrinsic semiconductor a semiconductor in a extremely pure form is known as intrinsic semiconductor.

In an intrinsic semiconductor even at the room temperature, hole electron pairs are created. when electric field is applied across an  intrinsic semiconductor, the current conduction take place by two processes, namely; by free electrons and holes.

The free electrons in the semiconductor are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, in semiconductor holes are created in the covalent bonds.So under the influence of electric field conduction through the semiconductor is by both free electrons and holes. Therefor, the total current inside the semiconductor is the sum of current due to the free electrons and holes.

Extrinsic semiconductor

The intrinsic semiconductor (extremely pure) has little current conduction capability at room temperature. So it to be useful in electronic devices, the pure semiconductor must be altered so as to significantly increase its conducting properties. This is achieved by adding a small amount of suitable impurity (pentavalent impurity,trivalent impurities) to a semiconductor. It is then called impurity or extrinsic semiconductor. The process of adding impurities to a semiconductor is known as dropping.

The amount and type of such impurities has to be closely controlled during the preparation of extrinsic semiconductor. Generally for 10^8 atoms of semiconductor, one impurity atom is added.
The purpose of adding impurities to increase either the number of free electrons and holes in the semiconductor crystal. As we see, if a pentavalent impurity (having 5 valency electron) is added to semiconductor, a large number of free electrons are produced in the semiconductor.

On the other hand adding a trivalent impurities (having 3 Valence Electrons) creates a large number of holes in semiconductor crystal. Depending upon the type of impurity added (trivalent, pentavalent), extrinsic semiconductor are classified into two types :-

1. N type semiconductor
2. P type semiconductor