A Semiconductor in its extremely pure form is said to be an intrinsic semiconductor. The properties of this pure semiconductor are as follows −

• The electrons and holes are solely created by thermal excitation.
• The number of free electrons is equal to the number of holes.
• The conduction capability is small at room temperature.

In order to increase the conduction capability of intrinsic semiconductor, it is better to add some impurities. This process of adding impurities is called as Doping. Now, this doped intrinsic semiconductor is called as an Extrinsic Semiconductor.

Doping

The process of adding impurities to the semiconductor materials is termed as doping. The impurities added, are generally pentavalent and trivalent impurities.

Pentavalent Impurities

·        The pentavalent impurities are the ones which has five valence electrons in the outer most orbit. Example: Bismuth, Antimony, Arsenic, Phosphorus

·        The pentavalent atom is called as a donor atom because it donates one electron to the conduction band of pure semiconductor atom.

Trivalent Impurities

·        The trivalent impurities are the ones which has three valence electrons in the outer most orbit. Example: Gallium, Indium, Aluminum, Boron

·        The trivalent atom is called as an acceptor atom because it accepts one electron from the semiconductor atom.