Physics Group - II (3110018)

Intrinsic semiconductor: A semiconductor in an extremely pure form is known as an intrinsic semiconductor.

• In an intrinsic semiconductor, even at room temperature, hole-electron pairs are created. When electric field is applied across an intrinsic semiconductor, the current conduction takes place by two processes, namely; by free electrons and holes.
• The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bonds.
• Under the influence of electric field, conduction through the semiconductor is by both free electrons and holes.
• Therefore, the total current inside the semiconductor is the sum of currents due to free electrons and holes.

Extrinsic semiconductor:If we are adding a small amount of suitable impurity to a semiconductor. It is then called impurity or extrinsic semiconductor.

• The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal.
• As we shall see, if a pentavalent impurity (having 5 valence electrons) is added to the semiconductor, a large number of free electrons are produced in the semiconductor.
• On the other hand, addition of trivalent impurity (having 3 valence electrons) creates a large number of holes in the semiconductor crystal.
• Depending upon the type of impurity added, extrinsic semiconductors are classified into:
  1. n-type semiconductor
  2. p-type semiconductor