Classification of semiconductors

Semiconductors are classified into two types.

a) Intrinsic semiconductors.

b) Extrinsic semiconductors.

a) Intrinsic semiconductors

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

Example: Silicon, germanium.

· Both silicon and Germanium are tetravalent (having 4 valence electrons).

· Each atom forms a covalent bond or e

lectron pair bond with the electrons of neighboring atom. The structure is shown below.

At low temperature

  • At low temperature, all the valence electrons are tightly bounded the nucleus hence no free electrons are available for conduction.

  • The semiconductor therefore behaves as an Insulator at absolute zero temperature.

At room temperature

  • At room temperature, some of the valence electrons gain enough thermal energy to break up the covalent bonds.

  • This breaking up of covalent bonds sets the electrons free and are available for conduction.

  • When an electron escapes from a covalent bond and becomes free electrons a vacancy is created in a covalent bond as shown in figure above. Such a vacancy is called Hole. It carries positive charge and moves under the influence of an electric field in the direction of the electric field applied.

  • Numbers of holes are equal to the number of electrons since, a hole is nothing but an absence of electrons.

Extrinsic Semiconductor

· When an impurity is added to an Intrinsic semiconductor its conductivity changes.

  • This process of adding impurity to a semiconductor is called Doping and the impure semiconductor is called extrinsic semiconductor.

  • Depending on the type of impurity added, extrinsic semiconductors are further classified as n-type and p-type semiconductor.