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.
