SEMICONDUCTOR : DEFINITION , TYPES

WHAT ARE SEMICONDUCTORS ?

• Semiconductors are those substances whose conductivity lies in between that of a conductor and Insulator.Example: Silicon, germanium, Cealenium, Gallium, arsenide etc.
• In terms of energy bands, semiconductors are those substances in which the forbidden gap is narrow.Thus valence and conduction bands are moderately separated .
• In semiconductors, the valence band is partially filled, the conduction band is also partially filled, and the energy gap between conduction band and valence band is narrow.
• Therefore, comparatively smaller electric field is required to push the electrons from valence band to conduction band . 
• At low temperatures the valence band is completely filled and conduction band is completely empty. Therefore, at very low temperature a semi-conductor actually behaves as an insulator.

TYPES OF SEMICONDUCTOR

a) Intrinsic semiconductors.

b) Extrinsic semiconductors.

a) Intrinsic semiconductors

• A semiconductor in an extremely pure form is known as Intrinsic semiconductor.
• Example: Silicon, germanium.

b) Extrinsic semiconductors

• 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.