Conduction in Semiconductors

The electrical conduction in a semiconductor is due to free electrons in the conduction band and holes [vacant sites] in the valence band.
The free electron current is in the opposite direction to the conventional current. But the hole current is in the same direction of the conventional current.
The movement of the free electron and holes are in the opposite direction in the presence of applied field .
The total current in the semiconductor is the sum of the free electron current and hole current.
The free electrons and holes are produced by thermal ionization of atoms by breaking the covalent bonds due to applied voltage.
There are intrinsic and extrinsic semiconductors.
Intrinsic semiconductors are pure form of semiconductors.
Extrinsic semiconductors are impure form of semiconductors [doped semiconductors].
By the process of doping, the impurities are added in the intrinsic semiconductors by which the holes and free electrons are produced.
If pentavalent (Phosphorous, arsenic, antimony) impurities are doped, then the free electrons are produced, this semiconductor is called N-type semiconductor.
If trivalent [Aluminium, Gallium,indium] impurities are doped, then the holes are produced, then this semiconductor is called P-type semiconductor.