Semiconductors

Semiconductors
Semiconductors are those materials whose conductivity lies between insulators and conductors. At 0 Kelvin semiconductors behave as insulators but at room temperature some of the valence electrons move towards the conduction band. This is property by which we can distinguish semiconductors from insulators and conductors. For conductors the valence band and conduction band overlaps and for the insulators the energy gap difference is 7eV but for the semiconductors it is approximately 1.107eV. Therefore at room temperature the valence electrons get enough energy to move from valence band to the conduction band.
Types of semiconductors:
There are two types of semiconductors
  * Intrinsic semiconductors
  * Extrinsic semiconductors
Intrinsic semiconductors:
Intrinsic semiconductors are those semiconductors in which no impurity is added. For example silicon semiconductor containing all the silicon atoms and not even a single impurity is added is intrinsic silicon semiconductor.

In this type of semiconductors all the conductivity is due to the rupturing of covalent bond. In other words due to heating when the covalent bond breaks and move towards the conduction band the valence electrons move towards the conduction band. For a single structure of silicon there is 5x1022/cm3 and only one of every billion atom is ionized.
Extrinsic semiconductors:
Extrinsic semiconductors are those semiconductors in which impurity is added and this process is called doping.
Types of extrinsic semiconductors:
There are two types of extrinsic semiconductors
  * P-type
  * N-type
P-type:
P-type semiconductors are that semiconductor in which trivalent impurity is added i.e. the impurity atom belongs to the 3-A group of the periodic table which has three electrons in the last shell. For example aluminum is selected.

It only form bond with the three valence electrons of silicon and three single bonds will be formed. There will be one...