Electrons and Holes in Intrinsic Semiconductor

Intrinsic Semiconductor

The pure semiconductors (impurity < 1 part in 10¹⁰) are called intrinsic semiconductor. The charge carriers (electrons and holes) are formed due to thermal excitation. Holes are vacancies of an electron in the bond of a covalently bonded crystal. It acts as a positive charge carrier.

As each free electron creates one hole, so in an intrinsic semiconductor, the number of free electrons (n_e) is equal to number of holes (n_h) is equal to the intrinsic charge carrier.

n_e = n_h = n_i 

Donor and Acceptor Impurities

Doping

In order to increase the conductivity of pure semiconductors a small amount of impurity atoms having valency different from 4 is added to the pure semiconductors. This process is called doping.

Two types of impurity atoms are added to tetravalent atom.

i)                    Trivalent                ii)         Pentavalent

Trivalent	Tetravalent	Pentavalent
B (Boron)	Si (Silicon)	P (Phosphorus)
Al (Aluminium)	Ge (Germanium)	As (Arsenic)
Ga (Gallium)		Sb (Antimony)
In (Indium)

Donor Impurity

When pentavalent atom is added to pure semiconductor, it forms bonds with neighbouring four tetravalent atoms. The fifth electron is not associated with any covalent bond and it remains loosely bounded to parent atom. The thermal energy at room temperature is practically enough to set all such electrons free from their atoms. Due to this, negatively charged electron is free to move in the lattice of semiconductor. A positive charge is acquired by pentavalent atom due to loss of one electron and it becomes positively charged ion. The pentavalent atom becomes positive ion by donating one extra electron from crystal. So, this type of impurity is called Donor Impurity.

Acceptor Impurity

When trivalent atom is added to pure semiconductor, it forms bonds with neighbouring three tetravalent atoms. It does not have any electron to form fourth bond. There is a deficiency of an electron around trivalent atom. The single electron in the incomplete bond has a great tendency to snatch an electron from neighbouring atom. The thermal energy at room temperature is enough to fill the incomplete bonds around all trivalent atoms. Due to this, a vacancy is created in adjacent bond from where the electron is jumped and has positive charge associated with it, hence it is a hole. Also, due to filling of incomplete bond, the trivalent atom becomes negatively charged ion. The trivalent atom becomes negative ion by accepting one electron from crystal. So, this type of impurity is called Acceptor Impurity.