N type and P type semiconductor

N type semiconductor

When is small amount of pentavalent impurity is added to a pure semiconductor, it is known as         N-type semiconductor.
The addition of pentavalent impurity in semiconductor provides a large number of free electrons in the semiconductor crystal. The examples of pentavalent impurities are arsenic (At. No. 33) and antimony (At. No.51).
Such impurities which produce n-type semiconductor are know as donor impurities becouse they donate or provide free electrons to the semiconductor crystal.
Suppose one pentavalent (arsenic) impurity is added to pure  (intrinsic) silicon.The impurity atom has 5 valence electrons. Four of the arsenic atom's valence electrons are used to form covalent bonds with 4 silicon atoms, leaving one extra electron.This extra electron become free electron becouse it is not attached to any atom. So, for each arsenic atom added, one free electron will be available in the silicon crystal. Though each arsenic atom added provides one free electron, yet an extremely small amount of arsenic impurity provides enough atoms to supply millions of free electrons. Since this type of extrinsic semiconductor provides a large number of free electrons, it is called n-type semiconductor (n stands for negative charge on an electron).

P type semiconductor

When a small amount of trivalent impurity is added in a pure semiconductor it is called P-type semiconductor.
The addition of trivalent impurity provides a large number of holes in the semiconductor the example of trivalent impurities are gallium (At. No.31) and Indium (At. No. 49).
 Search trivalent impurities which produce P type semiconductor are known as acceptor impurities because the holes created can accept the electrons.
Suppose one trivalent (gallium) impurity atom is added to pure (intrinsic) Silicon. The impurity atom has 3 Valence Electrons. Three of the gallium atom's Valence Electrons from covalent bonds with three silicone atoms, leaving one electron missing in the fourth band. This missing electron in the band is called hole.
The hole act as a positive charge therefore, for each gallium atom added provides one hole, yet an extremely small amount of gellium impurity provides enough atom to supply millions of holes. Since this type of extended semiconductor provide a large number of holes, it is called P-type semiconductor (P stands for positive charge on a hole)

P-type conductivity

The current conduction in a p-type semiconductor is predominantly by holes. Example positive charge and is called P-type or hole-type conductivity. When  potential difference is applied across the P-type semiconductor, the holes (produced by impurity) in the Crystal will drift towards the negative terminal, constituting what is know as hole current.So It may be noted that in p-type conductivity the valence electrons move from one covalent bond to another Unlike the n-type where current conduction is by free electrons.

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