Semiconductors

The atoms of these substance have their conductivity intermediate between conductors and insulators.
The resistivity of the substance is higher than that of conductors and lower than that of insulators.
The conductivity is lower than conductors and higher than insulators.
Resistance and resistivity decreases with rise in temperature, it means the temperature coefficient of the semiconductors are negative.
The conductivity of semiconductors can be increased by adding some impurities in it.
Example: Ge, Si, etc.

Types of semiconductors

A semiconductor in its natural pure form is called intrinsic semiconductor.
These semiconductors has four electrons in its outermost orbit, and hence they are tetravalent element.
On increasing the temperature, the valence electrons leave their position and create some vacant space, known as hole.
The hole has a positive charge (because this is the region, from where electron shifts).
The position of hole is filled by another electrons, and hence a new hole is created, and this new hole is again filled by next electrons. In this way hole changes its position continuously in a random manner.
The movement of hole is just like a free electron.
Since in pure semiconductor the number density of holes and electrons are very small, and hence the current produced in an intrinsic semiconductor can not be used as a practical purpose.

Motion of hole in intrinsic semiconductor

Structure of intrinsic semiconductor

When a small amount of impurity is added into a pure semiconductor, then its conductivity increases significantly.
The process of adding impurity into an intrinsic semiconductor is known as doping.
The semiconductor so obtained is known as doped semiconductor, impure semiconductor or extrinsic semiconductor.
Generally 1 impurity atom is added into 10⁸ atoms of given semiconductor.

When a pentavalent impurity atom (Sb, As, Bi, P) is added into a pure semiconductor, then the semiconductor so obtained is known as n-type (negative type) semiconductor.
Because a pentavalent atom has five valence electrons, and out of five four makes the covalent bond with the given semiconductor, and there is one free electrons.
In these semiconductors each impurity atom donates one free electrons, and hence it is donor type impurity.
Thus a semiconductor containing donor type impurity is n-type extrinsic semiconductor.
Here the majority charge carrier are electrons.

n-type extrinsic semiconductor

When a trivalent impurity atom is added into a pure semiconductor, then the semiconductor so obtained is known as p-type (positive type) semiconductor.
Because a trivalent atom has three valence electrons, and all the three electrons make the covalent bond with the given semiconductor, and there is deficiency of one electron.
This electron deficiency is known as hole.
In these semiconductor each impurity atom gains one electrons, and hence it is acceptor type of impurity.
Thus a semiconductor containing acceptor type impurity is p-type extrinsic semiconductor.
Here the majority charge carrier are holes.

p-type extrinsic semiconductor

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