Electrical Revolution

Intrinsic Semiconductor

The semiconductor material in its purest form is called as intrinsic semi conductor.
The electron – hole pairs are created even at room temperature in an intrinsic semiconductor.
When an electric field is applied to an intrinsic semi conductor material is applied, the conduction of current takes place by holes and electrons.
Due to thermal energy, some of covalent bonds are broken and free electrons and holes are produced.
Therefore the conduction of current is done by electrons and holes.
However the current through intrinsic semiconductor in the external circuit is done by only electrons.
The holes are move towards negative terminal as they are positive charged.
As the holes moves from semiconductor material to negative terminal, the electrons enter the semiconductor material from negative terminal of supply and combine with holes therefore cancelling each other.

Extrinsic Semiconductor

The conductivity of semiconductor is increased by adding some impurities in pure semiconductor.
When some impurities are added to pure semiconductor, it is called as extrinsic semiconductor.
The process of adding impurities in the semiconductor is called as doping.
One impurity atom is added for every 10⁸ atoms of semiconductor.
The number of free electron or hole in the semiconductor crystal increase by adding impurities.
However the free electrons or holes depend upon types of impurities added to pure semiconductor material.
When some Pentavalent impurities are added to pure semiconductor, large number of free electrons produced in the semiconductor.
Similarly when some trivalent impurities are added to pure semiconductor, large number of holes produced in the semiconductor.

There are two types of extrinsic semiconductor material

P type Semiconductor

When a small amount of trivalent impurities are added to semiconductor material, it is called as P type semiconductor material.
When a large number of trivalent impurities are added to semiconductor material, large number of holes is created in the semiconductor.
Some of examples of trivalent impurities are

Aluminium ( Atomic number 13 )

Boron ( Atomic number 5 )

Gallium ( Atomic number 31 )

Indium ( Atomic number 49 )

The semiconductor material in its purest form is shown in the figure A.
When a small amount of trivalent impurities are added to silicon crystal, large number of holes are created in the crystal ( Figure B ).
The gallium is trivalent it means that it has three valance electrons whereas the silicon has tetravalent it means that it has four valance electrons.
When a gallium impurity is added to silicon crystal, only three co – valent bonds are formed.
As there are three valance electrons in the gallium, one hole is created in the silicon crystal.
This short of electron is called as hole.
Therefore we can say that each gallium atom creates one hole.
As the impurities are added, more holes are created in the silicon crystal.
There is more number of holes rather than electrons in the P type semiconductor material.
As the holes are positive charged it is called as P ( positive ) type semiconductor material.

Majority carriers – Holes

Minority carriers – Electrons

N Type Material

When a small amount of pentavalent impurities are added to semiconductor material, it is called as N type semiconductor material.
As large number of pentavalent impurities is added to semiconductor material, large number of electrons is created in the semiconductor.

Some of examples of pentavalent impurities are

The semiconductor material in its purest form is shown in the figure D.

When a small amount of pentavalent impurities are added to silicon crystal, large number of electrons are created in the crystal.
The phosphorus is pentavalent it means that it has five valance electrons whereas the silicon has tetravalent it means that it has four valance electrons.
When a phosphorus impurity is added to silicon crystal, four covalent bonds are formed.
As there are five valance electrons in the phosphorus, one electron is created in the silicon crystal.
Therefore we can say that each phosphorus atom creates one electron.
As the pentavalent impurities are added, more electrons are created in the silicon crystal.
There is more number of electrons rather than holes in the N type semiconductor material.
As the electrons are negative charged it is called as N ( Negative ) type semiconductor material.

Majority carriers – Electrons

Minority carriers – Holes

Current flows due to N type material

The conduction of current in the N type material is done by electrons.
It does not mean that holes do not play any important role for flow of current in the N type material.
The holes are minority carriers whereas the electrons are majority carriers in the N type material.
As the charge of electron is negative, they move towards positive terminal of the supply which is known as electron current.

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