n-type semiconductor

Defined as :
“When a small amount of pentavalent impurity is added to a pure semiconductor , which provide a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n-type semiconductor”.

Let us consider what happen if a small amount of pentavalent impurity, for example  phosphorus is added to a sample of intrinsic germanium . Phosphorus atom has 5 Valence electrons, four of these form covalent bonds with neighbouring germanium atom.The fifth electron has no chance of forming a covalent bond. Since it is not associated with any covalent bond ,also it is quite far from the nucleus and it is very loosely bound. It requires very little energy from the attractive force of its nucleus (0.01 electron volt in case of Germanium and 0.05 electron volt in case of silicon). This energy is so small that at room temperature practically all such electrons become free. Or we can say at room temperature ,each impurity atom donates one free electron to the conduction band ,this is the reason why this impurity is called donar type.
Donor impurity :
(In other words,Such impurities which provide n-type semiconductor i.e. penatavalent impurity also known as Donor impurity, because each atom of them donate one free electron to the semiconductor crystal .)

Addition of pentavalent impurity provides a large number of free electrons in the semiconductor system.
Other examples: Arsenic , Antimony (having atomic number 33 ,51 resp.), provide a large number of free electrons in the semiconductor crystal .
When a small amount of pentavalent impurity like arsenic atomic number 33 electronic configuration (2 8  18  5)  having 5 valence electrons is added to Germanium crystal, each atom of the impurity fits in the Germanium crystal in such a way that its four electrons form covalent bond with four Germanium atoms where as the fifth electron finds no place in the covalent bonds and is thus free .Hence each arsenic atom provides one free electrons in the Germanium crystal .Since an extremely small amount of arsenic impurity has a large number of atoms, therefore it provides millions of free electrons for conduction .
See energy band diagram ,
With the addition of pentavalent impurity a large number of free electrons are made available in the conduction band these electrons are the free electrons which did not fit in the covalent bond of the Crystal (i.e. fifth electron of each arsenic at atom) .

However a minute quantity of free electrons are also available in the conduction band which are produced when thermal energy at room temperature is a imparted to the Germanium crystal forming hole electron pair .
So a large number of free electrons are made available by the addition of pentavalent impurity.
A minute quantity of free electrons are made available by the generation of electron pair when thermal energy at room temperature is imparted to the semiconductor crystal,these electrons leaves behind holes in the valence band .

Conduction through n-type semiconductor :

In n-type semiconductor a large number of free electrons (donated by the impurity atoms) are available in the conduction band .When a potential difference is applied across this type of semiconductor, the free electrons are directed towards the positive terminal constituting electric current. As the flow of current through the Crystal is constituted by free electrons which are carriers of negative charge, therefore this type of conductivity is called negative Or n- type conductivity .It may be noted that conduction through n-type semiconductor is similar to that of conduction through metal like Copper. It is seen that at room temperature ,electron-hole pair are formed .These holes which are available in minute quantity in valance band also constitutes a little current in a semiconductor. So this about n-type semiconductor.

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