Intrinsic semiconductor and Extrinsic semiconductor

Intrinsic Semiconductor

A semiconductor in an extremely pure form is known as an intrinsic semiconductor.
In an intrinsic semiconductor, even at room temperature, hole-electron pairs are
created. When electric field is applied across an intrinsic semiconductor, the current
conduction takes place by two processes, namely;

by free electrons and holes as
shown in Fig.1 the free electrons are produce due to the breaking up of some
covalent bonds by thermal energy. At the same time, holes ate created in the covalent
bonds, under the influence of electric field; conduction through the semiconductor is by
both free electrons and holes. Therefore, the total current inside the semiconductor is
the sum of currents due to free electrons and holes.

It may be note that current in the external wires is fully electronic e.i. by
electrons. What about the holes? Referring to Fig.1 holes being positively charge
move towards the negative terminal of supply.Intrinsic semiconductor and Extrinsic semiconductor.

As the holes reach the negative terminal
B, electrons enter the semiconductor crystal near the terminal and combine with holes,
thus cancelling them. At same time, the loosely held electrons near the positive terminal
A are attracte away from their atoms into the positive terminal. This creates new holes
near the positive terminal which again drift towards the negative terminal.

Extrinsic Semiconductor

The intrinsic semiconductor has little current conduction capability at room temperature.
To be useful in electronic devices, The pure semiconductor must be alter so as to
significantly increase its conducting properties. This is achieve by adding a small
amount of suitable impurity to a semiconductor.

Ti is then call impurity or extrinsic
semiconductor. The process of adding impurities to a semiconductor is know as
doping. The amount and type of such impurities have to be closely controlled during the
preparation of extrinsic semiconductor. Generally, for 10 8 atoms of semiconductor, one
impurity atom is added.

The purpose of adding impurity is to increase either the number of free electrons or
holes in the semiconductor crystal. As we shall see, if a pentavalent impurity (having 5
valence electrons) is added to the semiconductor, a large number of free electrons are
produced in the semiconductor. On the other hand, addition of trivalent impurity (having
3 valence electrons) creates a large number of holes in the semiconductor crystal.

Depending upon the type of impurity added, extrinsic semiconductors are classified into:

(i) .n-type semiconductor (ii) p-type semiconductor

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