Doping:
One way to raise conductivity is by doping.This means adding impurity atoms to a pure tetravalent crystal (intrinsic crystal). A doped material is called an extrinsic semiconductor.Impurityatoms added to the semiconductor change the thermal equilibrium density of electrons and holes. In
the case of silicon, the appropriate impurities are elements from the 5 and 3 columns of the periodic table, e.g. such as phosphorus and boron. By doping, two types of semiconductors may be produced.First of them are n-type semiconductors with a pentavalent (phosphorus) impurity where the stands n for negative (Fig. 1.3) because their conduction is due to a transfer of excess electrons.A pentavalent atom, the one that has five valence electrons is called a donor.Each donor produces one free electron in a silicon crystal. In an -type semiconductor, the free electrons are the n majority carriers, while the holes are the minority carriers because the free electrons outnumber the holes.
The addition of impurities to a semiconductor to achieve a desired characteristic, as in producing an n-type or p-type material. Also known as semiconductor doping.
Basic conditions that are required for the doping process are given below:
1.The atom which is to be doped in the crystal must be placed at the position same as that of the position of the semiconductor atom.
2.There should be no distortion in the crystal after the insertion of the dopants.
3.The size of the dopants should be exactly same as that of the size of the atom of the crystal.
4.In a crystal the percentage of doping should not be more than one percent.
Some basic doping techniques of doping are shown below:
1.First we have to heat up the semiconductor crystal. The heating must be at the place where the dopants are present in the atmosphere. After heating the diffusion of the dopants will take place in the lattice site of the crystal.
2.In the second method of doping, the semiconductor is bombarded with the ions of the semiconductor itself. Then the dopants are embedded.
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