The application of intrinsic semiconductor is restricted due to its low conductivity. In electronic device, high conducting semiconductors are more essential. The concentration of electrons or holes in a semiconductor is to be increased depending upon the requirements.
This can be carried out simply by adding impurities to the intrinsic semiconductor. The process of adding impurity to the intrinsic semiconductors is known as doping. The doped semiconductor is called an extrinsic semiconductor. The concentration of electrons and holes are not equal in an extrinsic semiconductor. The added impurity may be pentavalent or trivalent.
Depending upon the type of impurity added, the extrinsic semiconductor can be divided into two classes.
a) n-type semiconductors (pentavalent element as impurity) and
b) p-type semiconductors (trivalent element as impurity)
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