Sunday, May 20, 2012

Extrinsic Semiconductors

       In order to change the properties of intrinsic semiconductors a small amount of some other material is added to it. The process of adding other material to the crystal of intrinsic semiconductors to improve its conductivity is called doping. The impurity added is called dopant. Doped semiconductor material is called semiconductor. The doping increases the conductivity of the basic intrinsic semiconductors hence the extrinsic semiconductors are used in practice for manufacturing of various electronic devices such as diodes, transistors etc.
       Depending upon the type of impurities, the two types of extrinsic semiconductors are, 
       1. n-type    and   2. p-type
1.1 Types of Impurities
       The impurity material having five valence electrons is called pentavalent atom. When this is added to an intrinsic semiconductor, it is called donor doping as each impurity atom donates one free electron to an intrinsic material. Such an impurity is called donor impurity. The examples of such impurity are arsenic, bismuth, phosphorous etc. This create an extrinsic semiconductor with large number of free electrons, called n-type semiconductor.
       Another type of impurity used is trivalent atom which has only three valence electronics. Such an impurity. When this is added to an intrinsic semiconductor, it creats more holes and ready to accept an electron hence the doping is called acceptor doping. The examples of such impurity are gallium, indium and boron. The resulting extrinsic semiconductor with large number of holes is called p-type semiconductor.

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