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Classification of Semiconductors

Classification of Semiconductors

  1. Intrinsic
  2. Extrinsic

 

Intrinsic Semiconductor: A pure semiconductor is called intrinsic semiconductor. As already explained at the first even at a room temperature, some of the valence electrons may acquire sufficient energy to enter the conduction band to form free electrons. Under the influence of electric field, these electrons constitute electric current. A missing electron in the valence band leaves a vacant space there, which is known as hole. Holes also contribute to electric current.

In an intrinsic semiconductor, even at room temperature, electron-hole pairs are created. When electric fieldis applied across an intrinsic semiconductor, the current conduction takes place by two processes, namely, free electrons and holes. Under the influence of electric field, total currentthrough the semiconductor is the sum of currents due to free electrons and holes.

Though the total current inside the semiconductor is due to free electrons and holes, the current in the external wire is fully by electrons. Holes being positively charged move towards the negative terminal of the battery. As the holes reach the negative terminal of the battery, electrons enter the semiconductor near the terminal and combine with the holes.This create new holes near the positive terminal which again drift toeards the negative terminal.

Extrinsic Semiconductor: Due to the poor conduction at room temperature, the intrinsic semiconductor as such, is not useful in thr electronic devices. Hence, the current conduction capability of the intrinsic semiconductor should be increased. This can be achieved by adding a small amount of impurity to the intrinsic semiconductor, so that it becomes impure or extrinsic semiconductor. This process of adding impurity is known as doping.

The amount of impurity added is extremely small, say 1 to 2 atoms of impurity for intrinsic atoms.

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