# 1.17   What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.

Semiconductors are the solids with have conductivities in the range from $10^{-6}$ to  $10^{4}$  $ohm^{-1}\ m^{-1}$. In the case of semiconductors, the gap between the valence band and the conduction band is small. Thus, some electrons may jump to the conduction band and may show some conductivity.

Based on the doping process we have two types of semiconductors.

(i) n-type semiconductors:- In the case of n-type semiconductor doping is done by using group 15 elements like P(have 5 valence electrons).

Group 14 elements have 4 valence electrons; these all 4 valence electrons are bonded with 4 valence electron of group 15 element. One left out electron delocalised and increases the conductivity. Due to the presence of one extra electron, it is also called as electron-rich impurities.

(ii) p-type semiconductors:- In the case of p-type semiconductors we use group 13 impurities such as  Al (having 3 valence electrons). This results in the generation of electron hole due to the missing electron in 4th place. From a neighbouring atom, an electron can come and fill the electron-hole, but it will then create a new electron-hole at the position from where electron moved. In this circumstance, it would appear as if the electron-hole is moving in the direction opposite to that of the electron that filled it.

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