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Questions  

Choose the false statement from the following:

a
Substances with energy gap of the order of 10 eV are insulators.
b
The conductivity of a semiconductor increases with increase in temperature.
c
In conductors, the valence and conduction bands may overlap.
d
The  resistivity  of  a  semiconductor  increases  with increase in temperature.

detailed solution

Correct option is D

In insulators, energy gap  is  of  the  order  of  5  to 10 eV and it is practically impossible to impart this much amount of energy to the  electrons in valence band so as to jump to conduction band . So, choice (1) is correct.In semiconductors, with the rise in temperature more electrons  from valence band jump to conduction band and this results in increase in  conductivity.So, choice (2) is correct. In conductors, the conduction band is either partially filled or the conduction band overlaps on the valence band. So, choice (3) is correct. In semiconductor, resistivity decreases with increase in temperature. So, choice (4) is wrong.

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An n-type semiconductor has impurity level 20meV below the conduction band. In a thermal collision, transferable energy is KT. The value of T for which electrons start to jump in conduction band is :


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