Question

State true or false – The conductivity of an intrinsic semiconductor decreases with increase in temperature.
A) True
B) False

Answer 1

The conductivity of a material depends upon the number of free electrons present in the conduction band of the material. For electrons to be present in the conduction band, they must overcome the energy band gap between the valence band and conduction band and jump up to the conduction band.

Complete step by step answer:
The conductivity of a substance is a measure of how easily it allows current to flow through it. The greater the conductivity of a substance, more easily will current flow through it.
The conductivity of a material depends upon the number of free electrons present in the conduction band of the substance as these electrons are responsible for current flow. For electrons to reach the conduction band they must overcome the energy band gap between the conduction and valence band.
For semiconductors, the band gap between the conduction band and the valence band is low. Therefore, when the temperature rises, the electrons in the valence band gain thermal energy and can jump up to the conduction band by overcoming the energy gap. Therefore, this leads to an increase in the number of free electrons in the conduction band and eventually, increase in the conductivity of the semiconductor.

Therefore, the correct option is B) False.

Note:
The conductivity of semiconductors increases with rise in temperature but, for metals, it decreases with rise in temperature. This is because the energy gap between the conduction and valence band is zero as both the bands overlap. Therefore, no extra electrons can jump up to the conduction band by overcoming the energy gap since all of them are already present in it. However, the greater thermal energy leads to greater vibrations in the atoms of the metal, which hinder the flow of electrons and hence, hinder current flow. Therefore, the conductivity of metals decreases with temperature.
Students must be aware of this difference in the behavior of the conductivities of metals and semiconductors with increase in temperature.