The conductivity of the semiconductor is changed accordingly temperature variation. Now let’s see the effect of the temperature on semiconductors.
At Absolute Zero Temperature:-
At zero temperature all the electrons are tightly bound by the semiconductor atom. Inner orbit electrons are bound to the nucleus whereas valence electrons are engaged in covalent bonding. At absolute zero temperature, the covalent bonding is very strong therefore no free electrons are present. In this state, semiconductor behave like an insulator.
In terms of energy band, you can say that valence band is filled and there is a large energy gap between valence band and conduction band, therefore, electrons pass from valence band to conduction band so there is no current flow at absolute zero temperature.
Above Absolute zero Temperature:-
When the temperature is the increase some of the covalent bond break. For this reason, covalent bonds break and make electrons free for conduction. The result is few free electrons exist in the semiconductor which constitutes a tiny current.
As shown in the above figure potential difference is applied across the semiconductor which shows that the resistance of a semiconductor decrease with the rise in temperature. So you can say that semiconductor has a negative temperature. At room temperature semiconductor is too small for practical conduction.
As shown in the above energy band diagram. As the temperature increase, valence electrons acquire more sufficient energy to enter into the conduction band and become free. Under the influence of an electric field, these free electrons will constitute an electric current. Note that each time hole is generated when valence electron enters into the conduction band.