Effect of Temperature on Semiconductor Resistance & Conductivity
The conductivity of a semiconductor changes appreciably with temperature variation. This is a very important property of semiconductors.
To discuss this property of semiconductor let us consider two cases.
Case-1: At Absolute Zero
At absolute zero temperature, all the electrons are tightly held by the semiconductor atom. The inner orbit electrons are bound whereas the valence electrons are engaged in c0-valent bonding.
At this temperature, the co-valent bonds are very strong and hence there are no free electrons.
Therefore, at absolute zero temperature the semiconductor crystal behaves as a perfect insulator (as shown in Fig.1(i)).
In terms of energy band description (as shown in Fig.1 (ii)), the valence band is filled and the gap between the valence band and conduction band is large.
Hence, no valence electron can reach the conduction band to become free electron.
Therefore, due to the non-availability of free electrons the semiconductor behaves as an insulator at absolute zero temperature.
Case-2 : Above Absolute Zero
When the temperature is increased, some of the co-valent bonds in the semiconductor crystal breaks due to the supplied thermal energy.
The breaking of bonds set those electrons free which are engaged in the formation of these bonds. As a result few free electrons exist in the semiconductor.
These free electrons constitute a tiny electric current if potential difference is applied across the semiconductor crystal. This is shown in Fig.2 below.
However, at room temperature, current through a semiconductor is too small.
As the temperature is increased, some of the valence electrons acquire sufficient energy to enter into the conduction band and thus become free electrons.
Under the influence of electric field, these free electrons will constitute electric current.
It may be noted that each time a valence electron enters into the conduction band, a hole is created in the valence band.
In our next article we will see the contribution of holes to current. In fact hole current is the most significant concept in semiconductors.