Question

Show the effect of temperature on semiconductor materials. Why is a semiconductor an insulator at ordinary temperature?

Answer 1

Effect of temperature on the resistance and conductivity of semiconductor

At absolute Zero. (0 Kelvin) all electrons are held tightly in their orbit by the atom. The covalent bond is strong at this temperature and so there wont be any free electrons. Hence the semiconductor will behave as an insulator

As the temperature increases the electrons will gain energy and some of the electrons will move to the conduction band and so conduction will happen.

Also the bandgap decreases with temperature. This also incerases the conductivity

Silicon and Germanium are examples of such materials.

In such materials energy provided by heat at room temperature is sufficient to lift the electrons form the valence band into the conduction band. But at 0˚K, the semiconductor materials behave as perfect insulators. In case of semiconductors, the forbidden energy gap depends on temperature.

For Silicon, the energy gap is given by

For Germanium, the energy gap is given by

Where T is the absolute temperature in K.

Let us calculate the band gap of Si and Ge at absolute 0 and at room temperature, i.e., 27˚C = 300K,

For Silicon, at absolute 0K

For Silicon, at room temperature

For Germanium, at absolute 0

For Germanium, at room temperature

So as temperature increases, the band gap decreases. This will increase the conductivity of the semiconductor.

The conductivity of intrinsic(pure) semiconductor at room temperature is very small. Very few electron – hole pairs are generated at room temperature. Almost all the valence electrons are engaged in the formation of covalent bonds and there are practically very few free electrons. So it will act as an insulator.