We Know that some solids are good conductors of electricity and others are insulators. The difference in their behavior regards to the electrical conductivity can clearly understand by the energy bands. Now let’s see the definition of insulator and conductor with its energy band diagram.
What is insulator?
Insulators are the substance which does not allow electric current to pass through them. Glass, Wood rubbers are an example of the insulators. Insulators have a large energy gap approximately 15 eV as shown in the band diagram. In an insulator, the valence band is full while the conduction band is empty that’s why current is not flow from the insulator. A very high temperature is required to push the valence electrons to the conduction band.
In the above figure, you see that Forbidden energy gap is very large so at an ordinary temperature electron from the valence band cannot cross over to the conduction band. However when the temperature has raised the electron from the valence band cross over to the conduction band. The resistance of an insulator decreases with the increase in temperature so insulator has a negative temperature coefficient resistance.
What is a conductor?
Conductors are substance in which current is very easily passed. The valence band and a conduction band of the conductors are overlapping each other. That’s why current is easily flow through them. As shown in below figure there are a large number of free electrons are available in a conductor.
In terms of energy band, you can say that valence band and conduction band overlap each other. Due to this overlapping, a slight potential difference across the conductor causes the free electrons to constitute an electric current.