- Insulators have very wide forbidden energy gap nearly of the order of 5eV or more.
- Because of this very high energy gap it becomes impossible for electrons present in valence band to cross the gap and reach to the conduction band and this makes electrical conduction a practical impossibility in insulators at room temperature.
- However at very high temperatures or with very high voltage applied across the ends of the insulator , it may conduct and this is termed as breakdown of an insulator.
- Conduction band and valence band overlaps in case of a conductor.
- Value of forbidden energy gap is zero for conductors in other words it does not exists at all.
- For conductors or metals , valence band energies are same as conduction band energies and an valence electron can very easily become conduction electron (or, free electron) without any supply of heat energy.
- This is why metals contain large number of free electrons even at room temperature and are good conductor of electricity.