For any material to be classified as an Insulator, Conductor or Semiconductor it is important to consider the amount of available free electrons it has. According to the Energy Band theory, every substance has two bands of levels where electrons are placed, Valence Band and Conduction Band. Here, the Conduction Band, as the name suggests, is responsible for conduction of current as it provides free electrons. The more electrons a material has in the Conduction Band the better conductor it would be and vice versa.



Another thing that is of importance here is the the Energy Gap between the Valence and Conduction Band. Conduction Band is situated at a higher energy level than the Valence Band. If an electron had to jump to the Conduction Band and for it to be conducting, it would have to gain a specific amount of energy to do so.This is the Energy Gap, the difference between the energies of the two bands. The amount of Energy Gap is also an indicator of whether a material is a Conductor, Semiconductor or an Insulator. If the Energy gap is larger, it would be harder for an electron to jump upwards for conduction, the material would have tendency to be an insulator. On the other hand, smaller Energy Gap would mean easy access to the Conduction Band, typical for conductors. For semiconductors, the energy gap is somewhere in between the two.

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