Materials are classifieds into three categories according to their resistance. It can be understood by Band Theory. Band Theory is a useful way to visualize the difference between the type of materials is to plot the available energies of electrons in the materials. Two types of band used to classify the materials which are Conduction band and Valence band. All materials have full of electrons in Valence Band. The type of materials is classified as,

      1. Insulators.
      2. Semiconductors.
      3. Conductors.

1. Insulators:

In Insulator, Energy gap between the conduction band and valence band is large. The Electrons are packed strongly within atom so there is no movement of Electron.



The Large amount of energy needs to remove Electron from the atom of insulator materials. The Resistance value is too high. Examples of insulators are plastics, Styrofoam, paper, rubber, glass and dry air.

2. Semiconductors:

The Energy gap is low between the conduction band and valence band(~1eV). The Electrons are easily removed from an atom by a low amount of energy.



Normally semiconductors have no electron in the conduction band until gets energy from outside such as thermal or light energy. Resistance value lies in between insulators and conductors. Good examples of semiconductor materials are germanium, selenium, and silicon.

3. Conductors:

A Partial amount of the conduction band and valence band are combined together in the conductors. So the electrons are moving randomly in normal condition. This movement of electrons is electrical conduction. Conductors have low values of resistance. Examples of conductors include metals, graphite, aqueous solutions of salts (i.e., ionic compounds dissolved in water), and the human body.