Based on their conductivity, materials are classified as insulators, semiconductors and conductors.
Insulators are materials that offer a large resistance to the flow of current through them.
Typical resistivity levels of an insulator are of the order of 1010 to 1012 Ωcm.
Since, the resistivity levels of an insulator is very high, application of voltage across the insulator results in negligible flow of current.
Insulators have seven to eight valence electrons. These valence electrons are tightly bound to the atom, therefore, there are no free electrons that can move through the material.
Figure below shows the energy-band structure of an insulator.
Energy band diagram of an insulator.
Insulators have a large forbidden gap of greater than 5eV between the valence and the conduction energy bands. Hence, there are very few electrons in the conduction band and therefore the conductivity of an insulator is poor.
Increase in temperature is insufficient to transfer the electrons from the valence band to the conduction band. Therefore, there is not much change in the conductivity of an insulator with increase in temperature.
Some of the popular insulator materials are mica, glass and quartz.
Conductors are materials that offer very little resistance to the flow of current through them.
Resistivity level of conductors is of the order of 10–4 to 10–6Ωcm.
Since the resistivity levels of conductors are less, they support a generous flow of current when an external electric field is applied across their terminals.
Conductors, generally, have three or less than three valence electrons which are loosely bound and are free to move through the material.
Metals such as copper, aluminum, gold and silver are good conductors.
Figure below shows the energy band structure of a conductor. As we can see from the figure, the valence and conduction bands overlap with each other. Electrons are therefore free to move from the valence band to the conduction band and hence there are free electrons in the conduction band even at absolute zero temperature (0° k).
Energy band diagram of a conductor
When an external electric field is applied to a conductor, there is a large flow of current through it.
. In case of conductors, the current is due to the flow of electrons only.