Metals, Insulators And Semiconductors

 Metals, Insulators And Semiconductors

Metals, Insulators And Semiconductors

We already know that there are three types of materials

And they are Metals, Insulators and semiconductors,

 

A material is able to conduct electricity, if it contains movable or free electrons in it. As we already discuss in previous posts that the valence electrons are lying in the conduction band of an atom. (structure of an Atom)

When any external field is applied on these valence electrons then these valence electrons work as charge carriers.

 Silver and copper contains a large  number of free electrons at room temperature. There is no forbidden energy gap between the valence and conduction bands.

The following figure shows the energy bands of metals or conductors.

 

The valance band energy are the same as the conduction band energy in the metal. So that it is very easy to for a valence electron to become a conduction (free) electron.

Therefore, without applying any additional energy such as heat or light, a metal already contains a large number of free electrons and that is why it works as a good conductor.

In insulators there is a very wide forbidden energy gap (5ev or more). Because of this, it is practically impossible for an electron in the valence band to jump the gap, to reach the conduction band.

The following figure shows the energy band of insulators.

At room temperature, an insulator does not conduct because there are no conduction electrons in it. However, it may conduct if its temperature is very high or if a high voltage is applied across it. And this is called the breakdown the insulator.

Following figure shows the energy band of semiconductor.

 In semiconductors the forbidden energy gap is not very wide. It is of the order of 1 eV (for germanium E_G = 0.72 eV and for Silicon E_G = 1.12 eV). The energy provided by heat at room temperature is sufficient to lift electrons from the valence band to the conduction band. Some electrons do jump jump the gap and go into the conduction band. Therefore, at room temperature, semiconductor are capable of conducting some electrical current.