Semiconductor

Introduction to semiconductor

The material which has electrical conductivity between that of a conductor and that of an insulator is called as semiconductor. Silicon, germanium and graphite are some examples of semiconductors. Semiconductors are the foundation of modern electronics, including transistors, Light-Emitting diodes, solar cells etc.

In semiconductors, the forbidden gap between valence band and conduction band is very small. It has a forbidden gap of about 1 electron volt (eV).

At low temperature, the valence band is completely occupied with electrons and conduction band is empty because the electrons in the valence band does not have enough energy to move in to conduction band. Therefore, semiconductor behaves as an insulator at low temperature.

However, at room temperature some of the electrons in valence band gains enough energy in the form of heat and moves in to conduction band. When the valence electrons moves in to conduction band they becomes free electrons. These electrons are not attached to the nucleus of a atom, So they moves freely.

The conduction band electrons are responsible for electrical conductivity. The measure of ability to conduct electric current is called as electrical conductivity.

When the temperature is goes on increasing, the number of valence band electrons moving in to conduction band is also increases. This shows that electrical conductivity of the semiconductor increases with increase in temperature. i.e. a semiconductor has negative temperature co-efficient of resistance. The resistance of semiconductor decreases with increase in temperature. 

In semiconductors, electric current is carried by two types of charge carriers they are electrons and holes.