On the basis of band theory, solid is divided into three types. They are:
1. Conductor: The substance which contain relatively large number of free electrons is called conductor. In a conductor, the valance and conduction band overlap each other. Due to overlapping, even a slight potential difference across the conductor can cause the free electron to move and to generate current.
2. Insulator:The substance which contain relatively less number of free electrons is called insulator. In an insulator, the width of forbidden band is wide . The conduction band is full of electron while the valance band is empty.
3. Semi-conductor: The substance which contain relatively intermediate level of free electrons (less than conductor and more than that of insulator) is called semiconductor. Germanium and and silicon are the examples of semi-conductor. In a semi-conductor, the width of forbidden band is small and the valance band is almost filled with electron while the conduction band is empty.