When no of atoms is combining then the whole energy levels are dividing in sub energy levels and become overlapped. They make a band, which is called energy band. Remember that the energy of free electron is changing continuously.
In a solid there are three type of energy band.
1. Filled Band
As clear from its name it is that type of band, which is near to the nucleus, and it is completely full from electron. In that type of band there are no free electron.
2. Valance Band
The last band of the atom is called valance band and the electron of that band is called valance electron. When valance electron gain some energy then they leave that band and cross the energy gap which is also called forbidden energy gap and goes to conduction band then current flow starts from this material. The energy of the valance electron is more as compare to filled band.
3. Conduction Band
When free electrons are escape from its permanent atom then they to conduction band and such process is called conduction. The electron of such band is called free electron.
Insulator is that material in which current dose not flows easily e.g. Wood; paper, plastic, oil, mica etc The reason for insulation is the wide gape between the valance band and conduction band. A large amount of energy is required to shift electrons from the valence band in to the conduction band.
Conductor is those materials in which current flows easily. For example silver, cu and aluminum etc The reason for the conduction the absence of forbidden between the absence of forbidden band, so very small amount of energy is required for the flow of electric current. There are many free electrons in the conducting materials.
Semi conductor are those material which has the conduction property in between conductor and insulator. It means semi conductor do not allow the free electron to flow as conductor allow. In the same way semi conductor do not block the current as insulator do. For example silicon, boron, carbon etc. The reason for such type of conductor is the small gap between the valence band and conduction band. Semi conductors have comparatively less free electron than the conductor.