The lower completely filled band is called valence band and the upper empty band is called conduction band. The gap between the top of the valence band and bottom of the conduction band is called the energy band gap. It may be large, small, or zero depending on the material.
Valence Energy Band:
They have valence electrons. This band may be partially or completely filled with electrons. This band is never empty. In this band, electrons are not capable of gaining energy from the external electric field. Therefore, the electrons in this band do not contribute to the electric current.
Conduction Energy Band:
Here the electrons are rarely present. This band is either empty or partially filled with electrons. In this band, the electrons can gain energy from the external electric field. Electrons in this band contribute to the electric current.
Forbidden Energy gap:
The separation between the valence band and conduction band is known as forbidden energy gap. If an electron is to be transferred from valence band to conduction band, external energy is required, which is equal to the forbidden energy gap.