In solid-state physics, the Energy Bands i.e. valence band and conduction band are closest to the Fermi level and thus determine the electrical conductivity of the solid. In non-metals, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature, while the conduction band is the lowest range of vacant electronic states. On a graph of the electronic band structure of a material, the valence band is located below the Fermi level, while the conduction band is located above it. This distinction is meaningless in metals where conduction occurs in one or more partially filled bands, taking on the properties of both the valence and conduction bands.
Band gap in Energy Bands
The two bands are separated by a bandgap In semiconductors and insulators, while in semimetals the bands overlap. A band gap is an energy range in a solid where no electron states can exist. This happens due to the quantization of energy. The electrical conductivity of non-metals is determined by the susceptibility of electrons to excitation from the valence band to the conduction band.
In solids, the ability of electrons to act as charge carriers depends on the availability of vacant electronic states. When an electric field is applied then This allows the electrons to increase their energy (i.e., accelerate). Similarly, holes (empty states) in the almost filled valence band also allow for conductivity.
The electrical conductivity of a solid depends on its capability to flow electrons from the valence to the conduction band. Hence, in the case of a semimetal with an overlap region, the electrical conductivity is high. For small band gap (Eg), the flow of electrons is possible only if an external energy (thermal, etc.) is supplied. These groups with small band gap are called semiconductors. If the Eg is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible. These groups are called insulators.
There is some conductivity in semiconductors, however. This is due to thermal excitation—some of the electrons get enough energy to jump the band gap in one go. Once they are in the conduction band, they can conduct electricity, as can the hole they left behind in the valence band. The hole is an empty state that allows electrons in the valence band with some degree of freedom.