<P> In solids, the ability of electrons to act as charge carriers depends on the availability of vacant electronic states . This allows the electrons to increase their energy (i.e., accelerate) when an electric field is applied . Similarly, holes (empty states) in the almost filled valence band also allow for conductivity . </P> <P> As such, 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 . If there is a small band gap (E), then the flow of electrons from valence to conduction band is possible only if an external energy (thermal, etc .) is supplied; these groups with small E are called semiconductors . If the E is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible under normal conditions; these groups are called insulators . </P> <P> 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 some degree of freedom . </P>

What is meant by energy gap in band theory