In solids, the valence band is the highest range of electronenergies where electrons are normally present at zero temperature. In semiconductors and insulators, there is a bandgap above the valence band, followed by a conduction band above that. In metals, the conduction band is the valence band. The rest of this article refers to the valence band in semiconductors and insulators.
Semiconductors and insulators owe their low conductivity to the properties of the valence band in those materials. It just so happens that the number of electrons is precisely equal to the number of states available up to the top of the valence band. There are no available states in the bandgap. This means that when an electric field is applied, the electrons cannot increase their energy (i.e. accelerate) because there are no states available to the electrons where they would be moving faster than they are already going.
There is some conductivity in insulators, however. This is due to thermal excitation - some of the electrons get enough energy to jump the bandgap 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 which allows electrons in the valence band some degree of freedom.
It is a common misconception to refer to electrons in insulators as "bound" - as if they were somehow attached to the nucleus and couldn't move. Electrons in insulators are quite free to move - in fact they move at a speed on the order of 100 km (60 mi) per second! They are also delocalised, having no well defined position within the sample. It's just that for every electron moving left, there is another moving right - all the speeds cancel out precisely, leaving no overall current.
See also
Semiconductor for a full explanation of the band structure of materials.
Electrical conduction for more information about conduction in solids, and another description of band structure.
In solid state physics and related applied fields, the band gap is the energy difference between the top of the valenceband and the bottom of the conduction band in insulators and semiconductors.
The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap, which is defined as the energy level difference between the conduction band and the valenceband.
Band gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs.
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