Valence and conduction bands

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Filling of the electronic Density of states in various types of materials at equilibrium. Here the vertical axis is energy while the horizontal axis is the Density of states for a particular band in the material listed. In metals and semimetals the Fermi level EF lies inside at least one band. In insulators and semiconductors the Fermi level is inside a band gap; however, in semiconductors the bands are near enough to the Fermi level to be thermally populated with electrons or holes.

In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level and thus determine the electrical conductivity of the solid. 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 as the highest band is partially filled, taking on the properties of both the valence and conduction bands.

Band gap[edit]

Main article: band gap

In semiconductors and insulators the two bands are separated by a band gap, while in semimetals the bands overlap. A band gap is an energy range in a solid where no electron states can exist due to the quantization of energy in quantum mechanics. Electrical conductivity of non-metals is determined by the susceptibility of electrons to excitation from the valence band to conduction band.

Electrical conductivity[edit]

Semiconductor band structure (lots of bands 2).svg
Semiconductor band structure
See electrical conduction and semiconductor for a more detailed description of band structure.

In solids, the ability of electrons to act as charge carriers depends on availability of vacant electronic states. This allows the electrons to increase their energy (i.e., accelerate) when an electric field is applied. This condition is only satisfied in the conduction band as the valence band is full in non-metals.

As such, the electrical conductivity of a solid depends on its capability to flow electrons from valence band to conduction band. Hence in case of a semimetal with an overlap region the electrical conductivity is high. If there is a small band gap then the flow of electron from valence to conduction band is only possible if an external energy (thermal etc.) is supplied and these groups with small Eg are called semiconductors. If the Eg is sufficiently high then flow of electron from valence to conduction band become negligible under normal conditions, 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 some degree of freedom.

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