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Insolid-state physics, thevalence band andconduction band are thebands closest to theFermi level, and thus determine theelectrical conductivity of the solid. Innonmetals, thevalence band is the highest range ofelectronenergies in which electrons are normally present atabsolute zero temperature, while the conduction band is the lowest range of vacantelectronic states. On a graph of theelectronic band structure of a semiconducting material, the valence band is located below the Fermi level, while the conduction band is located above it.
The distinction between the valence and conduction bands is meaningless in metals, because conduction occurs in one or more partially filled bands that take on the properties of both the valence and conduction bands.
In semiconductors and insulators the two bands are separated by aband gap, while inconductors the bands overlap. A band gap is an energy range in a solid where no electron states can exist due to thequantization of energy. Within the concept of bands, the energy gap between the valence band and the conduction band is the band gap.[1] Electrical conductivity of non-metals is determined by the susceptibility of electrons to be excited from the valence band to the conduction band.
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In solids, the ability of electrons to act ascharge carriers depends on the availability of vacant electronic states. This allows the electrons to increase their energy (i.e.,accelerate) when anelectric field is applied. Similarly, holes (empty states) in the almost filled valence band also allow for conductivity.
As such, theelectrical 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 (Eg), 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 Eg are calledsemiconductors. If the Eg is sufficiently high, then the flow of electrons from valence to conduction band becomes negligible under normal conditions; these groups are calledinsulators.
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 thehole they left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom.
The edge shifting of size-dependent conduction and/or valence band is a phenomenon being studied in the field ofsemiconductor nanocrystals. The radius limit of occurrence of the semiconductor nanocrystal is the effectiveBohr radius of the nanocrystal. The conduction and/or valence band edges shift to higher energy levels under this radius limit due to discrete optical transitions when semiconductor nanocrystal is restricted by the exciton. As a result of this edge shifting, the size of the conduction and/or valence band is decreased. This size-dependent edge shifting of conduction and/or valence band can provide plenty of useful information regarding the size or concentration of the semiconductor nanoparticles or band structures.[2]