Valence and conduction bands

Filling of the electronic states in various types of materials atequilibrium.Here, height is energy while width is thedensity of available statesfor a certain energy in the material listed. The shade follows theFermi–Dirac distribution(black:all states filled,white:no state filled). InmetalsandsemimetalstheFermi levelE_Flies inside at least one band.

Ininsulatorsandsemiconductorsthe Fermi level is inside aband gap;however, in semiconductors the bands are near enough to the Fermi level to bethermally populatedwith electrons orholes."intrin." indicatesintrinsic semiconductors.

Insolid-state physics,thevalence bandandconduction bandare thebandsclosest to theFermi level,and thus determine theelectrical conductivityof the solid. Innonmetals,thevalenceband is the highest range ofelectronenergiesin which electrons are normally present atabsolute zerotemperature, while the conduction band is the lowest range of vacantelectronic states.On a graph of theelectronic band structureof 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.

Band gap

Main article:Band gap

In semiconductors and insulators the two bands are separated by aband gap,while inconductorsthe bands overlap. A band gap is an energy range in a solid where no electron states can exist due to thequantizationof energy. Within the concept of bands, the energy gap between the valence band and the conduction band is the band gap.Electrical conductivity of non-metals is determined by the susceptibility of electrons to be excited from the valence band to the conduction band.

Electrical conductivity

Semiconductor band structure
Seeelectrical conductionandsemiconductorfor a more detailed description of band structure.

In solids, the ability of electrons to act ascharge carriersdepends on the availability of vacant electronic states. This allows the electrons to increase their energy (i.e.,accelerate) when anelectric fieldis applied. Similarly, holes (empty states) in the almost filled valence band also allow for conductivity.

As such, theelectrical conductivityof 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_g), 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_gare calledsemiconductors.If the E_gis 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 theholethey left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom.

Band edge shifts of semiconductor nanoparticles

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 radiusof 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.

See also

• Electrical conductionfor more information about conduction in solids, and another description of band structure.
• Fermi sea
• HOMO/LUMO
• Semiconductorfor a full explanation of the band structure of materials.
• Valleytronics

References

General references

• Kittel, Charles (2005).Introduction to Solid State Physics.Wiley.ISBN0-471-41526-X.
• Kornic, Steve (11 April 1997)."The Valence Band".Chembio. Archived fromthe originalon 4 March 2021.Retrieved8 December2021.
• Nave, Carl R. (Rod) (1999)."Band Theory for Solids".Hyperphysics.Retrieved8 December2021.

External links

• Direct Band Gap Energy Calculator