Båndgap, i faststoff-fysikk, et spekter av energinivåer innenfor en gitt krystall som det er umulig for et elektron å ha. Generelt vil et materiale ha flere båndhull i hele båndstrukturen (kontinuumet av tillatte og forbudte elektronenerginivåer), med store båndhull mellom kjernebånd og gradvis smalere båndhull mellom høyere bånd inntil ikke mer oppstår. Fenomenet med båndgapet oppstår når to tilliggende tillatte bånd ikke er brede nok til å spenne over hele elektronområdeterginivået.
superledelse: Energihull
Som nevnt ovenfor, indikerer de termiske egenskapene til superledere at det er et gap i fordelingen av energinivået tilgjengelig
Fermi-båndgapet
I praksis er mest forskning fokusert på bare ett bestemt båndgap - det som omslutter Fermi-nivået (energinivået som elektroner eksisterer eller er under når et faststoff har absolutt null temperatur). Dette bestemte båndgapet er til stede i halvledere og isolatorer, og er dermed det eneste båndgapet som er relevant for diskusjonen om elektronikk og optoelektronikk (studiet av elektroniske apparater som interagerer med lys). Det er ikke til stede i metaller, hvor Fermi-nivået i stedet er lukket av et tillatt bånd. Derfor sies metaller å ikke ha noe båndgap, til tross for at de teknisk sett har båndhull lenger unna Fermi-nivået. I noen sammenhenger refererer begrepet båndgap til bredden på et materials båndgap, vanligvis rapportert i elektron volt (eV).
Materialklassifisering
Based on the absence or presence of a band gap and on band gap size, materials can be classified into metals, semiconductors, and insulators. Foremost, metals can be distinguished from semiconductors and insulators by their lack of a band gap. Semiconductors and insulators may be differentiated by the size of their band gaps, the former having narrower band gaps and the latter having wider band gaps. In some texts, 9 eV is designated as the cutoff band gap for being considered a semiconductor, though this is by no means universal.
Influence on conductivity
That metals are excellent conductors of electricity, insulators are poor conductors of electricity, and semiconductors are somewhere in-between is common knowledge. Lesser known, however, is that those properties are determined by the band gap in each of the different materials. In particular, metals have high electrical conductivity due to their lack of a band gap—with no band gap separating the valence band (normally occupied states) from the conduction band (normally unoccupied states; electrons in this band move freely through the material and are responsible for electrical conduction), a small fraction of electrons will always be in the conduction band (i.e., free). This results in a superior electrical conductivity in metals.
Insulators, on the other hand, owe their low electrical conductivity to wide band gaps separating the valence band from the conduction band. If their band gaps were narrower, it would be feasible for thermal excitations to raise electrons to the conduction band; however, they are simply too wide for this to occur appreciably. As a result, the conductivity of a good insulator can be as little as 24 orders of magnitude less than that of a good conductor.
Finally, semiconductors rank intermediate in electrical conductivity, because their narrow band gaps make it nontrivial, but not impossible, for electrons to be raised to the conduction band by way of thermal excitation. The result is conductivity in semiconductors that is about 4–16 orders of magnitude less than that of a good conductor.
Band gap tuning
The width of the band gaps in typical elemental and binary semiconductors are generally not optimized for specialized applications in electronics and optoelectronics. Thus it is often lucrative to tune, or engineer, the band gap of semiconductors. To that end, scientists have used techniques such as employing semiconductor heterojunctions and molecular beam epitaxy and, in doing so, unlocked the band gaps necessary to create heterojunction bipolar transistors, laser diodes, and solar cells.
