What is Band gap?
{{about|solid-state physics|voltage control circuitry in electronics|Bandgap voltage reference}} A '''band gap''', also called a '''bandgap''' or '''energy gap''', is an energy range in a [[solid]] where no [[electron]] states can exist. The term is used in [[Solid state|solid-state physics]] and [[chemistry]]. Band gaps can be found in [[Insulator (electricity)|insulators]] and [[semiconductor]]s. In graphs of the [[electronic band structure]] of solids, the band gap is the energy difference (in [[electron volt]]s) between the top of the [[valence band]] and the bottom of the [[conduction band]]. This is the same as the energy needed to free an [[Electron shell#Valence shells|outer shell]] [[Valence electron|electron]] from its orbit about the [[Atomic nucleus|nucleus]] to become a mobile [[charge carrier]]. The free electron is able to move freely within the solid material. So the band gap is a major factor determining the [[electrical conductivity]] of a solid. Substances with large band gaps are generally [[Insulator (electricity)|insulators]], those with smaller band gaps are [[semiconductor]]s. [[Electrical conductor|Conductors]] either have very small band gaps or no band gap if the energy levels of valence and conduction bands overlap. ==In semiconductor physics== [[File:Bandgap in semiconductor.svg|right|thumb|Semiconductor [[Electronic band structure|band structure]].]] Scientists use the band gap to predict if a solid will conduct [[electricity]]. Most electrons (called [[Valence electron|valence electrons]]) are attracted to the [[Nucleus (physics)|nucleus]] of just one atom. But if an electron has enough energy to fly away from its nearest nucleus, it can join in the flow of electric current across the many atoms that make up the solid. The electrons that are not tightly attached to just one nucleus are called the conduction band. In semiconductors and insulators, [[quantum mechanics]] shows that [[electron]]s are only found in some [[Electronic band structure|bands]] of energy. Electrons ar