In solid state physics (and related applied fields), the band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is often spelt "bandgap".
See electrical conduction and semiconductor for a more detailed description of band structure.
The band gap of a semiconductor is important for a number of reasons. An intrinsic (pure) semiconductor's conductivity is strongly dependent on the band gap. This is because the only available carriers for conduction are the electrons which manage to get enough thermal energy to be excited from the valence band into the conduction band. From FermiDirac statistics, the probability of these excitations occurring is proportional to:
 exp is the exponential function
 E_{g} is the band gap energy
 k is Boltzmann's constant
 T is temperature
Band gaps  

Common materials at room temperature  

The difference between semiconductors and insulators is rather ambiguous. Indeed, according to one definition, a semiconductor is a type of insulator. In general, a material with a sufficiently large band gap will be an insulator. The figure of 3 eV is sometimes given. Mobility also plays a role in determining a material's informal classification.
Band gap decreases with increasing temperature, in a process related to thermal expansion. Bandgaps can be either direct or indirect bandgapss.
See also
List of electronics topics Electronics
 Condensed matter physics
 Direct bandgap
 Electrical conduction
 Electron hole
 Field effect transistor
 Indirect bandgap
 Photodiode
 Photoresistor
 Solid state physics
 Semiconductor
 Semiconductor devices
 Valence band