Band gap
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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 spelled "bandgap".
Semiconductor band structure
See electrical conduction and semiconductor for a more detailed description of band structure.
An intrinsic (pure) semiconductor's conductivity is strongly dependent on the band gap. The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap, which is defined as the energy level difference between the conduction band and the valence band. From Fermi-Dirac statistics (to be precise the Boltzmann's approximation is actually used), the probability of these excitations occurring is proportional to:
- <math>e^\left(\frac{-E_g}{kT}\right)<math>
where:
- e is the exponential function
- Eg is the band gap energy
- k is Boltzmann's constant
- T is temperature
In many devices this intrinsic conductivity is undesirable, and larger band gap materials give better performance. In infrared photodiodes, a small band gap semiconductor is used to allow detection of low-energy photons.
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Band gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs. It is also possible to construct layered materials with alternating compositions by techniques like molecular beam epitaxy. These methods are exploited in the design of heterojunction bipolar transistors (HBTs) and laser diodes.
The distinction between semiconductors and insulators is a matter of convention. One approach is to consider semiconductors a type of insulator with a low band gap. Insulators with a higher band gap, usually greater than 3 eV, are not considered semiconductors and generally do not exhibit semiconductive behaviour under practical conditions. 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. It also depends on pressure. Bandgaps can be either direct or indirect bandgaps, depending on the band structure.