Indirect bandgap
In
semiconductor physics, an indirect bandgap is a
bandgap in which the minimum energy in the
conduction band is shifted by a k-vector. The k-vector difference represents a difference in momentum.
Semiconductors that have an indirect bandgap are inefficient at emitting
light. This is because any
electrons present in the conduction band quickly settle into the energy minimum of that band. Electrons in this minimum require some source of momentum allowing them to overcome the offset and fall into the
valence band.
Photons have very little momentum compared to this energy offset –.
Encyclopedia
In
semiconductor physics, an
indirect bandgap is a
bandgap in which the minimum energy in the
conduction band is shifted by a k-vector. The k-vector difference represents a difference in momentum.
Semiconductors that have an indirect bandgap are inefficient at emitting
light. This is because any
electrons present in the conduction band quickly settle into the energy minimum of that band. Electrons in this minimum require some source of momentum allowing them to overcome the offset and fall into the
valence band.
Photons have very little momentum compared to this energy offset –. The momentum "kick" of a photon being emitted or absorbed is negligible and direct transitions are essentially 'vertical' in k-space.
Since the electron cannot rejoin the valence band by
radiative recombination, conduction band electrons typically last quite some time before recombining through less efficient means.
Silicon is an indirect bandgap semiconductor, and hence is not generally useful for
light-emitting diodes or
laser diodes.
Likewise the absorption of light at an indirect gap is much weaker than at a direct one. As in the emission process both the laws of conservation of energy
and of momentum must be observed, the only way to promote an electron from the top of the valence band to the bottom of the conduction band is to simultaneously emit a
phonon that compensates for the missing momentum vector. However, such a combined transition has a much lower probablility. This means, for example, that silicon is at a disadvantage as a potential solar material compared to a direct gap material like CuInSe
2.
The absorption of an indirect bandgap material usually depends more on temperature than that of a direct material, because at low temperatures phonons are not available for a combined process. Silicon e.g. starts to transmit red light at these temperatures, because red photons do not have sufficient energy for a direct process.
In some materials with an indirect gap the value of the gap is negative, i.e. the top of the valence band is higher than the bottom of the conduction band in energy. Such materials are known as
semimetals.
External links
- at Electrical and Computer Engineering Department of University of Colorado at Boulder
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