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Burstein–Moss effect
The Burstein–Moss effect is the phenomenon of which the apparent band gap of a semiconductor is increased as the absorption edge is pushed to higher energies as a result of all states close to the conduction band being populated. This is observed for a degenerate electron distribution such as that found in some Degenerate semiconductors and is known as a Burstein–Moss shift.
The effect occurs when the electron carrier concentration exceeds the conduction band edge density of states, which corresponds to degenerate doping in semiconductors. In nominally doped semiconductors, the Fermi level lies between the conduction and valence bands. As the doping concentration is increased, electrons populate states within the conduction band which pushes the Fermi level higher in energy and in the case of degenerate level of doping, the Fermi level lies inside the conduction band. The "apparent" band gap of a semiconductor can be measured using transmission/reflection spectroscopy. In the case of a degenerate semiconductor, an electron from the top of the valence band can only be excited into conduction band above the Fermi level (which now lies in conduction band) since all the states below the Fermi level are occupied states. Pauli's exclusion principle forbids excitation into these occupied states. Thus we observe an increase in the apparent band gap. Apparent band gap = Actual band gap + Moss-Burstein shift (as shown in the figure).
Also we have negative Burstein shifts which are due to the interactions terms created by adding the extra charges through doping.[2]
References
Marius Grundmann (2006). The Physics of Semiconductors. Springer Berlin Heidelberg New York: Springer. ISBN 978-3-540-25370-9.
2- John.C Inkson (1984). "ch. 9.5, page 210". Many-Body Theory of Solids. ISBN 0-306-41326-4.
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