Discussion
An electron makes a transition from the valence band to the conduction band in an indirect band gap semiconductor. Which one of the following is true?
Answer: C
The true statement is a phonon is annihilated in the process.
In an indirect band gap semiconductor the bottom of the conduction band lies at a different crystal momentum from the top of the valence band. A photon carries almost no momentum, so the transition can only conserve momentum if a phonon takes part, either absorbed (annihilated) or emitted. Of the options, phonon absorption is the one that can occur.
The electron's energy increases, since it moves up into the conduction band, so 'energy decreases' is wrong. A photon is absorbed, not emitted or created, in a valence-to-conduction transition; emission belongs to the reverse process, recombination.
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