Which of the following is/are direct band semiconductor/s ? (1) GaAs (2) Si (3) Ge (4) InP
(1) and (4) only
In a direct band-gap semiconductor, the conduction-band minimum and the valence-band maximum occur at the same crystal momentum k, so an electron can recombine with a hole by emitting a photon without needing a phonon.
(1) GaAs: direct band gap of about 1.42 eV at the Γ point — direct.
(2) Si: conduction-band minimum lies near the X point, away from the valence-band maximum at Γ — indirect (about 1.12 eV).
(3) Ge: conduction-band minimum lies at the L point — indirect (about 0.66 eV).
(4) InP: direct band gap of about 1.34 eV at Γ — direct; it is widely used in lasers and photodetectors.
Hence, the direct band semiconductors are GaAs and InP, i.e. (1) and (4) only.
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