= Indirect band gap
An <indirect band gap> separates a <valence band> maximum and a <conduction band> minimum at different values of <crystal momentum>. A near-edge radiative transition then usually needs a <phonon> or another momentum-transfer process in addition to the <photon>, whose momentum is small on the <Brillouin zone> scale. A <direct band gap> has the extrema at the same <crystal momentum> and more readily supports efficient band-edge light emission. A material can have direct transitions at higher energy while its smallest gap is indirect.
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