= Radiative electron-hole recombination
{title2=$R_{\rm net}=B(np-n_i^2)$}
In a <semiconductor>, a conduction <electron> fills a vacant valence state and emits a <photon>. In a simple nondegenerate model the recombination rate is $Bnp$, while the equilibrium reverse generation rate is $Bn_i^2$; their difference gives the displayed net pair-loss rate. This subtraction enforces <detailed balance> at thermal equilibrium. Photon momentum is small on the scale of a <Brillouin zone>, so a <direct band gap> permits a direct transition while an <indirect band gap> generally requires a <phonon> or another source of momentum.
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