= Split-off valence band
<Spin-orbit coupling> separates the $j=1/2$ valence states from the upper $j=3/2$ manifold at the zone centre in common cubic <semiconductors>. If its maximum lies $\Delta_{\rm so}>0$ below the upper <valence band> edge, its thermal <electron hole> population is suppressed by $e^{-\Delta_{\rm so}/(k_BT)}$ apart from <effective density of states> factors. Neglect requires $k_BT\ll\Delta_{\rm so}$; it is not valid at every <temperature> or in every material.
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