= Solution
<forward bias> injects <electrons> and holes into a common active region. Their <electron-hole recombination> can emit a <photon> as an <electron> falls from a conduction-band state into an empty valence-band state, with \b[$h\nu$ approximately the <band gap> $E_g$] near the band edges. A <direct band gap> makes this radiative transition much more probable than an indirect gap requiring simultaneous <phonon> participation. Carrier confinement improves their overlap; <nonradiative recombination> and <photon> trapping reduce the external efficiency. This is spontaneous electroluminescence and does not require <population inversion>. The biased carriers have separate <quasi-Fermi levels>, rather than one global equilibrium <Fermi level>. The band-diagram downward arrow shows <photon> emission. As a concrete implementation, an InGaN/AlGaN double heterostructure was used for a bright blue junction LED.<Candela-class high-brightness InGaN⁄AlGaN double-heterostructure blue-light-emitting diodes by Shuji Nakamura and others>[1994, Applied Physics Letters 64, pp.1687–1689: double-heterostructure blue LED]{ref}
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