= Solution
At equilibrium, <electrons> initially diffuse from the n side and <holes> from the p side. Recombination leaves ionized donors and acceptors in a <depletion region>, producing an internal electric field and <band bending>. The <Fermi level> is constant across the equilibrium <p-n junction>. With p material on the left, both band edges lie higher on the p side, as in the left panel.
\Image[/past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2025/paper-2-gan-junction.png]
{title=Equilibrium and forward-biased direct-gap p-n junction energy bands, with injection and photon emission}
Forward bias, with the p side positive relative to n, reduces the injection barrier. <Electrons> and <holes> enter the junction region and can recombine radiatively, emitting <photons> with energy close to the <band gap>. Under bias, separate <electron> and <hole> <quasi-Fermi levels> replace a single equilibrium <Fermi level>.
Both <GaN> polymorphs have a <direct band gap>, so a conduction-band <electron> and valence-band <hole> can recombine without a large crystal-momentum change. <Silicon> and <germanium> have <indirect band gaps>, requiring <phonon> assistance for near-edge radiative recombination, which makes efficient band-edge light emission much harder. A direct gap enables the process; defects, carrier confinement and competing nonradiative channels still determine device efficiency.
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