= Solution
Two applications are \b[ultraviolet photodetection and ultraviolet/near-band-edge light emission]. The bulk edge corresponds approximately to
$$
\lambda_g=\frac{hc}{E_g}\simeq\frac{1240\ {\rm eV\,nm}}{3.2\ {\rm eV}}
\simeq388\ {\rm nm}.
$$
Above-gap absorption creates carriers that can be collected in a <photodetector>. Measure dark current and wavelength-dependent response to separate photocarriers from heating or leakage. An optically or electrically excited emitter instead uses <electron-hole recombination>. <quantum confinement> in sufficiently small dots can shift absorption and emission above the bulk <band gap>. Surface states can produce broad defect luminescence or nonradiative loss, so a direct bulk gap does not guarantee an efficient, narrow-band emitter.
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