= Semiconductor bands and efficient light emission
A <semiconductor> has a filled <valence band> and an empty <conduction band> at zero <temperature> in the intrinsic idealisation, separated by a modest <band gap>. Thermal excitation or deliberately introduced impurities can create mobile <Electrons> and <electron holes>. Its conductivity is consequently very sensitive to <temperature> and doping. A <metal> has available electronic states at the occupied <energy> boundary, through a partly filled band or overlapping bands, and need not thermally excite carriers across a gap.
For a <direct band gap>, the conduction-band minimum and valence-band maximum occur at the same <crystal momentum>. An <Electron> can recombine with a hole near these extrema and emit a <photon> while conserving crystal momentum, since optical photon momentum is small on the lattice scale. For an <indirect band gap>, the extrema lie at different crystal momenta. A band-edge radiative transition then ordinarily needs a <phonon> as well, making it less probable and allowing nonradiative processes to compete more effectively.
<gallium arsenide> has a direct fundamental gap, while <silicon> has an indirect one. This makes efficient band-edge emission and hence a <light-emitting diode> easier to obtain from gallium arsenide. It does not mean <silicon> has no optical transitions or that every direct-gap sample emits efficiently: defects, surfaces and carrier confinement also matter.
Back to article page
