In a semiconductor diode, there are plentiful electrons on the n-side and plentiful holes
on the p-side. They are separated by a potential hill, so that only a few electrons
(boltzmann factor) can climb it. This 'forward current' is cancelled by a few electrons
dribbling down the hill under no bias.
The hill is lowered by forward bias, whereupon electrons run up the hill in numbers,
(driven by thermal energy; diffusion) recombine with holes and give forward current. In
silicon, the energy of recombination goes into the lattice because momentum
conservation cannot be achieved in this type of material when a photon is emitted.
But the band structure in 'direct gap' semiconductors like GaAs is such that when the
electron recombines with a hole a photon may be emitted directly and still conserve
linear momentum, since the maximum of the valence band sits underneath the minimum
of the conduction and in momentum space.
--
Mike
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Mike Moloney moloney@nextwork.rose-hulman.edu
Dept of Physics & Applied Optics (812) 877 8302
Rose-Hulman Institute of Technology Terre Haute, IN 47803 http://www.rose-hulman.edu/~moloney/index.html