When a single electron moves from the conduction band to the valence band,
it can release its energy as a photon.This photon can stimulate a second electron
to fall into the valence band, producing a second photon by stimulated emission.
In this way, if the current through the junction is great enough, a chain reaction of
stimulated emission events can occur and laser light can be generated. To bring
this about, opposite faces of the p-n junction crystal must be flat and parallel, so
that light can be reflected back and forth within the crystal. (Recall that in the
helium–neon laser of Fig. 40-20, a pair of mirrors served this purpose.) Thus, a
p-n junction can act as a junction laser, its light output being highly coherent and
much more sharply defined in wavelength than light from an LED.