|MadSci Network: Physics|
Hello! Pumping a laser is usually done by a bright, non-coherent source that has a mix of photon energies in it. Those photons whose energy is greater than the bandgap can kick up electrons from valence to conduction band, and the electrons may end up anywhere within the conduction band, depending on how much more energy the kicking photon has than the bandgap energy. Once electrons are up in a conduction band, especially if "hot" (i.e. energy much above the lower band edge), they collide with the lattice. losing energy until they occupy the states at or very near the bottom of the conduction band.Almost all the transistions from conduction to valence band will occur from the band edge to band edge (requires the least energetic photon to be emitted).
Try the links in the MadSci Library for more information on Physics.