Quantum Theory of Free Carrier Absorption

The theory of free carrier absorption has been investigated to determine the adequacy of the current quantum theory at large photon energies, and the necessity of a quantum theory at small photon energies. It is found that even at appreciable photon energies virtual states in higher bands make a negligible contribution to the free carrier absorption. It is shown that the quantum theory becomes equivalent to the classical Boltzmann theory of free carrier absorption for photon energies that are small compared to the carrier energies, which is therefore a condition for the validity of the classical theory. The classical theory may, however, give reasonably good agreement with experiment over a much wider range of photon energies, since the inaccuracies involved in the classical theory of free carrier absorption tend to compensate each other. Hot electron effects on free carrier absorption are also discussed.