Download Lightwave Communications Technology: Photodetectors by W.T. Tsang (Eds.) PDF

By W.T. Tsang (Eds.)

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33, can be easily obtained when the coupling constant is independent of the initial and final states of the electron by integrating the Fermi golden rule equation over the final energies of the electron [Eq. (29)]. Thus, the scattering rates of Shichijo and Hess are derived from perturbation theory. The application of the golden rule in this high-scattering-rate region is questionable, as pointed out by Capasso et al. ( 1981a) and extensively discussed by Barker ( 1973, 1980). The reason for this is that the collision broadening of the initial and final states of the electron, implied by such high scattering rates is such that the assumptions behind the golden rule are no longer valid, and collisions can no longer be treated perturbatively.

This leads to a lowering and broadening of the absorption threshold below the band gap. Tunnel-impact ionization is very similar, with the difference that here the energy required for this field-assistedtransition is provided not by a photon but by a conduction-band electron with an energy below the ionization threshold (Fig. 1 1b). Subthreshold production of electron- hole pairs may be important in relatively low-gap materials with low effective masses. Kyuregyan ( 1976) has calculated the transition rate for this process and its effect on the ionization coefficientassuming parabolic bands.

If this is the case, one can then see that above threshold the ionization probability can increase extremely rapidly with energy and quickly exceed the phonon-scattering rate. This produces a dramatic drop of the magnitude of the distribution function above threshold. 1 eV or so). This is the reason for the assumption routinely made in the theory of ionization rates that carriers ionize as soon as they reach threshold and are therefore left with a negligible residual energy after impact ionization.

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