Interband scattering effects on secondary ionization coefficients in GaAs

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The ionization rates of holes and electrons in GaAs were measured experimentally over a wide doping range covering field values from 2.2×10 5 V/cm to 4.7×10 5 V/cm. As opposed to most experimemental measurements in GaAs, no assumption of equal ionization rates of the two carriers has been made. By using the conventional theoretical relationship between carrier ionization coefficients and multiplication data, the effective α is observed to be larger than β in lightly doped diodes while β is larger than α in heavily doped diodes. Previous theories of ionization rates utilizing just the normal conduction and valence bands do not show any possibility of such a crossover. It is suggested that electron transitions to higher conduction bands, which effectively increase the equilibration time of the electron distribution function, offer a resolution of this difficulty. The dependence of the effective electron ionization rate on doping can be explained by the requirement that electrons must make an interband transition before reaching the ionization threshold energy. This interband transition time estimated by this experiment is of the order of 10 −13 sec and is comparable to the transit time of electrons in the avalanching region. The breakdown voltages extrapolated from the measured α and β are consistent with those observed experimentally.