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Abstract--Collective impact ionization has been used to exT1ain lock-on in semi-insulating GaAs under highvohage bias. W7ehave used this theory to study some of the steady state properties of lock-on current filaments. In steady state, the heat gained from the field is exactly compensated by the cooling due to phonon scattering. In the simplest approximation, the carrier distribution approaches a quasi-equ~lbrium Ma.xwell-Boltzmann distribution. In this repom we examine the validity of this approximation. \Ye fmd that this approximation leads to a filament carrier density which is much lower than the high density needed to achieve a quasiequilibrium distribution. Further work on this subject is in progress.

Using a simple model, we have investigated the effect of deep level impact ionization on avalanche breakdown in GaAs, InP, and Si p±n junctions. The ionization coefficients are obtained using Robbins' formalism [phys. stat. sol. (b) 97, 9 (1980)]. Results are presented for trends in the dependence of the breakdown voltage and electric field on the doping densities and on the deep level density and energy. Our results show that, for fixed doping densities, the breakdown voltage and field both decrease with increasing deep level density. Further, we find that their sensitivity to the deep level density is much stronger for levels deep within the bandgap than for those near a band edge, correlating with the decreased ionization threshold energy for shallower levels. We also find that the breakdown voltage and critical field in Si p±n junctions are stronger functions of the deep level density and energy than in GaAs and InP junctions.

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