We present a theory of steadyhyphen;state impact ionization in GaAs including a realistic band structure and the electronhyphen;phonon interaction using a fieldhyphen;theoretic scheme that goes beyond first order processes. The theory explains most known experimental highhyphen;field transport data for GaAs satisfactorily, except for the anisotropy of the impact ionization rate measured by Pearsall. It is concluded that this anisotropy is not connected to quantum effects influencing the scattering rate and is probably caused by transient phenomena at the metallurgical junction of the samples used for investigating the anisotropy of the ionization rate.
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