A sequence of AlGaAs, GaAs heterojunctions can be used to generate a series of potential steps that are just large enough to accelerate conduction-band electrons to energies above the impact-ionization threshold while valence-band holes do not obtain this much energy. The smaller difference in the valence-band heterojunction discontinuity and the higher scattering rate for holes than conduction-band electrons are shown to suppress the initiation of impact ionization by valence-band holes in this structure. The resulting transit-time device is predicted to be a relatively low-noise amplifier or oscillator that can be used in the microwave region and will be insensitive to small fluctuations in the power supply voltage.
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Barnes et al. (1987) studied this question.
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