The current–voltage characteristics of n ^+-GaAs/ n ^0-GaAs/ N ^0-AlGaAs/ N ^+-AlGaAs/ n ^+-GaAs isotype heterostructures and n ^+-GaAs/ n ^0-GaAs/ n ^+-GaAs homostructures are studied. It is shown that, for a heterostructure under reverse bias providing the injection of electrons from n ^0-GaAs into N ^0-AlGaAs, the maximum operating voltage reaches a value of 48 V at a thickness of the N ^0-AlGaAs layer of 1 . 0 μm, and the current–voltage characteristic has no region of negative differential resistance. The operation of a homostructure is accompanied by a transition to the negative-differential-resistance region at a voltage of 10 V. Theoretical analysis in terms of the energy-balance model demonstrated that the reverse-biased isotype heterostructure has no negative-differential-resistance region because, in this case, the field domain does not collapse in contrast to what occurs in homostructures.