Miniband conduction in undoped GaAs/AlAs superlattices (SLs) has been investigated through current-voltage measurements on n+-SL-n+ structures. From the comparison with simulations based on an effective medium approximation for the conduction through the superlattice, we directly obtain the field dependence of the electron velocity perpendicular to the layers. Our data show strong evidence of negative differential velocity in a 35.5/20 Å (well/barrier width) SL.
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Sibille et al. (1989) studied this question.