Negative differential conductance (NDC) at 300 K inn+‐nn+‐GaAs/AlAs superlattice structures biased perpendicularly to the layers is demonstrated, and shown to be strongly enhanced at microwave frequencies close to the inverse transit time of electrons. The deduced electron velocities are in fair agreement with those independently determined in undoped superlattices where NDC was inhibited by the electric field nonuniformity. From the analysis of the experimental data, we show that NDC is a bulk superlattice effect, not related to ‘‘quantum defects,’’ e.g., enlarged barriers.