We report a combined theoretical and experimental study of electricaltransport in weakly-coupled doped superlattices. Our calculations exhibitnegative differential conductivity at sufficiently high electric fields for alldopings. In low-doped samples the presence of impurity bands modifies thecurrent-voltage characteristics substantially and we find two different currentpeaks whose relative height is changing with the electron temperature. Thesefindings can explain the observation of different peaks in the current-voltagecharacteristics with and without external THz irradiation in low-doped samples. From our microscopic transport model we obtain quantitative agreement withthe experimental current-voltage characteristics without using any fittingparameters. Both our experimental data and our theory show that absolutenegative conductance persists over a wide range of frequencies of thefree-electron laser source.
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