Generation of high-frequency radiation in semiconductor superlattices   with suppressed space-charge instabilities
  We theoretically investigated the scheme allowing to avoid destructive space-charge instabilities and to obtain a strong gain at microwave and THz frequencies in semiconductor superlattice devices. Superlattice is subjected to a microwave field and a generation is achieved at some odd harmonics of the pump frequency. Gain arises because of parametric amplification seeded by harmonic generation. Negative differential conductance (NDC) is not a necessary condition for the generation. For the mode of operation with NDC, a limited space-charge accumulation does not sufficiently reduce the gain. 
Kirill N. Alekseev, Natalia V. Demarina and Maxim V. Gorkunov