A novel concept for THz-photomixers based on semiconductur nanostructures with strongly increased conversion efficiency is presented. In THz-photomixers, typically based on photoconductivity in" Low-temperature-grown-GaAs"(LT-GaAs), the performance is typically limited by the (very low) photoconductive gain and depends critically on an extremely short lifetime of photo-generated charge carriers in LT-GaAs. In contrast to that devices, in our photomixer the gain does not depend on the recombination lifetime but only on the transit