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We report on a novel concept for THz-photomixers with strongly increased conversion efficiency. In the familiar THz photomixers, based on photoconductivity in "Low-temperature-grown-GaAs" (LT-GaAs) the performance depends critically on an extremely short lifetime of photo-generated charge carriers in LT-GaAs and is typically limited by the (very low) photoconductive gain. In our photomixer the gain does not depend on the recombination lifetime but only on the transit time and on the path length of the photo-generated electrons within suitably designed p-i-n _ nanostructures, which both can be optimized if the transport is (primarily) ballistic. In addition, impedance matching to the attached antenna can be achieved.