Negative differential resistance devices were fabricated from two epitaxial wafers with very similar GaAs/AlAs super-lattices and evaluated in resonant-cap full-height waveguide cavities. These devices yielded output powers in the fun-damental mode between 105 GHz and 175 GHz, with 14 mW generated at 127.1 GHz and 9.2 mW at 133.2 GHz. The output power of 4.2 mW recorded at 145.3 GHz constitutes a 50-fold improvement over previous results in the funda-mental mode. The highest confirmed fundamental-mode oscillation frequency was 175.1 GHz. In a second-harmonic mode, the best devices yielded 0.92 mW at 249.6 GHz, 0.7 mW at 253.4 GHz, 0.61 mW at 272.0 GHz, and 0.54 mW at 280.7 GHz. These powers exceed those extracted previously from higher harmonic modes by orders of magnitude. The power of 0.92 mW constitutes an improvement by 77% around 250 GHz. The second-harmonic frequency of 317.4 GHz is the highest to date for superlattice electronic devices and is an increase by 25% over previous results
