Negative differential resistance devices, based on two similar AlAs/GaAs superlattice structures, were fabricated using a standard selective etching and integral heat sink technology and evaluated in a resonant-cap full-height WR-15 waveguide cavity. Devices from both superlattice structures generated comparable output powers in the fundamental mode at 59–71 GHz. The best devices yielded >80 mW (with corresponding dc-to-RF conversion efficiencies of >4.5%) around 63 GHz. In a second-harmonic mode, these devices yielded up to 14 mW (1%) at 133 GHz. Higher harmonic frequencies were also observed with powers of >0.4 mW at 190 GHz and >0.1 mW at 260 GHz. The highest observed dc-to-RF conversion efficiency was 5.1% at 62.8 GHz.