The operation and FM noise of millimeter‐wave quantum well (QW) oscillators in a harmonic mode (relaxation oscillations) and a fundamental mode using post and cap waveguide mount structures are investigated. Harmonics up to the ninth harmonic was detected. State of the art results for QW oscillators registered were 84 μW (at 77 K physical temperature) and 60 μW at 90 GHz and 1 μW at 176 GHz (at 300 K physical temperature, respectively).