This letter presents a high-performance N-polar AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor grown by metal-organic chemical vapor deposition on sapphire substrate. The devices were passivated with SixNy deposited by plasma-enhanced CVD and consisted of a gate structure recessed through the SixNy passivation, with integrated slant field plates, to prevent dc to RF dispersion and improve the breakdown voltage. Devices with a drawn gate length of 0.7 μm and a gate drain spacing of 0.8 μm showed a breakdown voltage of 170 V, corresponding to a three-terminal leakage current of 1 mA/mm. Due to the high breakdown voltage of the devices, continuous-wave RF power measurements at 4 GHz could be measured at a drain bias of 50 V, yielding an output power density of 12.1 W/mm. To the best of our knowledge, this is the highest power density reported so far for an N-polar device and also matches the highest power density reported for a Ga-polar HEMT on the sapphire substrate.