Diamond possesses exceptional physical properties, such as a high breakdown field and carrier mobility. It is therefore expected to be highly efficient for high-power RF devices. We identify hole carrier doping in diamond using nitrogen dioxide (NO2). Furthermore, we find that an aluminum oxide (Al2O3) passivation layer greatly improves the thermal stability of the hole channel. These two technologies enable us to create thermally stable high-performance diamond field-effect transistors (FETs). The diamond FET shows a maximum IDS value of -1.35 A/mm, cut-off frequencies fT and fMAX of 35 GHz and 70 GHz, respectively, and an RF output power density of 2 W/mm at 1 GHz.