High performance AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy were fabricated. An integrated slant field-plate was used to suppress the DC-RF dispersion and enhance the breakdown. A recessed-gate structure was adopted to increase the cutoff frequency from 17.5 to 22 GHz. And a CF(4)-plasma treatment prior to gate metallization was found effective in reducing the gate leakage by one order of magnitude and improving the stability of the RF power operation. Treatment in a mixture of HF and HNO(3) acid was essential to ensure the effectiveness of the CF(4)-plasma treatment. Combination of these technologies yielded state-of-the-art MBE grown AlGaN/GaN HEMTs with stable high-power and high-efficiency operation. At 4-GHz frequency and 48-V bias, a CW power of 12 W/mm with (34% associated efficiency was achieved. (C) 2008 The Japan Society of Applied Physics.