Using our in-house 0.3 ¬Ķm mushroom gate process; AlGaN/GaN high electron mobility transistors (HEMTs) with total gate periphery up to 0.6 mm were fabricated and characterized. The transistors were processed on an AlGaN/GaN heterostructure grown by MBE on sapphire. Output current densities up to 1 A/mm and extrinsic DC-transconductances (gm) of 240 mS/mm were measured. Extrinsic cut-off frequencies (ft) of 35 GHz; maximum frequencies of oscillation (fmax) of 75 GHz; were calculated from S-parameters measurements. On wafer Load-Pull measurements were performed without any active cooling on HEMTs of different sizes. Continuous wave (CW) output power densities up to 3.2 W/mm at 6 dB compression and 1.9 W/mm at 3 dB compression at 6 GHz were achieved.