Recessed 0.15 µm gate-length AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated using inductively-coupled-plasma reactive ion etching (ICP-RIE) on sapphire substrate. These 0.15 µm gate-length devices exhibited maximum drain current density as high as 1.4 A/mm and peak extrinsic transconductance of 346 mS/mm. The threshold voltage was -4.1 V. A unity gain cut-off frequency (fT) of 80 GHz and maximum frequency of oscillation (fmax) of 73 GHz were measured on these devices. Pulsed I-(V) measurements did not show any significant dispersion. At 20 GHz, a continuous-wave (CW) output power density of 3.1 W/mm with power-added-efficiency (PAE) of 29.9% was obtained.