Power AlGaN/GaN heterojunction field effect transistors (HJFETs) on thinned sapphire substrates are demonstrated with improved power capability. A 16 mm-wide unpassivated HJFET on a 50 Pm-thick sapphire substrate exhibited a record output power of 15.9 W (on sapphire) with 9.0 dB linear gain, and 29.1 % power-added efficiency (PAE) at 34 V drain bias. A 8mm-wide SiN passivated HJFET on a 50 Pm-thick sapphire substrate exhibited 11.9 W, 13.3 dB linear gain, and 49.7 % power-added efficiency (PAE) at 26 V drain bias. These results demonstrate that the thinning process of a substrate enables the use of low cost sapphire for high-power applications.