State-of-the-art AlGaN/GaN high electron mobility structures were grown on semi-insulating 4H-SiC substrates by MOCVD and X-band microwave power high electron mobility transistors were fabricated and characterized. Hall mobility of 2291.1 cm²/(V·s) and two-dimensional electron gas density of 9.954 × 10¹² cm^–2 were achieved at 300 K. The HEMT devices with a 0.45-μm gate length exhibited maximum drain current density as high as 1039.6 mA/mm and peak extrinsic transconductance of 229.7 mS/mm. The f_T of 30.89 GHz and f_max of 38.71 GHz were measured on the device. Load-pull measurements were performed and analyzed under (–3.5, 28) V, (–3.5, 34) V and (–3.5, 40) V gate/drain direct current bias in class-AB, respectively. The uncooled device showed high linear power gain of 17.04 dB and high power-added efficiency of 50.56% at 8 GHz when drain biased at (–3.5, 28) V. In addition, when drain biased at (–3.5, 40) V, the device exhibited a saturation output power density up to 6.21 W/mm at 8 GHz, with a power gain of 11.94 dB and a power-added efficiency of 39.56%. Furthermore, the low f_max/f_T ratio and the variation of the power sweep of the device at 8 GHz with drain bias voltage were analyzed.