In this study, an S-band 300-W internally matched power amplifier was designed and fabricated using 80×350 μm GaN HEMTs, which were developed by the 0.4-μm GaN HEMT process(Wavice). A 4-inch GaN-on-SiC wafer was used in the 0.4-μm GaN HEMT process, and the developed GaN HEMT includes a field plate and backside via holes. An internally matched power amplifier was designed using the optimum input/output impedances, which were extracted from a hybrid load-pull measurement. The fabricated power amplifier was measured at 2.9~3.5 GHz in the pulsed mode with 15 % duty cycle, which included a 300-μs pulse width and 2-ms pulse period, and showed an output power of 295~336 W, power-added efficiency of 46.9~58.5 %, and power gain of 9.7~10.2 dB at the input power of 45 dBm.