A systematic experiment was designed and implemented to optimize the 0.25 pm gate InGaAs/AlGaAs pseudomorphic HEMT for the fabrication of high power X-band monolithic amplifiers. The material structure and gate recess process were engineered such that the device breakdown voltage was optimized without sacrificing gain and efficiency. A two-stage high power X-band monolithic amplifier based on the optimized device has been developed. When the amplifier was operated at vds = 10 V, an output power of 9 W was achieved across the 7 to 10 GHz frequency range. A peak saturated output power of 10 W, corresponding to a power density of 1 W/mm, occurred at 8.5 GHz. When biased at 7V, the amplifier generated a peak power of 6.7 W with an associated power added efficiency of 40% at 8.5 GHz.