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A backside illuminated mesa-structure InGaAs/InP modified uni-traveling-carrier photodiode (MUTC-PD) with wide bandwidth and high saturation power is fabricated and investigated. The device structure is optimized to reduce the capacitance and resistance. For the 22-μm-diameter device, the maximum responsivity at 1.55 μm is 0.5 A/W, and the 3-dB cutoff frequency reaches up to 28 GHz. The output photocurrent at the 1-dB compression point is measured to be 54 mA at 25 GHz, with a corresponding output radio frequency (RF) power of up to 15.5 dBm. The saturation characteristics of the MUTC-PD are also verified by the electric field simulation, and electric field collapse is found to be the cause of the saturation phenomenon.