A back-illuminated mesa-structure InGaAs/InP charge-compensated uni-traveling-carrier (UTC) photodiode (PD) is fabricated, and its saturation characteristics are investigated. The responsivity of the 40-\mu m-diameter PD is as high as 0.83 A/W, and the direct current (DC) saturation current is up to 275 mA. The 1-dB compression point at the 3-dB cutoff frequency of 9 GHz is measured to be 100 mA, corresponding to an output radio frequency (RF) power of up to 20.1 dBm. According to the calculated electric field distributions in the depleted region under both DC and alternating current (AC) conditions, the saturation of the UTC-PD is caused by complete field screening at high optical injection levels.