Deeper researches on the impacts of a cliff layer on uni-traveling-carrier photodetector (UTC-PD) are presented, especially the influence on 3 dB bandwidth. The results show that a cliff layer enhances the intensity of the electric field which extends to the absorption region in the heterojunction, makes the electron transit time and electron accumulation level reduce in the absorber, and eventually leads to the increase in 3 dB bandwidth and saturation current. The results also reveal that, both the doping concentration and thickness of cliff layer have a significant impact on 3 dB bandwidth. For a thicker cliff layer, as the product of the thickness and doping concentration is unchanged and the collection region remains depleted, the 3 dB bandwidth drops with the increase of cliff layer thickness. However, for a thinner cliff layer, typically ten to several tens of nanometers, whether the cliff layer thickness or the doping concentration is alternated, the decreases of 3 dB bandwidth can be ignored, that is, the 3 dB bandwidth is almost invariant.