A modified InGaAs/InP one-sided junction photodiode (MOSJ-PD) is presented for the first time. The MOSJ-PD is proposed from the one-sided junction photodiode by inserting a cliff layer into the absorption layer. Compared to the modified uni-traveling carrier photodiode, the MOSJ-PD has the advantages of simpler epitaxial layer structure and lower junction capacitance. In the MOSJ-PD, the space charge effect at high light intensity can be suppressed. Thus, both 3-dB bandwidth and output current can be improved simultaneously. The performance characteristics of the MOSJ-PD, including energy band diagram, internal electric field, frequency response, photocurrent, and responsivity, are carefully studied.