To simplify and improve Radio-over-Fiber (RoF) communication systems, a kind of modulation technology based on the variation of output photocurrent with bias of PIN photodiode (PIN-PD) and unitraveling-carrier-photodiode (UTC-PD) is proposed, which is called photodiode bias modulation. By using this modulation technology, both photodetection and optoelectronic modulation can be achieved simultaneously in one PD. Based on two analytical models, the modulation bandwidth of the PIN-PD is predicted as 800 MHz at a 10 GHz RF subcarrier, and the modulation bandwidth of the UTC-PD is predicted as 18.75 GHz at a 150 GHz RF subcarrier, when the incident optical power is both 2.93 dBm. The modulation bandwidth increases with the incident optical power, when the incident light power is 12.93 dBm, the modulation bandwidth of the UTC-PD can reach 25 GHz at a 150 GHz RF subcarrier. The modulation depth is associated with the minimum value of the sinusoidal bias modulation signal.