Silicon unique modulation mechanism based on free-carrier dispersion (FCD) effect determines that there is operation and performance difference from LiNbO(3) modulator when achieving various optical modulation formats. In this paper, the influence of nonlinear FCD and free carrier absorption (FCA) effect on the return-to-zero (RZ)-DPSK generation scheme is numerically analyzed. Silicon waveguide with p-n diode is adopted and the reverse bias is the key factor which should be chosen carefully. Performance analysis includes two parts: the property of the generated optical signal and the dispersion penalty which is related to chirp. The simulation results show that the output phase of the optical RZ-DPSK signal has undesirable distortion and the power has considerable loss. Furthermore, the simulation of modulator with 20 dB extinction ratio is also performed for relative analysis. The poor extinction ratio will further impact the characteristic. Even the push-pull operation is utilized, there is a residual chirp resulting from FCA and nonlinear FCD effect. This kind of chirp is characterized by the dispersion penalty.