A physics-based model incorporating the UTC (uni-traveling carrier) photodiode (PD) in the limit of weak nonlinearity was used in order to characterize the effects of nonlinearity on high speed and high power photodiodes. The combined influences of a) optical illumination, b) photocurrent, and c) interaction of the photodiode with an external circuit, were incorporated into three equations which described the phasor dynamics of the photodiode, which could be used to approximate the diode voltage, the depletion region thickness, and the electric field at the beginning of the depletion region by the Newton-Raphson Method. Then a frequency response plot as well as a third-order intermodulation distortion (IMD3) plot were obtained in order to evaluate the effects of nonlinearity on the photodiode. The third-order intercept point (IP3) was determined to be approximately 27.5 dB, illustrating its slight nonlinearity. For both the frequency and the IMD3 plots, it was shown that modulation bandwidth is predominantly RC-limited and that the stated assumptions were true: that the average electron transit time through the depletion region is expected to be significantly smaller than the period of the optical stimulus. Finally, nonlinearity was minimized by compensating the heavy loading and space charge effects on junction capacitance, and a surface plot was obtained demonstrating this behavior.