A nonlinear thermo-optic model for the characterization of electro-optic semiconductor probes used to simultaneously measure electric field and temperature is presented. Optical thermal self-heating of the semiconductor is shown to be the primary limiting mechanism for temperature dynamic range, electro-optic modulation power, temperature sensitivity, temperature contrast, and temperature invasiveness. Tradeoff considerations of these parameters as a function of optical wavelength and input optical power are discussed. Optical temporal defocusing is shown to minimize the effects of nonlinear absorption. Simulation results are compared with measurements.