Radiation sensing field effect transistors display a variety of nonlinear and saturating responses to ionizing radiation. A model involving the numerical solutions to the photoconductivity equations is presented which describes the response under a wide range of radiation doses and applied fields. The large, unexpected, response for zero bias demonstrates the importance of diffusion of the photocarriers. The simulations are consistent with the known transport parameters for SiO2, with the main unknowns being the concentration of electron and hole traps. A variety of data at different applied voltages and doses can be fit with the same transport parameters.