The magneto-photoconductivity due to 1s-2p+ optical transitions of shallow donors in n-GaAs has been investigated as a function of intensity for several bias voltages at low temperatures between 2K and 4.2 K. At low intensities a superlinear increase of the photoconductive signal with rising intensity has been observed which gets more pronounced at higher bias voltages and lower temperatures. The power broadening of the linewidth was found to be distinctly different from the behaviour expected for a two-level system. By a detailed analysis in terms of a nonlinear generation-recombination model it is shown that these effects may be attributed to impact ionization of the optically excited 2p+ states.