Experimental and theoretical results are presented to show the difference between electron- and hole-initiated avalanches on the microwave properties on impatt oscillators under optical illumination. This difference, arising from unequal electron- and hole-ionisation rates, is demonstrated with X-band Si impatt structures suitable for microwave-optical interactions. A large signal impatt model is extended to incorporate the difference between hole and electron photocurrent, with the intrinsic response time of the avalanche depending on photocurrent composition. The model gives good agreement with experimental results of the power dependence upon photocurrent, although additional power saturation mechanisms need to be considered at higher power levels.