Extensive simulation experiments are carried out for the first time, to study the optical modulation of the high- frequency characteristics of III–V GaN-(gallium nitride) based top-mounted and flip-chip IMPact Avalanche Transit Time (IMPATT) oscillators at MM-wave window frequency (140.0 GHz). It is found that the un-illuminated GaN IMPATT is capable of delivering a RF power of 5.6 W with an efficiency of 23.5% at 145.0 GHz. Frequency up-chirping of 6.0 GHz and a degradation of RF power output by almost 15.0% are further observed in case of photo-illuminated FC IMPATT. The study reveals that compared to predominate electron photocurrent in top-mounted IMPATT, photo-generated leakage current dominated by hole in flip-chip IMPATT has more pronounced effect on the GaN-based device as regards the frequency chirping and decrease of negative conductance and total negative resistance per unit area of the device. The inequality in the magnitudes of electron and hole ionization rates in the wide band gap semiconductor has been found to be correlated with the above results. The study reveals that GaN IMPATT is a potential candidate for replacing conventional IMPATTs at high-frequency operation. These results are useful for practical realization of optically controlled GaN-based high-power IMPATTs for application in MM-wave communication systems.