An attempt is made to study the shift of the avalanche transit time (ATT) phase delay and the modulation of the high frequency properties of single drift indium phosphide ATT diodes at 94 and 140 GHz window frequencies due to optical illumination. The study is based on a numerical simulation of the negative specific resistance profiles and the conductance-susceptance characteristics of the diode for different hole and electron current multiplication factors. The results indicate that the predominant hole photocurrent has a more pronounced effect in the 94 GHz diode as compared to the 140 GHz diode as regards the shift of avalanche phase delay, frequency chirping, narrowing of the avalanche zone, and decrease of the total negative resistance per unit area of the device.