The effect of photogenerated predominant electron or hole components of the leakage current on the admittance and negative resistance properties of a 94 GHz p+nn+ indium phosphide IMPATT diode under optical illumination is presented in this paper. A computer modelling and simulation technique has been used to study the above effect. The results show that the photogenerated leakage current dominated by holes is more important than that dominated by electrons in modulating the millimetre wave properties of the device and shifting its oscillation frequency. The magnitudes of electron or hole ionization rates in the semiconductor have been found to be correlated with the above effect.