Simulation experiments on Si, InP and wide band gap 4H-SiC based single drift IMPATT diodes operating at Ka-band clearly show the advantage of the SiC material system. RF power of 200 W with 10-15% efficiency at Ka-band can be expected from the 4HsiC based diodes. The effect of photo-illumination on 4H-SiC IMPATT is investigated by studying the role of enhanced saturation current on the mm-wave performance of the device. Under photo-illumination, (i) the negative conductance and (ii) the negative resistance of the device decrease, while, the frequency of operation and the device quality factor shift upward. The study indicates SiC IMPATT is a promising opto-sensitive high power mm-wave source.