The dynamic properties of a 4H-SiC DDR (p+ p n n+ type) IMPATT diode operating at 0.5 THz region are studied through DC and smallsignal analysis. The study indicates that 4H-SiC IMPATT is capable of generating high RF power (PRF) (2.70 W) at 0.515 terahertz with high efficiency (12 %). However, the parasitic series resistance is found to produce a 7 % reduction in the negative conductance and the PRF of the diode. The effect of photo-illumination on the device is also investigated by studying the role of enhanced saturation current on the THz frequency performance of this IMPATT device. A modified double iterative simulation technique developed by the authors is used for this purpose. It is found that (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 with increasing saturation current. The upward shift in operating frequency is found to be more (~ 65 GHz) when the device performance is controlled by the hole saturation current rather than by the electron dominated saturation current. These results thus indicate that 4H-SiC DDR IMPATT diode is highly photo-sensitive even at THz range of frequencies.