The dynamic properties of an InP p+pnn+ IMPATT diode at 0.5 terahertz are studied through a simulation experiment. The study indicates that the InP IMPATT may deliver 27.0 mW of RF power at 0.5 terahertz with an efficiency of 6.3%. However, the realistic consideration of series resistance has imitational effect on exploitable power level from the THz device. The effects of photo-illumination on the device are also investigated through a modified simulation technique. The study reveals that the optically induced leakage current degrades the high frequency properties but with an advantage of wider tuning range of 86 GHz. These studies reveal the potential of InP IMPATT in high-speed THz-optical switching tools. © 2007 IEEE.