This paper presents the simulation results of DC, small-signal and noise properties of GaP based DDR IMPATT diodes. In simulation study we have considered the flat DDR structures of IMPATT diode based on GaP, GaAs, Si and GaN(wz) material. The diodes are designed to operate at the millimeter window frequencies of 94 GHz and 220 GHz. The simulation results of these diodes reveal GaP is a promising material for IMPATT applications based on DDR structure with high break down voltage (V_B) as compared to Si and GaAs IMPATTs. It is also encouraging to worth note GaP base IMPATT diode shows a better output power density of 4.9×10⁹ W/m² as compared to Si and GaAs based IMPATT diode. But IMPATT diode based on GaN(wz) displays large values of break down voltage, efficiency and power density as compared to Si, GaAs and GaP IMPATTs.