An attempt is made in this paper to explore the potentiality of semiconducting type-IIb diamond as the base material of double-drift region(DDR) impact avalanche transit time(IMPATT) devices operating at both millimetre-wave(mm-wave) and terahertz(THz) frequencies. A rigorous large-signal(L-S) simulation based on the non-sinusoidal voltage excitation(NSVE) model developed earlier by the authors is used in this study. At first,a simulation study based on avalanche response time reveals that the upper cut-off frequency for DDR diamond IMPATTs is 1.5 THz, while the same for conventional DDR Si IMPATTs is much smaller, i.e. 0.5 THz. The L-S simulationresultsshowthattheDDRdiamondIMPATTdevicedeliversapeakRFpowerof7.79Wwithan18.17%conversion efficiency at 94 GHz; while at 1.5 THz, the peak power output and conversion efficiency decrease to6.19mWand8.17%respectively,taking50%voltagemodulation.AcomparativestudyofDDRIMPATTsbasedon diamond and Si shows that the former excels over the later as regards high frequency and high power performance at both mm-wave and THz frequency bands. The effect of band to band tunneling on the L-S properties of DDR diamond and Si IMPATTs has also been studied at different mm-wave and THz frequencies. An attempt is made in this paper to explore the potentiality of semiconducting type-IIb diamond as the base material of double-drift region (DDR) impact avalanche transit time (IMPATT) devices operating at both millimetre-wave (mm-wave) and terahertz (THz) frequencies. A rigorous large-signal (LS) simulation based on the non-sinusoidal voltage excitation (NSVE) model developed earlier by the authors is used in this study. At first, a simulation study based on avalanche response time reveals that the upper cut-off frequency for DDR diamond IMPATTs is 1.5 THz, while the same for conventional DDR Si IMPATTs is much smaller, ie 0.5 THz. The LS simulation results show that the DRD diamond IMIM ATT service deltaiversak RF Power of 7.79 Withan 18.17% conversion efficiency at 94 GHz; while at 1.5 THz, the peak power output and conversion efficiency decrease to 6.19 mW and 8.17% respectively, taking 50% voltage modulation. A Comparative Study of DDRIMPATTs based on diamond and Si shows that the former excels over the later as reg The effect of band to band tunneling on the L-S properties of DDR diamond and Si IMPATTs has also been at at different mm-wave and THz frequencies.