A quasi-static scheme is used to study the dynamic properties and optimum operation conditions of the DD X-band Lo-Hi-Lo silicon IMPATT diodes. The carriers ionization rates in a form α = A exp s(— B/E) are assumed the scattering-limited velocities of the carriers and are taken at an operation temperature of 150°C, for the analysis of the diode. Large signal values of admittance, efficiency, power output and phase relationship between the avalanche current and the oscillation voltage are calculated as functions of the d.c. bias current, the r.f. voltago swing, and the operation frequency. Current-tuning effect is inferred to play an important role in the device design. Furthermore, efficiencies and power outputs of diodes are also calculated as functions of the avalanche-to-drift voltage ratio VA/VD. The best diode we propose is ŋ = 20.3% and P = 3.7 watts at 50% modulation near VA/VD = 0.88.