Efficiency of x-band SD and DD silicon quasi-Read diodes as a function of modulation level are calculated taking into account the power losses in the unswept layer. It is found that the diode conductivity is the dominating factor in determining the power losses when operated in the small- to intermediate-modulation level, although the power losses increase with increasing r.f. voltage swing due to modulation ascending as well as mobility diminishing in this range of modulation. At large modulation condition, when the diode residual field is raised to such an extent that carrier drift velocity reaches its scattering limited value, the rapidly growing residual field results in serious losses independent of the material mobility. As a result, the optimum-modulation level falls in the intermediate-modulation region for each case. The optimum-modulation level almost remains unchanged when the bias is tripled from Jdc = 500 to 1500 A/cm2 in most cases, while there are trade-offs in output power and efficiency when the bias current of the diode is increased. The best x-band DD SI quasi-Read diode we explored shows that the device is capable of an optimum efficiency ηm = 20.9% and output power Pm = 3.2 W at bias density Jdc = 500 A/cm2 and an optimum modulation index m = 0.47.