The high‐speed microwave semiconductor switch, employing ann‐type germanium diode, has been further investigated to determine the ultimate switching speed. Data are presented which show that pulse rise and decay times are as fast as 3 m&mgr;sec. This limit is imposed by the capacitances and inductances of the equipment and is not intrinsic to the germanium diode. The ultimate rise and decay times are determined by relaxation processes in the germanium, and are expected to be faster than 1 m&mgr;sec.Two switching pulses of variable spacing in time were employed to show that there are no essential residual processes in the germanium following the application of a pulse. Since there is then no measured dead time of switching, the pulse‐repetition rate is limited by the rise times and decay times.Measurements have been made on a series of semiconductor switches using different concentrations of donors. As the concentration is decreased, avalanche breakdown occurs at higher negative voltages which allows switching of higher microwave powers. Successful operation has been obtained with microwave powers as high as one watt impinging upon the semiconductor switch. In general, rise and decay times tend to increase as donors decrease. Thus, rise times and decay times of 10 m&mgr;sec were obtained at one watt of microwave power.