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We report high-speed photoconductive switches based on low-temperature (LT) grown GaAs on Si substrate. Epitaxially grown LT GaAs was separated from its substrate, transferred on an SiO2-coated Si substrate and integrated with a transmission line. The 10×20-μm2 switches exhibit high breakdown voltage and low dark currents (<10/sup -7/ A at 100 V). The photoresponse at 810 nm shows electrical transients with /spl sim/0.55-ps full-width at half-maximum and /spl sim/0.37-ps decay time, both independent on the bias voltage up to the tested limit of 120 V. The photoresponse amplitude increases up to /spl sim/0.7 V with increased bias and the signal bandwith is /spl sim/500 GHz. The freestanding LT GaAs switches are best suited for ultrafast optoelectronic testing since they can be placed at virtually any point on the test circuit.