Response times of ∼60 and 25 ps (FWHM), respectively, have been measured for photoconductive detectors fabricated in GaAs layers grown by molecular beam epitaxy on silicon substrates and silicon-on-sapphire substrates. Photoconductive detectors, which can be readily combined with GaAs logic devices such as MESFETs to provide high-speed optical to electrical conversion, could be used in optical interconnects that are integrated with Si circuits on monolithic GaAs/Si wafers. Transconductance values of 120 mS/mm have been obtained for MESFET's fabricated in GaAs layers grown on silicon-on-sapphire substrates.