An interdigitated photoconductor (two terminal device) on GaA1As/GaAs heterostructure was fabricated and tested by an electro-optical sampling technique. Further, the photoresponse of GaA1As/GaAs HEMT (three terminal device) was obtained by illuminating the device with an optical signal modulated up to 8 GHz. Gain-bandwidth product, response time and noise properties of photoconductor and HEMT devices were obtained. Monolithic integration of these photodectors with GaAs microwave devices for optically controlled phased array antenna applications is discussed.