Metal-semiconductor-metal photodetectors with different submicron spacings (d= 100, 300, 500, 700 and 900 nm) were fabricated on GaAs with a carrier recombination time of similar to100 ps by electron beam lithography. Temporal responses of the detectors were measured by photoconductive sampling in order to identify factors which limits the response speeds. At a low excitation of <100 muW, the response speeds of 100, 300 and 500 nm spacing detectors are limited by parasitic capacitances of the submicron structures. The speeds of 700 and 900 nm spacing detectors are limited by an electron/hole transport in the semiconductor. At a high excitation of >100 muW, the response speeds of the all spacing detectors are limited by field screening caused by electron-hole plasma. (C) 2004 Elsevier Ltd. All rights reserved.