Interdigitated GaAs metalsemiconductormetal Schottky photodiodes have been studied experimentally and theoretically. The time evolution of the response current has been measured by means of photoconductive and electrooptic sampling with a time resolution of 0. 8 and 0. 3 ps respectively. The response current to a 70 fs laser pulse reaches maximum within 25 p5 then shows a fast decay of about 10 ps followed by a slower one. Selfconsistent twodimensional Monte Carlo particle simulation predicts that the former is due to electrons the latter to holes. With a sufficiently strong electric field the two species of carriers get separated. With increased light intensity a screened plasma forms that vanishes only through recombination which takes of the order of nanoseconds.