Results on GaAs planar photoconductive detectors, characterized by high speed and moderate gain, have been successively reported by many workers /1 to 6/. Under cw illumination these devices ex\hibit large gains, in excess of 10 , strongly depending on both light intensity and temperature. On the other hand, their dynamic gain decreases when the frequency of the illumination intensity modulation is increased. The mechanisms already proposed to explain this behavior overlook at least two not negligible factors. Precisely, for the behavior of the cw illumination gain only the spatial separation of the excess carriers by the surface potential barrier has been considered /3/ while the contribution of the active layer-substrate potential barrier has been neglected. Also, for the dynamic gain a simple model of generation-recombination has been proposed governed by a Poisson law /4/* which does not include the effect of minority carrier trapping. The aim of the present work is to take into account these parameters and investigate the contribution of each one to the device gain. The device dynamic gain G(w) at an angular frequency w , defined as the ratio of the number of electrons collected in the load to the number of incident photons on the active layer, can be expressed as /6/ 3 where% is the majority carrier effective lifetime and t the corresponding transit time through the device.