Sawtooth doping superlattices are composed of alternating n-type Si and p-type Be -doping layers with undoped GaAs layers (715 nm) between them. This doping profile leads to sawtooth-shaped conduction and valence bands in the growth direction. To investigate the photocurrent in these structures we made selective n- and p-type contacts to the sample, enabling us to measure the interesting behavior of the subband gap compared with the gap of the host semiconductor. The photocurrent is finite even when the photon energy is below the gap of GaAs and can be varied by applying a bias voltage between the n- and p-type contacts. We proposed a model based on the Franz-Keldysh theory of field-induced optical absorption which takes into account the finite voltage drop inside the sample and treats the internal field self-consistently. The relaxation process is investigated by considering the decay of the photocurrent after switching off the illumination. Our theory is in quite good agreement with the experimental results. © 1988 The American Physical Society.