A zero-bias photocurrent (ZBPC) is observed in Cr-doped semi-insulating (Si) GaAs bulk crystals under illumination in the subbandgap energy range at low temperatures. It is ascribed to photocarriers driven by local electric fields associated with internal potential barriers due to random defect density fluctuations in the sample. An interpretation of the ZBPC spectral behavior is proposed based on comparison with the conventional photocurrent spectrum. To our knowledge this is the first attempt to investigate the spectral behavior of the ZBPC in compensated semiconductors. The effect of the optical stimulated polarity changes of the ZBPC could be considered as an alternative approach to the analysis of photo-transport properties of Si semiconductor materials.