A new model of two species of electron–hole with shallow electron trap is presented. The model predicts that if the shallow trap accumulates large density of charge carriers, the photoconductivity can be a sublinear (or superlinear) function of light intensity, and the space-charge field increases (or decreases) and reaches a saturate value as intensity increases depending on the sign of dominant carrier. Comparison this model with previous ones shows that this model provides more common descriptions about photoconductivity and space-charge-field dependence of intensity. ©1997 American Institute of Physics.