Abstract The decay of photoinduced sub-bandgap absorption (PA) following pulsed excitation was studied as a function of cw bias illumination. Two temperature regimes were identified. At low temperature the dispersion parameter increases linearly with the steady state carrier population generated by the bias light. At high temperatures saturation of the change in the dispersion parameter is achieved. The low temperature result is explained by a bias controlled tunneling process while the high temperature results are explained by the multiple trapping process with the tail states under the saturation condition.