Transient Approach to Current Saturation in Photoconductive CdS

Abstract

Current saturation and the accompanying current oscillation in CdS are studied by measuring the spatial electric-field distribution as the current decays from its initial Ohmic value to its final saturated value. The initial saturation of the current and subsequent damped oscillation are found to be caused by the presence of moving high-field domains which propagate at the acoustic-wave velocity. The detailed behavior of these domains indicates that they are packets of high acoustic-energy density. As the oscillations damp and the current saturates, the moving domains apparently give way to a competing process. In the current-saturated condition, only a stationary high-field region remains. The field distribution in CdS crystals having the drift-current parallel to the hexagonal axis is also explored in some detail. The results indicate that in these samples the current saturation is caused by the amplification of shear waves which propagate predominantly at an angle of ∼30° with respect to the hexagonal axis.