The photodoping process in Al/AsS2/Ag tri-layer films has been studied through measurements of electrical impedance, photodoped-layer thickness, and photocurrent. Frequency dependence of the impedance suggests that the sample under photodoping can be approximated by an equivalent electrical circuit. A thickness of photodoped layers, which is estimated from the impedance, is in agreement with a geometrical thickness measured by an atomic force microscope for chemically etched samples. Under the photodoping (with zero bias voltages), a photocurrent remains constant at similar to 5 pA, and near the completion it increases to similar to 1 nA, which is followed with a gradual decrease. By applying a bias voltage between the top (semi-transparent Al) and bottom (Ag) electrodes, we can change a photodoping rate by an order. This rate change is attributable to the modulation of an effective electric field in the doped layer, which induces the motion of Ag ions. (C) 2010 Elsevier B.V. All rights reserved.