We have analyzed the effect of penetration of the electric field into the semiconductor on the electrical and mechanical characteristics of comb-drive actuators. In this study we found two specific effects due to the depletion layer: (1) the electro-mechanical conversion factor is a function of the depletion layer and becomes smaller than that for the “ideal conductor” assumption, and (2) an additional stiffness appears in the mechanical system. Following the change in the electro-mechanical conversion factor, the resonance peak becomes lower than that of an ideal conductor. These effects are relatively small when the gap between the comb-fingers and the substrate is of the order of a micrometer. However if the gap is 0.1μm, the electro-mechanical conversion factor decreases by about 2.8%. In this analysis, we have taken only the depletion effect into account; however, for a more complete analysis we should also consider the surface traps induced by unpassivated surface bonds.