Recently, the depletion-type IGFET became widely used as a load device in logic circuits. The purpose of this correspondence is to present a development of the current-voltage characteristic of a depletion-type IGFET based on the assumption that the non-linear semiconductor capacitance can be replaced by a constant average capacitance. This approximation is justified in light of the theoretical and experimental results obtained by many authors [1]-[4], indicating that the exact form of the semiconductor capacitance has little effect on the dc characteristics of a field-effect device.