This paper presents an analytical model for depletion-mode m.o.s. transistors which is particularly suitable for c.a.d. applications, and improves the current state of the art by including the effect of the surface-mobility degradation induced by the transverse field. The model is used to fit experimental data obtained on long-channel devices, and turns out to give good results; the mean square error over the wholeID/VDSplane, for several values of the gate- and bulk-source voltages, is of the order of 1%