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This paper reports on a semi-empirical model of the mobility in the inversion layer of enhancement-type MOSFET's operated at low temperatures. The n-channel model is based on three different scattering mechanisms important at cryogenic temperatures--phonon, Coulomb, and surface roughness scattering. It is shown that the degradation of the mobility with the vertical field is accelerated at low temperatures and has a different functional form compared to that at the above room temperature. The p-channel model is the extension of a high-temperature model. The simple analytical expression presented here is suitable for use in a circuit simulation program like SPICE. The definition and the temperature dependence of the effective normal field are reexamined for both n- and p-channel devices.