An analysis of silicon metal‐oxide‐semiconductor (MOS) transistor operation at liquid‐helium temperature is presented. More specifically, analytical models providing the drain current and transconductance characteristics in the linear and nonohmic regions are established in the presence of source‐drain series resistances. These models, which rely on a specific mobility law for very low temperature, enable a good description of MOS transistor operation to be obtained. They permit the understanding and the prediction of the effects of source‐drain series resistance both in the linear and nonlinear regions. Furthermore, they allow a suitable parameter extraction method to be developed for the liquid‐helium temperature range. In addition, peculiarities of the drain voltage dependence of the mobility at low longitudinal electric field are also pointed out and empirically accounted for in the modeling of the output characteristics.