The technology of modeling a field-effect transistor in the CAD system COMSOL Multiphysics is considered. The possibilities of CAD, its methods of graphical construction of the model and methods of modeling the behavior of the model are being studied. The object of study is the MOS transistor, its scope, operation and a mathematical model that can be used in designing its operation. The Shikhman-Hodges model, input and output parameters are determined, the degree of its adequacy to a real transistor is set, the main parameters are determined, with the help of which it is possible to conduct a study of a field-effect transistor, its current-voltage characteristic. A transistor model is built in the mode of operation in the mode of small-signal amplifiers, replacing it with a linear four-port model, it is described when this model can be applied when simulating the operation of the device.