Linear models for high-frequency transistors (bipolar and MESFET) at different biasing conditions are described. The model for a bipolar (MESFET) transistor shows good agreement between the measured and simulatedY(z) parameters. A discrete high frequency transistor is represented as a combination of two transistors and the bias dependence of the parameters of both intrinsic and extrinsic transistors are determined. A fully-automated equivalent-circuit parameter extraction program to fit the measured two-port MESFET data optimally up to 18 GHz is developed.