A computer-aided design and characterisation technique is presented which uses comprehensive physical device models to represent the active devices in monolithic microwave integrated circuits (MMICs). Time-domain device and circuit models are used to analyse and design monolithic circuit elements and circuits. This approach is particularly useful for the investigation of large-signal (nonlinear) circuits, and allows the optimal design of microwave oscillators, large-signal amplifiers and other monolithic devices. In particular this technique has been used to predict successfully the frequency, power, stability and noise properties of MMIC oscillators. This method also allows the optimum geometry of the active devices to be obtained for specific applications.