This contribution illustrates how a realistic nonlinear quasi-static model for FET-type devices can be extracted using an original frequency domain extraction technique. An ideal ‘made-up’ device is built from the measured bias dependence of a GaN medium power device. This ideal device is excited by two ideal voltage sources and its response (drain current) is used to illustrate how the extraction procedure can separate conduction and displacement current components provided the total current spectrum (or, alternatively, waveform) and control voltages are known.