Due to the demand for higher switching frequencies in switched-mode power supplies a new method to model and characterize power semiconductors under load conditions towards higher frequencies has been developed. The modelling is conducted based on measurements by combining a parameter extraction and a subsequent optimization step. A novel test setup has been designed, which allows for characterizing the transistor parameters within a frequency range of 500 MHz at a voltage of up to 500 V and with a load current of up to 90 A. The calibration for precise RF measurements at the ports of the device has been performed as a combination of measurements by means of a vector network analyzer and 3D field simulations of the setup. Selected results for a silicon transistor of manufacturer IXYS and type DE475-102N21A are presented.