The increase of the switching frequency of power electronic converter systems into the upper Megahertz range requires the consideration of RF design aspects as for example impedance matching. However, the data sheets of power semiconductors lack of the information required for such designs. Hence, a set-up for characterizing power transistors based on S-parameter measurements in the pinch-off and ohmic regions under load conditions has been assembled. It comprises as main elements power sources as to operate the device under test, and a network analyzer an oscilloscope for measurement and data acquisition. The set-up is controlled by a computer to allow for an automated and unattended operation. The set-up has been validated successfully by means of measurements with a resistor inserted instead of a transistor and comparing the measurement results with simulations. Moreover, as a first result, selected parameters of the characterization of a GaN transistor are presented.