For an, at best, first-time right circuit design, it is important that you have a good model for your device, whether this is a digital CMOS or a high power LDMOS FET. This is just as important for a GaN device in a DC power convertor or in an amplifier for THz radio. Note that even for digital circuits the device needs to be characterized as linear and then the circuit drives it from the extreme digital 0 to the extreme digital 1.
Several characterization techniques are described in why, how and what for, including the equipment available on the market to accomplish the task. Some buzz-words will come back in the talk: IV/CV measurement, Pulsed IV, S-parameters, noise characterization, and source/load pull.
n.n.b, BSW TestSystems and Consulting