A double-pulse technique for the I/V characterization of GaN-based transistors is adopted for the nonlinear modeling of a 0.25 μm AlGaN/GaN on SiC FET. Experimental validation is provided by means of large-signal PA performance prediction both at low-frequency, in order to outline the role played by the resistive drain current source, and at microwaves. Improved prediction accuracy is demonstrated with respect to the use of standard single-pulse I/V characteristics.