Nonlinear characterization techniques for improving accuracy of GaN HEMT model predictions in RF power amplifiers

In this paper, two vector nonlinear characterization procedures are presented, aimed at improving available GaN HEMT models for an accurate reproduction of the device behavior operating as a current source and in switched-mode RF power amplifiers (PAs). In the case of the more traditional linear amplifying classes, a technique for simultaneously extracting the higher order derivatives of the I_ds(V_gs), I_gs(V_gs) and C_gs(V_gs) transistor nonlinearities, along a desired load line, is described. This procedure conveniently isolates and models their contributions to the device intermodulation distortion (IMD) behavior. In the case of highly efficient switched-mode PAs, employed under drain modulation condition, a modified procedure for isodynamically measuring the higher order derivatives of the V_dd-to-AM and V_dd-to-PM amplifier profiles is put into consideration, as a way to refine the triode region reproduction for ON-OFF operation. The test set-ups, based on the combination of vector signal analysis and vector signal generation capabilities, are described in detail, together with some analytic expressions for the parameter extraction. Measured results for a 15 W device from Cree are finally included.