Modeling of kink-effect in RF behaviour of GaN HEMTs using ASM-HEMT model

In this paper, a physics-based compact model is reported that captures the kink-effect observed in S22 for AlGaN/GaN HEMTs. The presence of this kink in the Smith-plot of S22 severely affects the design of the output matching network for amplifiers based on these devices which calls for a precise consideration of this effect. The kink-effect originates due to the ambivalent nature of the output impedance of the intrinsic device, wherein it changes its nature from a low frequency series-RC network to a high frequency parallel-RC network, giving rise to a kink at the frequency where the contours corresponding to low and high frequency approximations intersect each other. The output impedance is a complicated function of the various intrinsic elements of the device small signal model, and all the device intrinsic characteristics in our model arise from a physics-based framework, therefore, making multi-bias validation of the kink-behaviour with measured data possible with sufficient ease.