Pole-Zero Approach to Analyze and Model the Kink in Gain-Frequency Plot of GaN HEMTs

In this letter, we present a novel approach toward understanding the Kink effect (KE) in the bode plot of short circuit current gain (h
₂₁
) observed for microwave transistors, particularly gallium nitride (GaN) HEMTs. We ascribe the origin of the KE to the presence of a pair of complex conjugate poles at the frequency of interest, introduced due to the extrinsic parasitic inductances and their interaction with the device intrinsic elements, such as the capacitances and transconductance, and develop simplified mathematical expressions that govern the behavior of the kink. We also present a physics-based compact model that is capable of capturing the KE and extensively validate the model against measured data for a GaN device under multibias conditions, thereby advocating the strong physical background of the model. We conclude by demonstrating the impact of various elements of the small signal model on the kink based on the developed mathematical hypothesis for KE.