A Comprehensive and Critical Overview of the Kink Effect in S22 for HEMT Technology

This invited contribution is aimed at presenting a thorough knowledge, a critical understanding, and new insights into the kink effect in the output reflection coefficient (S₂₂) of field-effect transistors (FETs). To accomplish this challenging goal, we report a measurement-based investigation, using the high-electron-mobility transistor (HEMT) technology as a case study. The kink effect is investigated over a wide range of operating conditions, by changing bias point, ambient temperature, device size, and semiconductor technology. The origin of the kink phenomenon and its dependency on the operating condition are analyzed by using a lumped-element equivalent-circuit model. The achieved findings represent a powerful know-how to entitle microwave engineers to take properly the kink effect into account in fabrication, modeling, and design phases.