Observation of ID-VD Kink in N-Polar GaN MIS-HEMTs at Cryogenic Temperatures

This paper reports on the hot-carrier effects and semi-on-state behavior of nitrogen-polar GaN MIS-HEMTs at cryogenic temperatures (from 300 K down to 100 K). In the semi-on-state (V-G approximate to -2 V), holes are generated by impact ionization in the high field region at the drain-side of the gate-edge. At room temperature, holes overcome the SiN/AlGaN stack and are collected at the gate-terminal, resulting in measurable hole gate-current (similar to 11 nA/mm). Conversely, at cryogenic temperatures, the top SiN/AlGaN stack confines the holes within the GaN channel, thus inducing a negative threshold voltage shift (-0.4 V) and a sharp increase in drain current (0.18 A/mm). This behavior, referred to as kink, is readily observable on the ID-VD characteristics. We demonstrated that the kink is related to impact ionization and follows a non-monotonic behavior maximized in the semi-on-state. Our interpretation is supported by a quantitative analysis based on the latest experimental impact-ionization coefficients available in the literature.