Reliability of T‐gate AlGaN/GaN HEMTs

Abstract

Abstract For the first time, we report on degradation mechanisms of short gate length (L g = 0.15 µm) T‐gate AlGaN/GaN HEMTs, and compare to previously reported findings for GaN field plate devices of the same gate length. Both types of studied devices were subjected to temperature‐stress tests in air. T‐gate devices were stressed with a drain bias of 15 V, while field plate devices were stressed with a drain bias of 30 V. Some T‐gate devices were also stressed in dry nitrogen to observe any impact of the environment. The devices stressed in air failed faster and at a lower junction temperature than the devices stressed in nitrogen. FIB‐STEM, EDS and EELS analysis of devices that reached failure criteria revealed that the devices stressed in air may have failed faster due to an oxide formation. For a T‐gate device stressed in air, an oxide formation was observed directly underneath the gate foot in a TEM image. For a field plate device stressed in air, an oxide was observed at the interface between the gate foot metal and the field plate dielectric on the drain side of the gate in a TEM image. Gate sinking into the barrier was observed in a TEM image for a T‐gate device stressed in dry nitrogen, while no gate sinking was observed or has been reported for field plate devices. Physics‐based device modeling of the two structures at the different stress bias conditions reveals that the electric field under the gate is much higher for the T‐gate device compared to the field plate device, despite the much higher drain bias for the field plate device. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)