Analysis of barrier inhomogeneities in AlGaN/GaN HEMTs' Schottky diodes by I-V-T measurements

Gate design and process is a major reliability issue in AlGaN/GaN HEMTs, arousing the setup of accurate tools and models to determine the Schottky diode parameters. This paper proposes the study of the Schottky diode on a set of two HEMT structures featuring different gate pad connections: as some electrical differences can be found out from the behavior of leakage currents (and associated reliability consequences), it is of prime importance to detect if the extrinsic design of the gate pad impacts the Schottky barrier behavior and hence the physical and electrical parameters. Therefore, forward gate I-V measurements in the temperature range of 100K-400K are presented: the ideality factor and the Schottky barrier height are extracted. An investigation on the Schottky barrier height by using the conventional Richardson plot from the well known physical equations gives an effective Richardson constant (A*) value far from the theory. The model can be greatly improved by applying Werner's model which leads to A* in good agreement with the theory. A small difference is found between the structures, due to a weak difference between the real effective masses. The study reveals a spatial barrier inhomogeneity under the gate at the metal-semiconductor interface on the two sets of devices featuring an extrinsic difference at the interconnection between the gate finger and the pad. However, the Schottky parameters are the same for all tested devices using Werner's model, instead of that from the classical physical equation of the Schottky diode.