Reverse Gate-Current of AlGaN/GaN HFETs: Evidence of Leakage at Mesa Sidewalls

Reverse gate-current of AlGaN/GaN heterostructure field-effect transistor is studied over a wide range of lattice-temperatures from 150 to 490 K. For gate-source voltages approaching zero, volt signatures of gate-to-2-D electron gas leakage through the sidewalls of the mesa are observed. For exploring this leakage path, a set of devices built on a number of alternative isolation features of different geometries, and with different number of gate-covered sidewalls, are investigated. Among these devices, which were realized on an identical layer structure (produced following the same fabrication technology with identical gate-length and -width), as the number of isolation-feature sidewalls overlapping with the gate metal increases, an increase in the gate-current is observed. The identified sidewall leakage is not only consequential in devices built on isolation-feature geometries presenting more than two sidewalls, but it can also compete with mechanisms such as Poole-Frenkel (PF), Fowler-Nordheim, and trap-assisted tunneling (TAT), which are traditionally considered in devices built on cubic mesas. In this paper, the relevance of these other transport mechanisms is also re-evaluated. It is observed that at temperatures below 320 K, among all of the explored devices, sidewall leakage becomes more dominant than the PF and trap-assisted tunneling (TAT) processes.