Degradation Mechanisms of GaN-Based Vertical Devices: A Review

This article reviews most recent results on the reliability of vertical GaN-based devices, by presenting a few case studies focused on the stability and degradation of high-voltage GaN-on-GaN diodes and of GaN-based field-effect transistors (FETs). With regard to diodes, two relevant stress conditions are investigated. The first is operation at high forward current that can induce a degradation of the electrical properties of the devices, mostly consisting in an increase in the operating voltage, well correlated to a decrease in the electroluminescence signal emitted by the diodes. This degradation process is ascribed to the diffusion of hydrogen from the highly p-type doped regions toward the junction, with consequent compensation of the acceptor (Mg) dopant. The second stress regime investigated on diodes is avalanche: specifically, it is shown that polarization-doped GaN devices may show avalanche capability, and the stability of diodes in avalanche regime is investigated in detail. With regard to transistors, the analysis is focused on GaN-on-GaN vertical Fin-FETs. First, the stability of the threshold voltage under positive gate stress is analyzed, and the role of interface/oxide traps is discussed by experimental characterization. Then, the degradation under positive gate or high-drain stress is investigated, to provide information on the dominant degradation processes.