Reliability Issues in GaN HEMTs related to traps and gate leakage current

In this paper we present the results of an experiment carried out on HEMT test structures, which were submitted to a reverse-bias step stress up to -100 V. A set of AlGaN/GaN HEMT wafers was manufactured using a common fabrication process and device layout, and similar epitaxial structures. Different suppliers for substrate and epitaxial layers were adopted, in order to compare the influence of material quality on device degradation. Main results are: (i) all devices show common failure modes, consisting in an increase of the reverse gate current, IG, a (slight) decrease of drain saturation current, IDSS, and transconductance, gm, and a worsening of current collapse phenomena; (ii) as already observed in [1][2], a "threshold" reverse voltage value for degradation exists; this threshold (as well as the amount of gate current increase) remarkably depends on the wafer quality, ranging from ≈15 V to more than 100 V: devices belonging to the same wafer show similar threshold values; (iii) during tests, the increase in gate current occurs suddenly, and is correlated in all wafers with the growth of localized "hot spots", which can be detected by electroluminescence, EL, microscopy; (iv) the degradation of IDSS and gm is gradual, and is accompanied by a decrease in the intensity of the EL signal measured in on-state conditions and by a large increase in the device breakdown voltages ("breakdown walkout" [6]). With respect to preliminary data reported in [3][7], we show here clear evidence of the link between reverse bias degradation and material quality, together with the first observation of breakdown walkout effects in GaN HEMTs.