Impact of Structural and Process Variations on the Time-Dependent OFF-State Breakdown of p-GaN Power HEMTs

In this article, we present an extensive investigation of the time-dependent drain breakdown occurring in GaN-on-Si power HEMTs with p-GaN gate under long-term OFF-state stress. In particular, the time-dependent breakdown induced by high-temperature-reverse-bias stress is investigated as a function of different process and structural variations. Main results demonstrate that, by varying the gate-to-drain distance (LGD) and the field plates configuration, the physical location of failure changes as well. If LGD is relatively short (3 μm), the time-dependent breakdown occurs through the GaN channel layer between drain and source. In this case, a thinner GaN layer significantly improves the device robustness to long-term OFF-state stress. If LGD is relatively long (≥ 4 μm), the failure occurs between the two-dimensional electron gas (2DEG) and the source field plates. In this second case, the GaN layer thickness and LGD have no significant impact on the time-dependent breakdown, whereas the field plate lengths can be optimized to reduce the area exposed to high electric fields, hence limiting the probability of failure. Finally, the role of the AlGaN barrier layer has been analyzed as well. If LGD = 3 μm, a thinner AlGaN layer is preferred, whereas if LGD ≥ 4 μm, a thicker layer with lower aluminum content gives rise to longer time to breakdown under OFF-State stress.