Investigation of ALD Dielectrics for Improved Threshold Voltage Stability and Current Collapse Suppression in AlGaN/GaN MOS-HFETs

Owing to a high critical electric field and high electron mobility, GaN based lateral Heterojunction Field Effect Transistors (HFETs) are sought after for high voltage power and RF applications. But, device reliability continues to be a critical challenge to be overcome before successful commercialization. In this work, different dielectrics deposited by Atomic Layer Deposition are investigated for improving the threshold voltage stability and dynamic reliability of AlGaN/GaN based Metal-Oxide-Semiconductor-HFETs (MOS-HFETs). But more importantly, this work includes a first-of-its-kind comprehensive analysis of electrical characterization techniques and physics-based models required to evaluate and recommend any dielectric for mitigating surface trapping phenomena in the gate stack or the accessregions.;In an investigation of the impact of MOSHFET device structure on the efficacy of different methods for characterization of dielectric/AlGaN interface traps, it is found that the popular conductance method has a severely constrained detection limit when the AlGaN barrier offers high resistance to the de-trapping electrons. A capacitance-based method is immune to the issue of barrier resistance, but is still restrictive in its range. To improve the range and accuracy of trap detection, a novel pulsed-IV-based methodology is developed and demonstrated to be applicable for detecting both shallow and deep traps.;Identical electrical thickness of different high-k and low-k ALD dielectrics are evaluated for gate leakage and magnitude and stability of threshold voltage. It is established that the high-k dielectrics (Al2O3, HfO2 and HfAlO) harbor a high density of shallow traps at the dielectric/AlGaN interface. In contrast, ALD SiO2, annealed in N2 at 700°C, creates a very low density of interface traps (< 2x10 12 cm-2) and is an excellent candidate for a gate dielectric. It provides strong gate leakage suppression, minimal threshold voltage shift and highly reliable ON-state characteristics.;With the use of physics-based simulation models, it is identified that the leakage at the surface of the AlGaN, whether through the passivation dielectric bulk or the dielectric/AlGaN interface, must be minimized to restrict the formation of a "virtual gate" and minimize current collapse. An optimal passivation dielectric must also create a high density of shallow interface donor traps to quicken the de-trapping of electrons from the "virtual gate" and the recovery of the channel underneath.;In order to create a high density of shallow interface donor traps a thin ALD HfAlO film is used. Surface leakage is also minimized by capping with a thick layer of PECVD SiO2 and annealing in N2 at 700°C. The effectiveness of the resulting optimal dual dielectric passivation stack in mitigating current collapse and ensuring contact isolation is also demonstrated.;Therefore, the optimal ALD dielectrics for a reliable gate stack and access-region passivation in an AlGaN/GaN MOSHFET are identified to be SiO 2 and HfAlO, respectively, annealed at 700°C in N2.