0741-3106 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LED.2019.2925776, IEEE Electron Device Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract — In this letter we investigate by means of experimental results and TCAD simulations the threshold voltage instability due to off-state drain stress in p-GaN gate AlGaN/GaN-on-Si HEMTs. When the drain of the p-GaN HEMT is biased in the off-state the threshold voltage (Vth) shows a linear increase up to ~ 40%. This increase saturates at drain bias voltages above 50V. The positive Vth shift is attributed to the ionization of acceptor traps in the AlGaN region below the p-GaN gate with the source of these trapping sites suggested to be the p-GaN gate out-diffused Mg dopant atoms. The ionization of the Mg acceptors due to high electric field during off-state bias and the removal of the generated holes from the AlGaN region through the gate contact creates the charge conditions for a positive Vth shift. The sharp decrease in the gate drain capacitance (Cgd) for VD < 50V, the simulated gate edge electric field reaching its peak for a drain voltage bias VD ~ 50V and the positive threshold voltage shift observed for negative gate stress further validate the proposed model. Index Terms— GaN, p-GaN gate, normally-off HEMT, threshold voltage instability, reliability I. INTRODUCTION ALLIUM Nitride due to its spontaneous and piezoelectric polarization, superior intrinsic properties and its ability to form hetero-structures is an exciting material for high power and high frequency applications. The high critical electric field of the material in association with the formation of a high carrier density, high mobility 2-Dimensional Electron Gas (2DEG) in AlGaN/GaN High Electron Mobility Transistors (HEMTs), enables the development of power switching devices with very competitive specific on-state resistance values. The presence of the 2DEG at the AlGaN/GaN interface leads to the more straightforward design of normally-on devices however, for low static power dissipation and safety in high power applications normally-off HEMTs are preferred. Among several normally-off HEMT solutions, the p-GaN gate/AlGaN/GaN-on-Si HEMT is considered the leading structure for commercialization. In recent years, a significant amount of effort has been T his paragraph of the first footnote will contain the date on which you submitted your paper for review. L. Efthymiou, K. Murukesan, G. Longobardi and F. Udrea are with the Electrical Engineering Department, University of Cambridge, Cambridgeshire, UK (email: le257@cam.ac.uk). directed towards understanding the behavior of the p-gate for achieving normally-off GaN HEMT devices [1][2][3]. Several studies in literature have dealt with the stability of the threshold voltage (Vth) of these devices under forward gate bias stress. Different studies attribute the threshold voltage shift observed to the accumulation or depletion of holes in the p-GaN region [4][5] or electron trapping in the AlGaN region [6][7]. The out- diffusion of Mg doping from the p-gate in the AlGaN and GaN layers underneath has been identified as a potential source of the trapping phenomena observed [6][7]. Both positive and negative threshold voltage shifts have been reported depending on the process and design [4]-[7]. Alternatively, a large amount of research has been directed at understanding the effects of off-state bias stress on the dynamic on-state resistance of GaN HEMTs. In the early stages of research in GaN HEMTs, trapped charges at the AlGaN/passivation interface were often reported to be the cause of current collapse [8][9]. However, in recent years, with improvements in the passivation of GaN HEMTs the dynamic Ron degradation observed is less commonly attributed to trapped charges on the surface of the device [10]. Dynamic Ron degradation during off-state stress is instead often considered the result of electron trapping due to the presence of acceptor traps in the carbon doped GaN buffer [11]-[13]. In this study, we demonstrate that off-state drain bias stress does not only affect the dynamic Ron of the p-GaN HEMT but also impacts on the stability of the threshold voltage. Here we discuss in detail the mechanisms behind this observation and validate it with suitably calibrated TCAD simulations. II. EXPERIMENTAL METHODS Normally-off lateral AlGaN/GaN-on-Si HEMT devices (650V, 15A) based on p-gate technology (see Fig. 1(a)) were used in this study. Standard dynamic Ron measurements were first performed using a Keysight B1505A power device analyzer and a N1265A fast switch. The drain terminal of the device was biased at various OFF-state bias voltage levels (Vdstress) for 1 second. Post-stress, the N1267A fast switch was used to switch the device to ON-state within a time interval of 20μs and the output characteristics at various gate bias levels were recorded. The measurement methodology is illustrated in A. Shibib and K. Terrill are with the Vishay Siliconix, San Jose, CA, USA. Loizos Efthymiou, Member, IEEE, Karthick Murukesan, Member, IEEE, Giorgia Longobardi, Member, IEEE, Florin Udrea, Member, IEEE, Ayman Shibib, Fellow, IEEE, Kyle Terrill Understanding the threshold voltage instability during off-state stress in p-GaN HEMTs G