Impact of the Al Mole Fraction in the Bulk- and Surface-State Induced Instability of AlGaN/GaN HEMTs
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Impact of the Al Mole Fraction in the Bulk- and Surface-State Induced Instability of AlGaN/GaN HEMTs S. DasGupta1, M. Sun2, A. Armstrong1, R. Kaplar1, M. Marinella1, J. Stanley1, M. Smith1, S. Atcitty1 and T. Palacios2 1 Sandia National Laboratories, Albuquerque, NM 87185 USA 2 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139 USA. ABSTRACT Charge trapping and slow (10 s to > 1000 s) detrapping in AlGaN/GaN HEMTs designed for high breakdown voltage (> 1500 V) are studied to identify the impact of Al molefraction and passivation on trapping. Two different trapping components, TG1 (Ea = 0.62 eV) and TG2 (with negligible temperature dependence) in AlGaN dominate under gate bias stress in the off-state. Al0.15Ga0.85N shows much more vulnerability to trapping under gate stress in the absence of passivation than does AlGaN with a higher Al mole fraction. Under large drain bias, trapping is dominated by a much deeper trap TD. Detrapping under illumination by monochromatic light shows TD to have Ea § 1.65 eV in Al0.26Ga0.74N and Ea § 1.85 eV in Al0.15Ga0.85N. This is consistent with a transition from a deep state (Ec - 2.0 eV) in the AlGaN barrier to the 2DEG. INTRODUCTION AlGaN/GaN High-Electron-Mobility Transistors (HEMTs) have traditionally been the device of choice in RF electronics. More recently, the ease of achieving a low on-state resistance due to high channel mobility (ȝch) coupled with a wide bandgap (Eg(GaN) = 3.4 eV) has led to significant advancements in developing the GaN HEMT as a device for the next generation of high voltage power electronics [1-2]. While greatly detailed studies of defects in the AlGaN/GaN system exist, much of the focus in HEMTs has been on specific kinds of trapping that limit RF performance, such as traps causing gate lag. In studies analyzing the effects of high DC gate or drain voltage stress (including permanent degradation) the traps that have been studied in great detail have short detrapping time constants (< 1 s at room temperature) [3-4]. In spite of evidence of trapping that shows much slower (10 s to > 1000 s) detrapping behavior in most stress experiments, relatively few studies analyzing traps causing this kind of behavior exist [5-6]. In this work, we investigate the effects of different bias conditions on the detrapping rate in AlGaN/GaN HEMTs. Traps with room-temperature time constants ranging from 10 to 1000 s were characterized thermally using the current transient method, and deeper traps were characterized optically using sub-bandgap monochromatic light. The effect of passivation on individual trapping components was analyzed for different mole fractions of Al in AlGaN. DEVICE DETAILS Four different sets of devices were fabricated at MIT on silicon (111) substrates for two different breakdown voltage ranges. One set of devices were fabricated to achieve a maximum breakdown voltage of 500 V. These used Al0.26Ga0.74N in the barrier and had a threshold voltage
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(Vth) of approximately -1.8 V. The o
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