(Invited) AlGaN Transistors for Digital Logic Applications in High-Temperature Environments

Abstract

Al-rich AlGaN HEMTs (high electron mobility transistors) are promising for extreme environment applications1. Over wide temperature ranges III-N HEMTs with ternary channels (AlGaN) exhibit more stable device behavior, compared to GaN, due to their alloy scattering-dominant transport, rather than the polar optical phonon scattering that dominates binaries (GaN). Additionally, Al(x>50%)Ga(1-x)N’s Ultra-Wide Band Gap enables superior suppression of intrinsic carrier density effects and thermionic emission-induced leakage2, making them prime candidates for high temperature electronics. While wide band gap (WBG) materials, such as SiC and GaN, have been previously investigated for high temperature device operability, the ultra-wide bandgap of Al-rich AlGaN potentially offers further advantages. We report on AlGaN logic development, using combined enhancement- and depletion-mode transistors to fabricate inverters with operation from room temperature up to 500°C. Our development approach included: (a) characterizing temperature dependent carrier transport for different AlGaN compositions, (b) developing a suitable gate metal scheme for use in high temperatures, and (c) over-temperature testing of discrete devices and inverters.For the carrier transport characterization over temperature, Hall mobility data were collected for four different (Al)GaN HEMT structures with different Barrier/Channel AlxGa1-xN aluminum compositions. As the temperature increased from room temperature to 500°C, the GaN-channel HEMT experienced a 6.9× reduction in mobility, whereas the AlGaN channel HEMTs experienced about a 3.1x reduction. A greater aluminum contrast between the barrier and channel enabled higher carrier densities in the two-dimensional electron gas for all temperatures. The combination of reduced variation in mobility with temperature and high sheet carrier concentration showed that an Al-rich AlGaN-channel HEMT with a high barrier-to-channel aluminum contrast is the best option for an extreme temperature HEMT design. For the gate metal scheme three stacks selected for low resistivity, high melting point, low thermal expansion coefficient, and high expected barrier height were evaluated. HEMTs were fabricated on Al0.85Ga0.15N barrier / Al0.7Ga0.3N channel epitaxy and used a Ti/Al/Ni/Au Ohmic metal (annealed at 1100°C). The gate metal stacks evaluated were: (1) 20 nm Pt / 200 nm Au, (2) 200 nm Pd, and (3) 200 nm W. The impact of thermal cycling was examined through electrical characterization of samples measured before and after rapid thermal anneal (500°C, 10 minutes, 1 mTorr nitrogen atmosphere). The 200 nm tungsten gate metallization was the top performer with minimal reduction in drain current, a slightly positive threshold voltage shift, and about an order of magnitude improvement in on-to-off current ratio. After incorporating the tungsten gate metal stack in device fabrication, characterization of transistors and inverters from room temperature up to 500°C was performed. The enhancement-mode (e-mode) devices’ resistance started increasing at about 200°C, resulting in drain current degradation. This phenomenon was not observed in depletion-mode (d-mode) devices but highlights a challenge for inverters in an e-mode driver and d-mode load configuration. P. H. Carey, F. Ren, A. G. Baca, B. A. Klein, A. A. Allerman, A. M. Armstrong, E. A. Douglas, R. J. Kaplar, P. G. Kotula and S. J. Pearton, Journal of the Electron Devices Society 7, 444-452 (2019).P. G. Neudeck, R. S. Okojie and L.-Y. Chen, Proceedings of the IEEE 90 (6), 1065 - 1076 (2002). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND2021-4920 A