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1. AlN/AlGaN/AlN quantum well channel HEMTs

   https://doi.org/10.1063/5.0145582  

   Singhal, Jashan; Kim, Eungkyun; Hickman, Austin; Chaudhuri, Reet; Cho, Yongjin; Xing, Huili Grace; Jena, Debdeep (May 2023, Applied Physics Letters)

   We present a compositional dependence study of electrical characteristics of AlxGa1−xN quantum well channel-based AlN/AlGaN/AlN high electron mobility transistors (HEMTs) with x=0.25,0.44, and 0.58. This ultra-wide bandgap heterostructure is a candidate for next-generation radio frequency and power electronics. The use of selectively regrown n-type GaN Ohmic contacts results in contact resistance that increases as the Al content of the channel increases. The DC HEMT device characteristics reveal that the maximum drain current densities progressively reduce from 280 to 30 to 1.7 mA/mm for x=0.25,0.44, and 0.58, respectively. This is accompanied by a simultaneous decrease (in magnitude) in threshold voltage from −5.2 to −4.9 to −2.4 V for the three HEMTs. This systematic experimental study of the effects of Al composition x on the transistor characteristics provides valuable insights for engineering AlGaN channel HEMTs on AlN for extreme electronics at high voltages and high temperatures. 

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2. Toward AlGaN channel HEMTs on AlN: Polarization-induced 2DEGs in AlN/AlGaN/AlN heterostructures

   https://doi.org/10.1063/5.0121195  

   Singhal, Jashan; Chaudhuri, Reet; Hickman, Austin; Protasenko, Vladimir; Xing, Huili_Grace; Jena, Debdeep (November 2022, APL Materials)

   Due to its high breakdown electric field, the ultra-wide bandgap semiconductor AlGaN has garnered much attention recently as a promising channel material for next-generation high electron mobility transistors (HEMTs). A comprehensive experimental study of the effects of Al composition x on the transport and structural properties is lacking. We report the charge control and transport properties of polarization-induced 2D electron gases (2DEGs) in strained AlGaN quantum well channels in molecular-beam-epitaxy-grown AlN/AlxGa1−xN/AlN double heterostructures by systematically varying the Al content from x = 0 (GaN) to x = 0.74, spanning energy bandgaps of the conducting HEMT channels from 3.49 to 4.9 eV measured by photoluminescence. This results in a tunable 2DEG density from 0 to 3.7 × 1013 cm2. The room temperature mobilities of x ≥ 0.25 AlGaN channel HEMTs were limited by alloy disorder scattering to below 50 cm2/(V.s) for these 2DEG densities, leaving ample room for further heterostructure design improvements to boost mobilities. A characteristic alloy fluctuation energy of ≥1.8 eV for electron scattering in AlGaN alloy is estimated based on the temperature dependent electron transport experiments. 

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3. Large Signal Response of AlN/GaN/AlN HEMTs at 30 GHz

   https://doi.org/10.1109/DRC52342.2021.9467196  

   Hickman, Austin; Chaudhuri, Reet; Moser, Neil; Elliott, Michael; Nomoto, Kazuki; Li, Lei; Hwang, James C.; Grace Xing, Huili; Jena, Debdeep (June 2021, 2021 Device Research Conference (DRC))
4. High Breakdown Voltage in RF AlN/GaN/AlN Quantum Well HEMTs

   https://doi.org/10.1109/LED.2019.2923085  

   Hickman, Austin; Chaudhuri, Reet; Bader, Samuel James; Nomoto, Kazuki; Lee, Kevin; Xing, Huili Grace; Jena, Debdeep (August 2019, IEEE Electron Device Letters)
5. Electron mobility enhancement by electric field engineering of AlN/GaN/AlN quantum-well HEMTs on single-crystal AlN substrates

   https://doi.org/10.1063/5.0190822  

   Chen, Yu-Hsin; Encomendero, Jimy; Savant, Chandrashekhar; Protasenko, Vladimir; Xing, Huili Grace; Jena, Debdeep (April 2024, Applied Physics Letters)

   To enhance the electron mobility in quantum-well high-electron-mobility transistors (QW HEMTs), we investigate the transport properties in AlN/GaN/AlN heterostructures on Al-polar single-crystal AlN substrates. Theoretical modeling combined with experiment shows that interface roughness scattering due to high electric field in the quantum well limits mobility. Increasing the width of the quantum well to its relaxed form reduces the internal electric field and scattering, resulting in a binary QW HEMT with a high two-dimensional electron gas (2DEG) density of 3.68×1013 cm–2, a mobility of 823 cm2/Vs, and a record-low room temperature (RT) sheet resistance of 206 Ω/□. Further reduction of the quantum well electric field yields a 2DEG density of 2.53×1013 cm–2 and RT mobility > 1000 cm2/V s. These findings will enable future developments in high-voltage and high-power microwave applications on the ultrawide bandgap AlN substrate platform. 

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