(Invited) the Outlook for Al-Rich AlGaN Transistors

High critical electric field (E_C) and a large saturation velocity in ultra-wide bandgap transistors (UWBG) such as Al-rich AlGaN and ß-Ga₂O₃ is stimulating research into these devices [1,2]. This research is aimed at producing devices that may one day excel in power switching [3] and RF applications [4]. Figure-of-merit (FOM) analysis supports the possibility that Al-rich AlGaN transistors will outperform other WBG and UWBG transistors [5,6]. In spite of this promise, the UWBG devices do not yet perform well enough reach the promise evidenced by the FOM analysis. The favorable resolution of several important issues may be the key to broad acceptance of Al-rich AlGaN transistors. Until that time, Al-rich AlGaN transistors will begin to find utility in less mainstream uses. Recent developments are encouraging for the acceptance of Al-rich AlGaN in harsh environments.

This talk will describe multiple research efforts to develop Al-rich AlGaN into the material of choice for next-generation power electronics applications and compare these research results to other WBG transistors where appropriate. These transistors generally benefit from the materials and fabrication similarity to AlGaN/GaN. They are grown epitaxially in the same metal organic chemical vapor deposition reactors and on mostly the same substrates. They will also benefit from similar fabrication infrastructure that is in place for AlGaN/GaN. Likewise, the AlGaN alloys support a wide diversity of device types. Depending on the targeted application, the AlGaN transistors may be depletion-mode (normally-on) or enhancement-mode (normally-off) transistors. Diversity in device types also includes various types of device designs that have been investigated, including metal-semiconductor field effect transistors, high electron mobility transistors (HEMTs), metal-insulator-semiconductor transistors, metal-oxide transistors, and polarization doped transistors.

We will also describe the progress in maturing this transistor technology along a number of fronts. They are improving towards high current density at a rapid rate through efforts to improve the mobility, efforts to improve the Ohmic contacts, and by means of aggressively scaling the device dimensions. The particular strengths of the Al-rich AlGaN transistor include high electric field at breakdown, which is already more than 2-times that of GaN, the high I_ON/I_OFF ratio in the transistor, now exceeding 10⁹ in multiple devices, and in their high temperature operation with multiple reports in the 300-500°C range. A recent comparison of an Al_0.85Ga_0.15N/Al_0.7Ga_0.3N HEMT against AlGaN/GaN showed a superior I_ON/I_OFF ratio at 500°C and also shows a favorable extrapolation to the 500-800°C temperature range for Al_0.7Ga_0.3N but not for GaN [7]. Such results are beginning to suggest that AlGaN may one day become the preferred transistor for operation at extreme temperatures. Details regarding ongoing work to optimize device performance for Al-rich AlGaN transistors will be presented in the talk.

[1] A.G. Baca et al., J. Vac. Science and Technology, 40, 1600501 (2020).

[2] M. Higashiwaki and G. H. Jessen, Appl. Phys. Lett. 112, 060401 (2018).

[3] R. J. Kaplar et al., J. Solid-State Sci. Technol., 6, Q3061 (2017).

[4] R. J. Kaplar et al., Proc. GOMAC, Albuquerque, NM, USA (2019), pp. 587-591.

[5] Coltrin et al., J. Solid-State Sci. Technol., 6, S3114 (2017).

[6] S. Bajaj et al., Appl. Phys. Lett. 105, 263503 (2014).

[7] P. H. Carey et al., J. Electron Devices Society, 7, 444 (2019).

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and 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-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.