With the advancement of communication technology and the use of higher frequency bands for greater bandwidth, there is an increasing need for power amplifiers that can simultaneously provide high power, linearity, and efficiency. High electron mobility transistors (HEMTs) based on the AlGaN/GaN heterostructure provide state-of-art performance for high frequency power amplifiers [1]. For mm-wave and higher frequencies, significant improvement of performance is necessary to achieve the required performance. This invited talk will address new channel designs that could provide improvement in power density and linearity for future power amplifiers.

The first part of this talk will discuss new channel designs to improve the linearity performance of III-Nitride transistors. Transconductance drop is a major source of non-linearity for AlGaN/GaN HEMTs and has been shown to originate from the fundamental transport properties in the 2-dimensional electron gas that forms the conducting channel in such transistors [2]. In this work, we show that replacing the 2D electron gas channel with a 3-dimensional electron gas (3DEG) channel using polarization grading can enable constant transconductance and power gain profiles over the operating regime of the transistor, and therefore improve the linearity performance [3]. We will describe different variations of these polarization graded FETs (PolFETs) including InGaN-based structures, composite 2D/3D channels, and designs for passivation and low leakage [4]. Our results show that novel channel design can enable record performance in terms of two-tone linearity at X-band (10 GHz) frequencies.

The second part of the talk will focus on ultra-wide band gap (UWBG) high Al-composition AlGaN based transistors. AlGaN is promising as a high frequency channel material due to its high critical breakdown field and high saturated velocity [5,6]. We will discuss, using detailed simulations and calculations how the high field strength in AlN and AlGaN (above 8 MV/cm) could enable higher power density for mm-wave and THz applications. Achieving the projected performance in actual devices is challenging due to fundamental issues related to the low mobility, charge injection, and electrostatic field management [7]. We will discuss novel designs being pursued to overcome these challenges, including the use of novel heterostructure contacts, scaled devices to enable high-velocity transport, and the integration of extreme dielectric constant materials such as BaTiO₃/AlGaN to realize high breakdown fields. These efforts have enabled current state-of-art performance for ultra-wide band gap AlGaN transistors, including 60% AlGaN channel HEMT devices with record cutoff frequency f_T up to 40 GHz.

References:

1. Kobayashi et al., IEEE Journal of Solid-State Circuits, vol. 47, p. 2316 (2012).
2. Bajaj et al., IEEE Transactions on Electron Devices, vol. 64, p. 3114 (2017).
3. Sohel et al., IEEE Electron Device Letters, vol. 39, p. 1884 (2018).
4. Sohel et al., IEEE Electron Device Letters, vol. 40, p. 522 (2019).
5. Hudgins et al., IEEE Transactions on Power Electronics, vol. 18, p. 907 (2003).
6. Farahmand et al., IEEE Transactions on Electron Devices, vol. 48, p. 535 (2001).
7. Bajaj et al., Applied Physics Letters, vol. 109, p. 133508 (2016).