Polarization Engineering in AlGaN/GaN High Electron Mobility Transistors for High Linearity, High-Power RF Applications

The demand for efficient and reliable wireless systems is driving rapid advancements in the field of broadband communication, calling for increased research in novel semiconductor materials and devices capable of achieving exceptional performance at high frequencies. These technological developments are also steered by increasing awareness of the need for a more sustainable future with higher efficiency. Wide bandgap III-Nitride semiconductors like GaN and AlGaN are excellent candidates for use in high power RF systems due to their large bandgaps, and consequently high breakdown fields, as well as the excellent electron transport properties of polarization-induced two-dimensional electron gases that form at heterointerfaces in these materials. The combination of these effects leads to devices that can achieve high currents, high power, and operate over a wide range of frequencies, making them attractive candidates for RF, microwave, and millimeter wave applications. This work presents studies of polarization engineering in AlGaN/GaN high electron mobility transistors (HEMTs), and how polarization grading can be used to tailor transistor performance. In particular, the impact of channel grading on device linearity, speed, and output power density is explored. Our simulation-based studies show that the use of polarization-graded structures is promising for increasing the linearity figure of merit, LFOM= OIP3/PDC, to ~20 dB or more, an enhancement of ~7 dB over the conventional abrupt-interface HEMT. This has also been validated experimentally. Our studies also show that channel engineering results in increased effective carrier velocity over a wide range of device bias conditions, helping to increase the speed performance of the devices. This is consistent with RF measurements made on polarization-graded devices, which show that high ft and fmax can be achieved and maintained over larger Vds bias ranges than is typical of conventional HEMTs. Adding to the improved DC and RF performances, polarization-graded structures also enable lateral electric field engineering, due to the modified polarization fixed charge distribution. In contrast to traditional abrupt-interface AlGaN/GaN HEMTs that employ field plates to manage the gate-to-drain electric field, polarization-graded HEMTs can achieve electric fields that are reduced by approximately 23% compared to abrupt-interface HEMTs. This enables a new approach to channel field control and has significant implications for improving device breakdown and output power scaling. It is demonstrated that polarization grading of the channel enhances breakdown voltage (VBD) while preserving high ft, resulting in a Johnson's figure of merit (JFOM = ft x VBD) that is ~2.4x that of a conventional HEMT. This improvement represents a significant advancement in device performance. In addition, this improvement can be combined with conventional field management strategies (e.g. field plates). Based on these findings, polarization grading emerges as a compelling device design concept for high-speed and high-power millimeter-wave applications.