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.高效可靠的无线系统需求正推动宽带通信领域快速发展，促使学界加大对新型半导体材料与器件的研究力度——此类材料器件需能在高频下实现优异性能。与此同时，人们对高效可持续未来的认知不断深化，也为这类技术发展提供了重要导向。以氮化镓（GaN）、铝镓氮（AlGaN）为代表的宽禁带III族氮化物半导体（Wide bandgap III-Nitride semiconductors），凭借其大禁带宽度带来的高击穿电场，以及此类材料异质界面处形成的极化诱导二维电子气所具备的优异电子输运特性，成为高功率射频（RF）系统的理想候选材料。这些特性的协同作用，使得基于此类材料的器件可实现大电流、高功率输出，并能在宽频段范围内稳定工作，因此在射频、微波及毫米波应用领域极具吸引力。 本研究针对铝镓氮/氮化镓高电子迁移率晶体管（HEMTs）中的极化工程展开探讨，分析极化渐变结构如何用于调控晶体管性能。其中重点研究了沟道渐变对器件线性度、速率及输出功率密度的影响。基于仿真的研究结果表明，采用极化渐变结构可将线性优值（linearity figure of merit，LFOM=OIP3/PDC）提升至约20 dB及以上，相较于传统突变界面高电子迁移率晶体管，性能提升约7 dB，该结论已通过实验验证。研究同时发现，沟道工程可在宽器件偏置条件范围内提升有效载流子迁移率，从而改善器件的速率性能，这一结果与极化渐变器件的射频测试结果一致：相较于传统高电子迁移率晶体管，此类器件可在更宽的漏源电压（Vds）偏置范围内实现并维持高特征频率（ft）与最大振荡频率（fmax）。除直流与射频性能提升外，由于极化固定电荷分布得到优化，极化渐变结构还可实现横向电场工程调控。与传统依靠场板调控栅漏电场的突变界面铝镓氮/氮化镓高电子迁移率晶体管不同，极化渐变高电子迁移率晶体管可将电场强度降低约23%，这为沟道电场调控提供了全新方案，对提升器件击穿性能与输出功率缩放能力具有重要意义。研究证实，沟道极化渐变可在维持高特征频率ft的同时提升击穿电压（VBD），使得约翰逊优值（Johnson's figure of merit，JFOM=ft×VBD）达到传统高电子迁移率晶体管的约2.4倍，该性能提升是器件领域的一项重要进展。此外，该优化方案还可与传统电场调控策略（如场板）结合使用。基于上述研究结果，极化渐变已成为面向高速高功率毫米波应用的极具吸引力的器件设计方案。