The Sun Qian team at the Suzhou Institute of Nano-Tech and Nano-Bionics has achieved new progress in the field of low-voltage RF devices based on gallium nitride.

In recent years, 5G communication applications have been deeply expanding into mobile terminal scenarios such as smartphones, driving an increasingly urgent demand for low-voltage, high-performance radio-frequency power amplifiers (PAs). Gallium nitride (GaN) high-electron-mobility transistors (HEMTs), with their high electron saturation velocity and excellent two-dimensional electron gas (2DEG) transport characteristics, have emerged as an ideal choice for realizing high-performance RF frontends. However, conventional GaN devices suffer from a relatively high knee voltage, which severely limits their RF output performance under low-voltage conditions. To address this issue, the conventional approach internationally is to employ strongly polarized heterojunctions (such as InAlN/GaN and AlN/GaN) and adopt a "deep scaling" process, both of which place stringent requirements on fabrication precision.

Recently, the research team led by Researcher Qian Sun at the Suzhou Institute of Nano-Tech and Nano-Bionics successfully developed a silicon-based gallium nitride “near-zero access region” metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) device based on an AlGaN/GaN heterojunction. Thanks to innovative structural design, this device effectively reduces the knee voltage even under relatively relaxed fabrication tolerances and demonstrates outstanding DC and RF performance in low-voltage applications.

By extending the source-drain ohmic contact region to the gate edge and employing an n⁺⁺-GaN secondary epitaxial growth technique, the research team effectively suppressed the access-region resistance caused by the non-gate-controlled 2DEG region typically found in conventional devices, thereby significantly enhancing the device’s output characteristics. At a gate length of 550 nm, this device achieved a saturation current density of 2.11 A/mm, a peak transconductance of 300 mS/mm, and a low on-resistance of 0.61 Ω·mm, with an ohmic contact resistance as low as 0.09 Ω·mm.

In load-pull tests conducted in the 3.5 GHz band, the device achieved an output power density of 0.81 W/mm at a drain voltage of 5 V, with a linear gain of 10.5 dB and a peak power-added efficiency (PAE) of 19.4%. Particularly noteworthy is that, under RF power application conditions, the knee voltage of the device—obtained via linear extrapolation—was only 0.75 V, representing the best value among low-voltage GaN RF devices reported to date. The next step will involve further enhancing device performance through research on selective-area growth of high-resistivity GaN and the optimization of T-gate structures.

Figure 1: Schematic cross-sectional view of the MIS-HEMT device structure with a nearly zero-access region based on silicon-gallium nitride.

Figure 2: (a) Transfer characteristics, (b) Output characteristics, (c) Three-terminal leakage characteristics, and (d) Three-terminal off-state breakdown characteristics of the silicon-based GaN MIS-HEMT device with a near-zero access region.

Figure 3: Structure of the Si-based GaN MIS-HEMT device in the near-zero-access region (a) RF power characteristics based on load-pull measurements at 3.5 GHz (b) Extrapolation results for the knee-point voltage

The research findings, titled “AlGaN/GaN MIS-HEMT Nominally Free of Access Regions on Si for Low-Voltage RF Applications,” have been published in IEEE Microwave and Wireless Technology Letters, a prestigious international journal in the field of microwave and radio frequency engineering. The first author of the paper is Dr. Zhang Xinkun, who is currently a postdoctoral fellow working with the team.

This study is closely related to another research paper previously published by our team in the international microelectronics journal Semiconductor Science and Technology (Impact of eliminating ungated access regions on DC and thermal performance of GaN-based MIS-HEMT). In the earlier study, our team systematically elucidated the mechanism by which access regions influence the electrical and thermal performance of GaN devices, pointing out that suppressing the access regions holds promise for significantly enhancing the low-voltage output characteristics of these devices. Building on this foundation, this study further proposes and validates a low-voltage RF device fabrication process route that differs from conventional “deep scaling.” By effectively reducing the resistance of the access regions, we have achieved precise control over the knee voltage and even improved the overall RF output characteristics of the devices, thus providing new insights for the development of low-voltage Sub-6 GHz RF front-end chips.

The related work has been supported by several projects, including the National Key Research and Development Program, the National Major Science and Technology Special Project, the National Natural Science Foundation, the Strategic Leading Science and Technology Special Project of the Chinese Academy of Sciences, the Jiangsu Provincial Key R&D Program, and the Suzhou City Science and Technology Program. The fabrication and testing of the relevant devices were carried out at the Nanofabrication Platform, the Nanotest and Analysis Platform, and the Nano-Vacuum Interconnection Experimental Station (Nano-X) of the Suzhou Institute of Nanotechnology.

Source: Suzhou Institute of Nanotechnology, Chinese Academy of Sciences