In a landmark achievement for 5G mobile technology, researchers from Singapore have set a new record for saturated output power (Psat) using gallium nitride (GaN) on silicon (Si) high-electron-mobility transistors (HEMTs). This breakthrough, led by scientists from the National GaN Technology Centre (NGTC), the Institute of Microelectronics (IME), the Singapore-MIT Alliance for Research and Technology, and the National University of Singapore, opens new avenues for 5G frequency range 2 (FR2) mobile handsets, which operate in the 24.25–71.0 GHz range.
The team’s innovative approach uses a double-heterostructure (DH) design, contrasting with conventional single-heterostructure (SH) devices. Their findings, published in IEEE Electron Device Letters, reveal how this novel structure achieves exceptional low-voltage, high-frequency performance without requiring deeply scaled gates, which often fall below 100nm in length. Instead, the Singaporean team’s DH HEMTs achieved outstanding performance with a gate length (Lg) of 120nm, combined with conventional ohmic and gate processes.
Double-Heterostructure Design Sets New Benchmark for GaN on Si
The DH structure developed by the research team features a GaN channel encased between an aluminum nitride (AlN) top barrier and an aluminum gallium nitride (AlGaN) bottom barrier. This configuration provides stronger carrier confinement compared to SH devices, which have only a single top barrier. The increased confinement in DH devices enables high-frequency stability, essential for 5G applications that demand both high data rates and reliable connections.
“Previous work has studied this heterostructure for high-voltage power amplification,” the researchers explained, “but the potential of DH HEMTs in low-voltage power amplification remains largely unexplored.” By pioneering this application for low-voltage power in 5G, the team is expanding the use cases for DH HEMTs beyond traditional high-voltage applications.
Key Performance Metrics of the DH HEMT Structure
To construct the device, the researchers grew the double-heterostructure material on high-resistivity silicon (HR-Si) using metal-organic chemical vapor deposition (MOCVD), incorporating an in-situ silicon nitride (SiN) layer to reduce gate leakage. Their process produced a GaN channel with 1.7x10¹³ cm² carrier density and 1400 cm²/V-s mobility, as confirmed by Hall measurements, while sheet resistance stood at 260 Ω/square.
The device’s source and drain contacts were formed with a titanium/aluminum/nickel/gold metal stack, and isolation was achieved through plasma mesa etching. The gate structure included nickel/gold T-gates, with a thin layer of aluminum oxide (Al2O3) applied using atomic layer deposition (ALD) for passivation. This ALD-Al2O3 layer helps reduce parasitic capacitance while avoiding potential damage from plasma processing.
The device exhibited impressive low-voltage characteristics, with a maximum drain current of 1.9 A/mm, ON-resistance of 1.5 Ω-mm, and a peak transconductance of 0.66 S/mm. Its threshold voltage of -2.9V places it in normally-on or depletion-mode operation. Testing revealed a three-terminal breakdown voltage of 49V, equating to an estimated breakdown electric field of 0.327 MV/cm. The breakdown mechanism was attributed to source-drain punch-through, which is mitigated by the design’s floating gate structure and high-quality SiN insulation.
High-Frequency Performance Suitable for 5G FR2
Frequency testing of the DH HEMT devices yielded notable results, with cut-off (fT) and maximum oscillation (fmax) frequencies reaching 145 GHz and 195 GHz, respectively, at a 5V drain bias and -2.5V gate potential. The frequency-gate length products—17.4 GHz-µm for fT and 23.4 GHz-µm for fmax—demonstrate the device’s suitability for FR2’s high-frequency demands. The team calculated a carrier saturation velocity of approximately 1.1 x 107 cm/s, a critical factor in achieving stable high-speed operation in 5G mobile devices.
Large-Signal Power Capability at 30 GHz
The research team also tested the device’s large-signal power capability at 30 GHz, an essential frequency for FR2 applications. On-wafer load-pull measurements showed a saturated output power (Psat) of 1.3 W/mm, with power-added efficiency (PAE) at 32% and gain at 3.7 dB. The peak PAE reached 42%, with a 1.1 W/mm output power and 7.3 dB gain. Lowering the bias to 3.5V yielded a slightly higher PAE of 43%, with an output power of 0.52 W/mm and 7.2 dB gain.
Increasing the bias linearly up to 15V raised the Psat value to 4.0 W/mm, although peak PAE decreased slightly due to increased trap effects and limited load impedance tuning. The researchers see potential for further improvements in device performance through optimized passivation, scaling of device dimensions, and refining ohmic contact designs.
Click here to read the Research Paper.
Future Outlook for GaN on Si HEMTs in 5G Handsets
The team’s findings mark a critical milestone in GaN on Si HEMT technology for mobile applications. Achieving a positive threshold voltage for normally-off (enhancement-mode) operation could further expand its deployment in consumer 5G handsets, where low-power, reliable operation is paramount. Additionally, future optimizations in passivation and contact design could push device performance even further.
With 5G standards continuing to evolve and the potential for FR3 (7–24.25 GHz) under discussion, this advancement could position GaN on Si HEMTs as a leading technology for the next generation of high-performance, high-frequency mobile devices.
Click here for more information on 5G FR2 Frequency Bands.
