GaN MISHEMTs in RF PAs exhibit a stable operating range
GaN MISHEMTs are being investigated for application in 5G+/6G RF systems due to their outstanding efficiency and power-handling capabilities. However, they encounter challenges, particularly regarding positive gate bias instability, where shifts in the threshold voltage under certain conditions can affect the power amplifier’s performance and long-term reliability.
In a breakthrough at the IEEE International Reliability Physics Symposium (IRPS) 2025, imec demonstrated that, despite their positive bias (on-state) instability, GaN MISHEMTs (Metal-Insulator-Semiconductor High Electron Mobility Transistors) can maintain consistent performance when operating within a well-defined range.
The ability to design reliable GaN-based power amplifiers to mitigate positive bias instability enables handset applications for 6G communication.
Gate bias instability in GaN MISHEMTs is a complex and largely unexplored phenomenon that can occur in the different operational states —off, semi-on, and on state— each exhibiting distinct instability mechanisms. Further, as traditional RF power amplifiers typically use GaAs HBT or HEMTs without a dielectric gate, the role of positive gate bias instability has not been a key consideration. The performance requirements of evolving wireless standards now render GaN MISHEMTs a compelling option.
Imec researchers introduced a pragmatic analytical approach that directly compared a stable range of gate voltages in DC conditions with the actual gate modulation range in the RF power amplifier operation. Their analysis revealed a strong overlap between these two ranges, confirming that GaN MISHEMTs remain stable within the typical voltage swing of RF power amplifiers. This allows linearly operating power amplifiers to be designed that avoid positive gate bias instability.
“Our research goes beyond merely identifying challenges; it also offers practical solutions, showing that GaN MISHEMTs can be reliably used in power amplifier applications for 5G+/6G technology. By integrating fundamental device reliability research with real-world RF system assessments, our team at imec bridges the gap between theoretical studies and practical applications, ensuring that GaN technology meets the demands of next-generation communication systems,” says Hao Yu, Principal Member of the Technical Staff at imec.
The researchers also show that naturally occurring positive interfacial polarisation charges at the material interface play a key role in preventing unwanted shifts in operating voltage over time. This unique feature of GaN MISHEMTs makes them more immune to on-state threshold voltage instability than other gate dielectric-contained devices, such as MISFETs. Simulations further demonstrate that even under real-world RF input signals, the on-state threshold voltage instability is mitigated.
The research was presented at the 2025 IEEE IRPS conference alongside three complementary papers from Imec’s advanced RF program in collaboration with Prof. Tian-Li Wu’s team at National Yang-Ming Chiao Tung University (NYCU) in Taiwan. These additional papers delve into fundamental research, exploring different degradation mechanisms and reliability challenges of GaN HEMTs and MISHEMTs under various stress conditions, including semi-on, off, and half-on states, and for both high- and low-voltage applications. Together, they contribute to a comprehensive understanding of GaN-Si device performance.
Image: Simulated probability profiles indicating the operational range of a power amplifier. The region where Nch<NFB is the safe operating range of a MISHEMT PA without on-state threshold voltage instability concern.
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