The Start of GaN Technology
In this past couple of years, more commercial products using Gallium Nitride (GaN) based High-Electron-Mobility-Transistors (HEMTs) have started to emerge. For example, fast chargers and adapters for smartphones, laptops and game consoles using GaN can offer faster charging speed with smaller size and weight as compared to the current silicon-based technology. However, these applications are just the tip of the iceberg of the wide range of applications which can leverage on GaN. These include dc-dc converters and traction inverters in electric vehicles, data centres, light detection and ranging (lidar) systems for autonomous cars, robots, drones, and security systems which can take full advantage of GaN’s high-speed switching and high-power abilities. In short, we are witnessing the start of GaN technology which is ready to transform the entire power electronics industry.
Extending Operating Frequency of GaN-on-Si HEMTs
In view of the huge economic potential, increasingly more silicon wafer fabs have also started to invest and build up their capabilities to enable high-volume GaN HEMTs on low-cost silicon substrates leveraging on their existing 8” CMOS lines. By adopting this approach, it will make GaN products more affordable and thus more readily adopted by the mass consumer market. Hence, to be ready for these burgeoning demands, the focus for these wafer fabs will be to develop devices targeting high-power and high-speed switching applications which typically have operating frequency in the KHz to MHz range. At such a lower frequency regime, standard low-resistivity silicon substrates are still able to meet the technical requirements as the substrate loss is still acceptable. However, does that mean GaN-on-Si HEMT will only be limited to low-frequency applications? What about other applications such as >5G wireless broadband communications, or ultra-high frequency test systems, which need to operate at much higher frequency? To enable GaN-on-Si HEMT to extend its operating frequency capability, researchers worldwide have adopted high-resistivity silicon substrates which have lower substrate loss at higher frequency. Using this approach, extremely high frequency HEMTs have been reported. For example, a research team from Cornell University has recently reported a 55 nm HEMT fabricated on 200-mm-diameter 725-μm-thick high-resistivity (3000 Ω·cm) Si substrates with cutoff frequencies >250 GHz .
GaN-on-Si HEMT can now be applied to other applications such as >5G wireless broadband communications
Research in Singapore
In Singapore, the Nanyang Technological University (NTU)’s Electrical and Electronic Engineering (EEE) research team led by Prof. Ng Geok Ing has been working on GaN-on-Si technology since 2005. Recently, his team in collaboration with the researchers at SMART LEES are embarking on an A*STAR AME IRG project to develop a GaN HEMT for E-band (60-90 GHz) applications. Using a more advanced HEMT structure, namely InAlN/GaN, and with deeply-scaled 40 nm gate-length, the team has reported a record-high cutoff frequency of 310 GHz . The team has also reported an 80 nm GaN HEMT with high Johnson’s Figure-of-Merit (product of cutoff frequency and breakdown voltage) of 8.8 THz-V using a CMOS-compatible non-gold process . These results also rival those GaN HEMTs fabricated on more expensive silicon carbide substrates using non-CMOS-compatible gold-based processes. Thus in contrast to the early beliefs that GaN-on-Si is limited to low-frequency applications, these promising preliminary results have demonstrated the great potential of GaN HEMTs on high-resistivity silicon substrates using CMOS-compatible processes to extend their operation beyond 100 GHz, which will enable their use in many new application domains in the near future.
Fig.1. Transmission Electron Microscopy picture of the GaN-on-Si HEMT with gate length of 40 nm and source-drain spacing of 400 nm.
Fig. 2. Comparison of the achieved fT versus Lg of NTU’s devices with other published state-of-the-art GaN-on-Si HEMTs results.
- Li, K. Nomoto, M. Pan, et. al, “GaN HEMTs on Si with regrown contacts and cutoff/maximum oscillation frequencies of 250/204 GHz”, IEEE Electron Device Letters, vol. 41, no. 5, pp. 689-692, May 2020.
- Xie, Z. Liu, Y. Gao, K. Ranjan, K. E. Lee and G. I. Ng, “Deeply-scaled GaN-on-Si high electron mobility transistors with record cut-off frequency fT of 310 GHz”, Applied Physics Express, vol. 12, no. 12, pp. 126506-1–126506-5, Dec. 2019.
- H. Xie, Z. Liu, Y. Gao, K. Ranjan, K. E. Lee and G. I. Ng, CMOS-compatible GaN-on-Si HEMTs with cut-off frequency of 210 GHz and high Johnson’s figure-of-merit of 8.8 THz·V, Applied Physics Express, vol. 13, no. 2, pp. 026503-1–026503-4, Jan. 2020.
ABOUT THE AUTHORS
Geok Ing Ng
School of Electrical and Electronic Engineering, Nanyang Technological University
Hanlin Xie, Kenneth Lee
Low Energy Electronic Systems, Singapore-MIT Alliance for Research and Technology Centre