Transphorm Inc is a leading developer of HEMT GaN technology. They recently won a new DARPA research contract worth $1.4 million for research on nitrogen-polar (N-polar) HEMT GaN devices based on sapphire substrates.
According to reports, the new project builds on Transphorm’s history and continuous cooperation with the Office of Naval Research (ONR) to establish domestic resources and supplies for RF GaN epitaxial wafers, with a focus on N-pole GaN. This technology has been proven to bring more innovation. The big advantage is gallium polar (Ga polar) GaN, which is more commonly used today for radio frequency and millimeter wave applications. Compared with traditional Ga polar solutions on silicon carbide (SiC), Transphorm will explore the use of sapphire substrates to achieve higher cost efficiency of N polar GaN solutions. It is expected that the work output will produce a stable, high-quality thin epitaxial structure whose capabilities are established by high-performance transistors.
Transphorm’s team will meet the following planning goals for N-polar GaN-on-Sapphire: establish an overall value proposition, define a high-performance parameter space, and define the feasibility of constructing epitaxial wafers.
Transphorm focuses on the development of nitrogen polar GaN epitaxial wafers on various substrates including silicon carbide (SiC). However, the company now intends to explore an alternative to sapphire substrates that will be analyzed based on performance, cost and manufacturability for use in RF/millimeter wave radio technology.
Dr. Mishra, CTO and co-founder of Transphorm, said: “The goal now is to lay the foundation so that our RF epitaxy customers can achieve more efficient RF power in US dollars.” “For this purpose, sapphire is an attractive It’s a powerful material choice, but because of its low thermal conductivity, it has traditionally been ignored. We believe that through innovative engineering, the project team can overcome this limitation and are pleased to have the opportunity to set this benchmark for the GaN RF industry.”
N-pole GaN technology was originally developed under the leadership of Dr. Umesh Mishra of the University of California, Santa Barbara (UCSB) as a variety of high electron mobility transistors (HEMT). In 2007, Dr. Mishra continued to create Transphorm to further develop this technology. They further pointed out that N-polar GaN has extraordinary efficiency at frequencies up to 94 GHz, and its potential value in RF/millimeter wave applications and future power Electronic equipment is very attractive. It is ready to directly benefit DoD systems and 5G, 6G and higher applications.
GaN epitaxial wafers can be grown on a variety of substrates, including sapphire, silicon, SiC, and pure GaN. As expected, each substrate has advantages and disadvantages. The silicon carbide substrate is expensive and the wafer size is limited. In contrast, sapphire is said to be more cost-effective and less affected by temperature during the manufacturing process.
The biggest problem with sapphire is its lack of thermal conductivity.
“Historically,[蓝宝石] It has been overlooked because of its low thermal conductivity,” said Dr. Mishra. “We believe that through innovative engineering, the project team can overcome this limitation and are pleased to have the opportunity to set this benchmark for the GaN RF industry. “
Another problem Transphorm may need to overcome with sapphire is the lattice mismatch with GaN, especially when compared to SiC. This mismatch can adversely affect device performance.
A preliminary research paper (2007) led by Dr. Misra demonstrated the early potential of developing HEMT N-polar GaN devices. The device built by the team is grown on a C-plane SiC substrate and consists of a GaN/AlGaN/GaN heterostructure.
When developing the device, the researchers had to address several flaws, including pulsed large signal current collapse and gate leakage.
In 2013, a UCSB team published a second paper that discussed the application of N-polar GaN in RF/mixed signal applications. The advantages of N-pole devices over Ga-pole devices include a strong back barrier (which leads to better pinch-off characteristics), low resistivity, ohmic contact, and improved scalability.
According to the paper, the transition from phase-assisted molecular beam epitaxy (MBE) to metal organic chemical vapor deposition (MOCVD) epitaxy has led to the development of higher performance radio frequency devices.
In 2019, the Mishra Research Group (UCSB) demonstrated the record-setting performance of millimeter wave power transmission using N-polar GaN technology.
The team is said to provide low gate leakage, enhanced 2D Electron Gas (2DEG) and charge limiting, and DC-RF dispersion control for the progress made in the HEMT structure.
Fundamentally speaking, N-pole HEMT is said to have significantly better high-frequency millimeter wave characteristics than traditional Ga-pole devices. Specifically, it is said that it “breaks through” the POUT saturation point of Ga polar devices.
GaN-based semiconductors have begun to play an increasingly important role in the field of millimeter wave radio technology, including radar, power electronics, and lighting applications.
The performance characteristics of N-polar HEMT GaN devices certainly look promising. Transphorm’s growth indicates the market’s acceptance of its technology. Finally, establishing a semiconductor manufacturing base in the United States will help bring stability and reliability to the industry.