DARPA Commissions Transphorm to Develop N-polar GaN on Sapphire Substrates

October 15, 2021 by Adrian Gibbons

Disrupting the Ga-polar GaN technology tradition with N-polar GaN devices, Transphorm is evaluating the cost vs. performance of sapphire substrates with a new DARPA contract.

Transphorm Inc, a leading developer of HEMT GaN-based technology, has recently been awarded a new DARPA research contract, valued at $1.4M USD, to investigate nitrogen polar (N-polar) HEMT GaN devices based on sapphire substrates.

This research agreement follows a 2019 agreement valued at $18.5M USD to establish a domestic US-based source for GaN epiwafer production

Transphorm specializes in developing nitrogen-polar GaN epiwafers on a variety of substrates including silicon carbide (SiC). However, the company now intends to explore a sapphire substrate alternative, which will be analyzed based on performance, cost, and manufacturability for use in RF/mm-wave radio technology.


Transphorm’s product categories are available in several package types

Transphorm’s product categories are available in several package types. Image used courtesy of Transphorm


The N-polar GaN technology was originally developed under the leadership of Dr. Umesh Mishra at the University of California, Santa Barbara (UCSB) as a variety of high electron mobility transistors (HEMTs). In 2007, Dr. Mishra went on to found Transphorm to further develop this technology.


Why Transition From SiC to Sapphire?

GaN epiwafers can be grown on various substrates including sapphire, silicon, SiC, and pure GaN. As expected, each substrate has advantages and disadvantages. SiC substrates are costly and are limited in wafer sizes. Comparatively, sapphire is said to be much more cost-effective and less affected by temperature in manufacturing.

The biggest problem with sapphire is that it lacks thermal conductivity. 

“Historically [sapphire has] been dismissed as it has low heat conductance,” says Dr. Mishra. “We believe that, with innovative engineering, the program team can overcome that limitation and are excited for the chance to set that benchmark for the GaN RF industry.”

Another issue Transphorm may need to overcome with sapphire is the lattice mismatch to GaN, especially compared to SiC. This mismatch can adversely affect device performance. 


UCSB's Progress With N-polar GaN 

An initial research paper (2007) led by Dr. Misra demonstrated the early potential for the development of a HEMT N-polar GaN device. The device the team built was grown on a C-face SiC substrate and consisted of GaN/AlGaN/GaN heterostructures. 


Structure of the N-polar epitaxial layer

Structure of the N-polar epitaxial layer (2007). Image used courtesy of Mishra et al.


While developing the device, researchers had to address several deficiencies, including pulsed large-signal current collapse and gate leakage.


I/V curve shows significant dispersion under 80 µs pulse conditions  

I/V curve shows significant dispersion under 80 µs pulse conditions. Image used courtesy of Mishra et al.


In 2013, a second paper was published by a team at UCSB, exploring the application of N-polar GaN for radio-frequency/mixed-signal applications. The advantages of N-polar over Ga-polar devices included a strong-back barrier (which resulted in 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 led to the development of higher performance RF devices. 

In 2019, the Mishra Research Group (UCSB) presented a record-setting performance for mmWave power transfer using N-polar GaN technology.


The Importance of N-polar GaN for RF/mmWave Radio

The advances the team made to HEMT structure are said to provide low gate leakage, enhanced 2D electron-gas (2DEG) and charge confinement, and DC-RF dispersion control. 


N-polar deep recess structure

N-polar deep recess structure (2019). Image used courtesy of UCSB


Fundamentally, N-polar HEMTs are said to have significantly better high-frequency mmWave characteristics over traditional Ga-polar devices. Specifically, it is said to "breakthrough" the POUT saturation point for Ga-polar devices. 


Record setting 8W/mm power output at 94 GHz

Record-setting 8W/mm power output at 94 GHz. Image used courtesy of UCSB


GaN-based semiconductors have begun to play an increasingly important role in the fields of mmWave radio technology including radar, power electronics, and lighting applications. 

The performance characteristics of the N-polar HEMT GaN devices certainly appear promising. Transphorm's growth indicates market acceptance of its technology. Finally, the development of a US-based semiconductor manufacturing base could help bring stability and reliability to the industry.