Transphorm: Faster FETs coming soon

Transphorm is set to commercialise GaN epiwafers with a difference, reports Rebecca Pool.

As Transphorm adds more and more GaN FETs to production lines, it recently revealed it is set to manufacture a new kind of transistor technology.

In a $15.9 million project with the US Office of Naval Research, the US-based GaN device manufacturer will fabricate nitrogen polar GaN epiwafers, with a view to providing the first commercial source of material for domestic RF and power electronics markets.

As Transphorm co-founder and chief operating officer, Primit Parikh, tells Compound Semiconductor: “Now the science has been proven, the Department of Defense wants a commercial source of this material.”

“Many of its clients are working on GaN devices, so a manufacturing-scale supply of the nitrogen-polar GaN epitaxial materials is going to be important,” he adds. “We already have multiple MOCVD reactors but through this program we will be able to add to our existing capability.”

Making progress

While high voltage Ga-polar GaN devices are making in-roads into myriad RF and power electronics applications, N-polar versions have lagged in development. When growing GaN on a foreign substrate, the material naturally nucleates to form Ga-polar GaN. What's more, the N-face of GaN is less thermally stable than the Ga-face making subsequent N-polar material growth on N-polar GaN more difficult.

However, researchers from the US-based University of California, Santa Barbara, led by Professor Umesh Mishra, and also sponsored by the US Office of Naval Research (ONR)and Defense Advanced Research Projects Agency (DARPA), have spent more than a decade honing this technology. And crucially, they have now demonstrated mm-wave devices with, as they claim, record power densities and high efficiencies.

“[We have reversed] the N-polar orientation of the material from the traditional Ga-polar GaN, currently used in base station and Department of Defense applications,” highlights Mishra. “This flip produces radical benefits in output power along with groundbreaking efficiencies to frequencies as high as 94 GHz.”

“Applications span the frequency range of interest for 5G and 6G, and beyond, and also fill a critical technological void for DoD systems,” he adds.

Details on the polarisation engineering behind the development of N-polar structures and devices are scant, but Parikh is confident that he and colleagues can now make the material manufacturable on a commercial-scale.

“A lot of research has gone into producing a good quality material that can be manufactured on large area wafers, and this is the crux of what Transphorm is now doing with this contract,” he says.

According to the CTO, Transphorm will grow N-polar GaN layers on SiC and sapphire substrates for RF applications, but switch to silicon for power electronics applications. “We are working with GaN-on-silicon as power electronics devices [follow] a different kind of performance-cost roadmap ,” he says. “And because of the SiC shortages we also want to work on developing this with sapphire substrates as well [for RF applications].”

Substrate sizes will include four and six inch wafers, and ultimately eight inch wafers, although work on the largest wafer size will be, as Parikh says, be dictated by market demand.

“In the future, [development on eight inch substrates] will be market driven and this is going to take several years,” he says. “At Transphorm, when we say we are ready, we mean we are ready for manufacturing and not just demonstrations, so work has to be done but this is a natural progression and all the proof-points exist.”

So what now? According to Parikh, technology and manufacturing will be prioritised in the project's first year, with production scaling following. “We will be scaling up our uniformity and yield, and demonstrating this, especially on the RF side,” he says.

Come the end of the project, 2022, technology and manufacturing maturity will have transitioned from the DoD's early-stage Technology Readiness Levels (TRL) and Manufacturing Readiness Levels (MRL) to the latter, proven levels of MRL8, 9 and TRL8, 9. And after this time, the mature devices will undergo qualification.

“We expect that after the project, the devices made by our customers will get traction and continue to mature as more and more devices are qualified and released,” says Parikh. “The government's long-term vision is to see these devices making their way into systems.”

And crucially for Transphorm, the project allows the company to not only expand MOCVD production of GaN-based devices but also develop vertical epiwafer sales for DoD customers as well as RF and 5G markets.

“We are already seeing demand and are excited to go from purchase to production in less than 36 months, a key program goal,” says Parikh.