Polar: Another convert for GaN-on-silicon

Licensing technology from imec expands Polar Semiconductor’s GaN portfolio with the addition of E-mode devices operating at various voltages.

BY RICHARD STEVENSON, EDITOR, CS MAGAZINE

For many years, our industry has banged the drum for GaN-on-silicon power electronics. We’ve been championing this technology by highlighting the low on-resistance that boosts efficiency, and the capability to switch at very high frequencies, opening the door to smaller and lighter power supplies.

Recently, the compelling case for GaN has been winning converts in the silicon industry. They include Polar Semiconductor, a US-owned foundry that’s based in Bloomington, Minnesota, which is near Minneapolis.

Polar has a rich history within the semiconductor industry. Founded in 1962 under the moniker Control Data, it initially focused on the manufacture of ICs for the hard-disc-drive industry.

Construction of a 2-inch fab followed in the late 1960s, enabling the facility to operate as a captive foundry. Over the intervening years two additional fabs have been built on the site, wafer size has increased – including the introduction of a 200 mm line in the early 2000s – and ownership has changed hands. Sanken Electric acquired the company in 2005, prior to shared ownership, with Sanken accounting for 70 percent and Allegro the other 30 percent.

Among the many changes over the years, arguably the biggest is the recent transformation. This includes a take-over by private equity firms, and the drawing of funding from the US CHIPS Act that is designed to help drive Polar’s expansion into GaN power devices, and ultimately strengthen domestic production of GaN HEMTs within the US.

Supported by a total investment in the facility of up to $525 million, Polar has changed its role to a merchant foundry, and updated its 100,000 ft² facility. The injection of cash is aiding efforts to double production capacity from 20,000 wafers per month.

Helping to grow this capacity, according to Polar Vice President of Marketing, Ken Obuszewski, is the purchasing of about 150 new tools. “We have added automation and overhead transport,” says Obuszewski, who explains that visitors are impressed by the modernisation of the 200 mm fab.

Alongside new tools has been a rise in head count, increasing by about 20 percent to more than 600 staff.

Construction work on the site is now winding down, but at times this added another 200 workers to the facility, supporting the local economy.

Turning to GaN
Over many decades, Polar has solidified its position as an expert in power semiconductors. This includes discretes, such as MOSFETs and IGBTs, and a technology referred to as BCD, which is short for bipolar-CMOS-DMOS.

“We do BCD up to 900 volts, and even up to 1,200 volts; IGBTs at 1,200 volts,” says Obuszewski, who claims that GaN fits very well into this power legacy.

Polar also has a sensor portfolio, supported by Allegro’s leadership in Hall effect sensors. On that side of the business, the company is just starting a MEMS programme, and engaging with the photonics industry.

Driving Polar’s interest in wide bandgap technology is the opportunity to gain an edge over its silicon-centric rivals.

“Silicon discretes is a very mature technology, so it's harder to differentiate yourself. You're more likely to be competing on pure price,” says Obuszewski, who argues that by diversifying into GaN, Polar has the opportunity to develop unique expertise.

The company has decided not to broaden its capabilities through the addition of SiC, viewed as a more maturity technology that’s suffering from an over-investment in capacity.

What’s not in Polar’s DNA is developing its own technology from scratch, and bringing this to market.

“We've taken the smart approach to work with people who have expertise in GaN, and to leverage their learnings and apply it to our foundry.”

Polar enjoys frequent engagement with Renases and Imec. At times, engineers are on site, helping to solve more challenging problems.

“But it's not to the point where we need somebody co-located, constantly,” says Obuszewski. “We've been growing our own GaN team as well. We're now up to four integration engineers.”

At present, Polar does not produce GaN-on-silicon epiwafers in house. Instead, it sources from a third party. Due to this, there’s a need for incoming inspection, partly due to concerns related to bow. In this regard, Renases offers salient advice.

In addition, the Japanese chipmaker has helped to offer solutions to address the threat of contamination. This has been a concern, given that Polar is not dedicating a line to just GaN.

According to Obuszewski, what’s been the most challenging task to date is the development of the ohmic contact and its etching, to optimise for on-resistance.

Polar Semiconductor plans to generate 10 percent of its revenue from GaN-on-silicon D-mode and E-mode HEMTs by the end of this decade.

GaN foundries
Against a backdrop of growing sales for GaN-on-silicon technologies, the decision by TSMC to exit this business will have raised a few eyebrows.

For those offering these services, or intending to do so, the move by TSMC gives them more opportunity, since a strong ecosystem of GaN foundries is yet to be established.

Today there are not that many GaN foundries, according to Obuszewski. Off the top of his head, he lists Vanguard and Power Chip in Taiwan, the German chipmaker Global Foundries, and X-fab, which is based in the US.

“The world is bifurcating. Any large semiconductor company needs a China supply chain and a Western supply chain. We fit very well with the Western supply chain.”

From D-mode to E-mode
Thanks to the licensing agreement with Renesas, Polar has established a 650 V D-mode technology. The deal with imec is now broadening that portfolio considerably – as well as adding E-mode, there’s an expansion of the voltage range. Initially at 600 V and 100 V, there’s the potential to go further in both directions.

“The current 100-volt can scale down to 40 volts,” says Obuszewski, adding that this is then expected to reach to 24 V. “Low voltages are very interesting, especially around the data centre architectures.”

At the other end of the spectrum, there’s an opportunity to use imec’s technology for 1200 V devices.

“We're actually involved in an R&D programme that's funded by the US government for a lab-to-fab transfer to do 1200-volt.”

Another advance Obuszewski anticipates is the introduction of monolithic integration. This move, involving the introduction of logic components, would eliminate the requirement for a separate drive chip.

As well as those goals, Polar plans to move from outsourcing GaN-on-silicon epiwafers to in-house production.

Understandably, a ramp in GaN revenue is also on the roadmap.

“We have clear line of sight to 10 percent, by the end of the decade,” says Obuszewski. “But we have the opportunity to take that to 20 percent, based on the demand we're seeing.”