Carolina's big guns board the GaN train

Much hope, and indeed hype, has long accompanied the prospects for wide-bandgap RF technology based on GaN and SiC transistors. This now seems set to become a commercial reality. Not only have industry big guns RF Micro Devices (RFMD) and Cree both launched a wide range of high-power GaN-based transistors, but their North Carolina neighbor Nitronex has cornered another $22 million in equity finance.

With all three now set to plow greater effort and cash into pushing the technology at customers in the broadband wireless access and infrastructure sector, 2006 looks likely to go down in history as the turning point for GaN microelectronics.

That s largely because Cree and RFMD are such key players in the compounds business, and have been central to the success of GaAs and SiC-based electronics. The success or otherwise of their GaN offerings may well determine the fate of the overall sector.

RFMD s involvement is arguably the most significant development. The Greensboro-based device maker is the world s major GaAs wafer producer and not a company that launches a range of new products based on a new material platform without a great deal of forethought. "RFMD is fairly conservative," admits Jeff Shealy, head of RFMD s wireless infrastructure unit, lending weight to the theory that GaN s time has now come. He says that while the market opportunity for GaN has been there all along, only now has the reliability of products reached a point where customers are serious about deploying it. Coupled with this is the widespread availability of 3 inch SiC substrates on which to manufacture GaN devices.

Proving reliability is perhaps the most critical problem that GaN has faced. Armed with a substantial data set from a long run of GaN-on-SiC wafers, Shealy is in the process of educating potential customers in the base-station business who want, unsurprisingly, to see watertight stats before they commit to a new line of products. Shealy believes that they are now finally waking up to the advantages that the wide-bandgap technology can bring, and that the cellular industry in particular is committed to backing GaN technology for applications in wideband-CDMA base stations.

Shealy says that RFMD is looking to compete in the high-end cellular sector for final-stage, linearized amplifiers. Already sampling to the top-tier vendors of cellular base-station equipment, its new family of GaN HEMT transistors features nine different products. Transistors for cellular applications include four HEMTs, the most powerful being a 120 W device. On the WiMAX front, RFMD launched new 50 W products for both the 2.5 and 3.5 GHz frequency bands.

Of the three North Carolina companies, RFMD looks to have the best chance of success within the cellular business. Crucially, it is in a position to exploit its experience in high-volume GaAs manufacturing to give it the edge over a relative upstart such as Nitronex. And although Cree also has plenty of volume-manufacturing expertise thanks to its great success with GaN-based LEDs, it is not nearly so well established in cellular infrastructure. "RFMD has great customer relationships and a qualified high-volume facility," said Shealy. Although its GaN process will need individual qualification, which is set to begin in September, cellular customers are already convinced that RFMD can deliver as a company.

Having now converted much of its 4 inch GaAs capacity to 6 inch, RFMD has been able to apply its smaller manufacturing line for back-end processes such as metalization and lithography to 3 inch GaN-on-SiC without the need for costly redevelopment. "We plan on competing [for this market] in the context of our GaAs fab," said Shealy. "It means that GaN looks no different to any other wafer."

A key difference is at the epitaxy stage, where RFMD uses MOCVD for GaN deposition rather than MBE. The latter is its standard method for GaAs wafer epitaxy. The company uses both internal and external sources of GaN epitaxial material and has two MOCVD machines of its own. Durham-based Cree is also targeting WiMAX and cellular applications with its own family of 2.4-3.9 GHz HEMTs. At the MTT-S exhibition in mid-June, Cree released higher-power versions, including a 120 W device.

Meeting as competitors for the first time, one side issue that Cree and RFMD might need to resolve is the supply of SiC substrates. Despite the locality of the two companies and its dominance of the SiC market, Cree is not currently RFMD s primary supplier of wide-bandgap material. Even though the two will compete directly for the first time with their GaN products, the market for these new devices is at such an early stage that for Cree to supply RFMD with material should be viewed as a minor conflict of interest.

Nitronex, based just down the road from Cree in Raleigh, is the young pretender that stands against the two established chip makers. The key difference is its technology, GaN-on-silicon. The thinking here is that silicon wafers can already be scaled much larger than SiC, and ultimately provide a more economic materials solution with tougher thermal problems dealt with through innovative packaging solutions. Having recently sealed an additional $22 million in venture capital, Nitronex is poised to move into a new state-of-the-art facility in early 2007, in preparation for device production.

While acknowledging the potential threat posed by its near neighbors, Nitronex CTO Kevin Linthicum is happy to see their involvement: "Commercial customers like Nokia and Siemens will want more than one supplier," he said. "The GaN train is leaving the station and Cree and RFMD don t want to be left behind."

Of course, the relative merits of the North Carolina trio will be rendered irrelevant if silicon LDMOS continues to dominate the infrastructure scene. As always for III-V developers, silicon will pose the biggest threat of all.