The GaN Plan

Companies develop ways to grow gallium nitride on silicon; market expected to jump over next couple years.

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By Ann Steffora Mutschler
Given the need to control power in high-end market segments such as servers, notebooks, mobile handsets and wired communications equipment, the market for gallium nitride (GaN)-based chips is poised for explosive growth.

Case in point: Server farms are using more and more electricity, and the cost of that power is getting to be a significant fraction of the operating cost of major companies like Yahoo and Google. If $65 billion in servers are purchased this year, it is estimated that the cost of the electricity to power and cool those same servers will be $45 billion. And, in a typical server, of the 450 watts consumed, 163 watts come solely from the delivery of power through the AC-DC and DC-DC power supplies.

Clearly, extensive power management is needed, which is precisely what GaN promises. For this reason, iSuppli Corp. projects that the GaN market will reach $183.6 million in revenue by 2013, up from essentially nothing this year.

Market researchers at Strategy Analytics are more optimistic, viewing this year as the start of a ramp-up of GaN microelectronic products from multiple vendors for commercial applications, including cable TV and power management. The firm forecasts the market for GaN microelectronic devices will grow to approximately $375 million by 2014.

Until now, GaN has had success in the RF space in products made by vendors such as Nitronex Corp. and Freescale Semiconductor. iSuppli pointed out that now is prime time for GaN, given that the use of silicon has reached its practical limits in power management semiconductors and that major breakthroughs have taken place in terms of growing GaN layers on silicon.

Component suppliers with their feet already in the water are International Rectifier, which announced its first GaN technology-based Point-of-Load (POL) products in February, along with Efficient Power Conversions Corp., which is headed up by former IR CEO Alex Lidow. Efficient Power announced 10 power MOSFET devices last month.

Why it matters
What’s so great about GaN and why is it important for low-power applications?

“The benefits of GaN in low power are density, efficiency and cost. Ultimately, it gives you a better trade-off for all three of those compared to silicon and the key is to make it below a certain cost threshold and growing GaN on silicon allows you to get below that cost threshold. That’s the exciting part about it,” noted Tim McDonald, vice president of the emerging technologies group at International Rectifier.

McDonald admits that today GaN benefits are limited to a couple of applications where it is cost effective. But he added, “Over time, as we mature the technology and the cost goes down and the performance goes up, that performance/cost ratio will cover a lot of applications.”

“At the low power end, one of the benefits that GaN has is there’s a very high ratio between the ‘on’ current and the ‘off’ current, at least with the device that we’ve demonstrated—something like 11 or 12 orders of magnitude, and that compares with 6 or 7 orders of magnitude for silicon. What that means is that in the ‘off’ state it can be consuming a lot less silicon for its ability to process a certain amount of current. So if people are really concerned about power sensitive applications, then GaN has the potential to be very beneficial,” he continued.

Changing materials
In terms of manufacturing GaN devices, given that it is composed of a different material than silicon, it has its own set of properties that drive the benefits discussed above, McDonald explained. As such, when comparing the physical properties of GaN versus silicon, the actual device structures are quite different. GaN has a traditional power MOSFET structure for low voltage and has some form of a MOSFET—literally with a source, a drain, and a gate—with diffusions that are used, he said.

The GaN device is what’s called a HEMT (high electron mobility transistor), which basically works on a sea of electrons in a very narrow region that can conduct electricity. That makes its device physics quite different compared to a silicon device.

“This gives a lot of benefits to the application because it has a great combination of a figure of merit called RQ (resistance x gate charge) and of the switching: it is very superior to silicon in that regard,” he said. “From a processing point of view, it’s a very simple device because there are no diffusions. The really important part is growing GaN on top of silicon and in that area.”

There are some challenges in growing GaN on silicon, though. First, GaN expands and contracts at a different rate in temperature than silicon does, i.e., the thermal coefficient of expansion is different. This is important because the film of GaN on top of silicon tends to be grown at 900 or 1000 degrees, and by the time it cools there are two films that are expanding at a different rate. As a result, the GaN wants to crack because it is a big mismatch with silicon. Second, the crystal lattice of silicon and of GaN have different interatomic distances, so defects tend to create at the interface between the two.

The benefit of GaN on silicon is low cost, of course, because a low-cost silicon substrate is used, but all the magic goes into being able to grow that GaN on top of the silicon to overcome those two effects, McDonald said. “A lot of the work we’ve done has been focused in getting good, high quality, low cost, and very reliable GaN-on-silicon epi organic chemical vapor deposition growth to solve those two problems.”

Specifically, IR has done proprietary work on how to transition from the silicon—having layers and structures to transition from the silicon—to the electrically active GaN region. “Those transition layers have to function to overcome the two limitations of the thermal coefficient of expansion mismatch and the lattice mismatch,” McDonald added.

To really find its niche, GaN devices must have improved efficiency and small form factors enabled by the material to address the power management needs of portable electronic products such as mobile PCs and smart phones, as well as enterprise servers and wired communications infrastructure equipment, according to Marijana Vukicevic, iSuppli’s principal analyst for power management.

Adoption of GaN technology for these applications this year and next year are expected to be slow due to the high cost of parts using the material. However, as the technology advances and the cost of manufacturing GaN technology drops in 2012 and 2013, the technology will begin to take market share from conventional MOSFETs, driver ICs and voltage regulator ICs, Vukicevic said.

She concluded that the first adoption of GaN devices most likely will be among servers, which always demands high-performance devices and often are one of the first product areas to accept new technologies that improve performance. Then, over the next three years, the bulk of GaN device volume will be driven by notebooks, as the power savings and smaller form factor delivered by the technology is expected to be in high demand.