GaN: The dawn of a new era?
In comparison to silicon and GaAs, GaN compares favourably in terms of power density and power levels, but it is not without its own technical limitations. Noting that GaN power transistors are now capable of power density >10 W/mm² and power levels in excess of 500W, Douglas H. Reep, PhD, senior director of research at TriQuint Infrastructure and Defense Products says, “In a theoretical sense, the technical limitations on GaN are strictly the fundamental material properties and our creativity utilising them.”
Reep suggests that the most important factor to consider with GaN is the relationship between transistor speed and operating voltage, as captured by Johnson’s figure of merit. He notes that in this type of comparison, GaN demonstrates an order of magnitude advantage over GaAs, and two orders of magnitude over silicon. He adds that R&D for GaN is frequently focused on thermal management at the semiconductor and package level.
With regard to high voltage parts, GaN Systems recently announced five normally-off 650V GaN transistors optimised for high-speed system design. These 650V devices have reverse current capability, zero reverse recovery charge, and source-sense. (Earlier this year, the company announced 100V GaN power transistors, as well.)
Current technical limits involve pushing operating voltages higher while maintaining reliability, according to Tim Boles, distinguished technology fellow, MACOM (see MACOM commits to GaN at IMS2014). Boles articulates a desire to try to push to higher voltage bias points in order to achieve higher power added efficiency (PAE) and increased power density. “PAE in excess of 70% at 2.5 to 3.5 GHz can reasonably be achieved,” he says. “It is expected that higher bias voltages will improve this parameter.”
Girvan Patterson, president of GaN Systems, agrees that voltage breakdown is key for GaN. He reports that by using GaN on SiC, they have demonstrated more than 2000V in the lab. However, he points out that with today’s GaN on silicon technology, the breakdown voltage is limited by vertical breakdown through the silicon substrate.
“This gives us our [present] operating voltage limit of 650V,” Patterson explains, “We anticipate increasing this to 900 or even 1200V over the next couple of years with further substrate development." As for efficiency, Patterson says that users have demonstrated conversion efficiencies of 99% with GaN versus silicon’s <95%.
And then, there’s the heat. Felix Ejeckam, head of defence and aerospace business at Element Six Technologies, notes that the present limitation of GaN transistors is that the intrinsic power density maximum cannot be realised until heat can be effectively extricated from the heat-generating junction. Until heat is fully extricated, parameters such as power, efficiency, size/weight, and reliability will suffer significant penalties.
Applications
Today, those working in GaN are perhaps looking less at it as a substitute for silicon or GaAs and more as a unique material that will enable new applications. As a result, GaN is a material of interest for high frequency, high voltage, and high power density applications.
Alex Lidow, CEO of Efficient Power Conversion (EPC) and keynote speaker at DesignCon 2015, notes that new applications are emerging daily. He points to high-volume envelope tracking and LIDAR, as well as recent volume ramp ups in DC-DC converters for servers, class D audio, wireless power charging, and medical applications. Enhancement mode GaN transistors have shown a high tolerance for radiation such as experienced in space.
GaN Systems’ Patterson feels that the number of applications that will benefit from GaN is enormous. He points to efficient power conversion in the alternative energy market, electric and hybrid vehicles, transportation, and high-efficiency power supplies, noting that these applications are an excellent match for the 98-99% efficiency that can be achieved with GaN. He also observes that this year has seen growing interest from designers in the consumer sector. “For example, within the next 12 months we are likely to see the introduction of a new generation of TVs that are far slimmer in depth than current models, due to the very significant space savings enabled by using GaN power transistors,” Patterson predicts.
In the RF sector, GaN on diamond is seeing some traction as RF power amplifiers for cellular base stations and radar for military applications, according to Element Six’s Ejeckam. He also sees interest in GaN-on-diamond for very high-voltage power (several kV) inverters used in wind/PV grid generators.
GaN-on-diamond wafer uses Element Six’s synthetic diamond. Source: Element Six Technologies
Enabling new designs
If we want to look beyond GaN as a replacement for other materials, then the question becomes, what will GaN enable designers to do that they couldn’t before?
“Simply put, GaN enables designers to go to faster switching speed and higher frequencies. This results in smaller products (think diagnostic systems in pill form), higher resolution sensors (think autonomous vehicles), and lighter weight products (think micro-satellites). All are possible today. As GaN improves, these products will be more ubiquitous,” says Lidow.
GaN enables operation at significantly higher voltages compared to alternative technologies. Higher bias voltage translates to higher power added efficiency, lower parasitic reactance, and wider operating bandwidths. In addition, MACOM’s Boles observes that GaN has an unparalleled combination of electron mobility and saturated drift velocity, enabling operation at frequencies and power levels unachievable by any other technology today. “The final result,” says Boles, “will be that GaN will enable designers to conceive of and realise very complex microwave and MMICs that are not possible today. It is expected that complex, fully integrated high-power and high-frequency MMIC solutions will be achievable within the next two to five years.”
Reep concurs with the potential for more RF/microwave/mm-wave applications, noting that GaN allows designers to reach higher levels of RF power with reduced DC power, circuit area, component count, and, ultimately, expense. “The systems that take advantage of this are wide ranging today and will only expand in the future – a key part is the dramatic expansion of communication density between both people and things. The Internet of Things needs GaN capabilities to reach full potential,” says Reep.
Zeroing in on a specific GaN technology, Ejeckam explains that GaN-on-diamond enables GaN engineers to triple the RF output power density of a power amplifier while simultaneously reducing the GaN’s junction temperature rise by 40-50%, as compared to industry-standard GaN-on-silicon carbide materials. “These benefits have dramatic downstream effects on reliability, system energy consumption, system costs, system size/weight, and overall performance,” he says, noting that engineers at Raytheon Corporation, AFRL, and others have announced these “3X” power-density and thermal benefits in recent months/years.
Advice from the experts
So, what should EEs designing new products know about GaN?
“GaN will displace silicon in power conversion. To avoid obsolescence, EEs need to be gaining experience in GaN now.”—Lidow
“GaN outperforms all other RF materials for very high voltages (scores to hundreds of V), high power density (tens of kW/sq.cm), and high-frequency (GHz) applications. GaN-on-Diamond based systems outperform all other types of GaN by many fold, in particular GaN-on-SiC and GaN-on-Si – even after including the added material costs of diamond.”—Ejeckam
“GaN is available today and useful in largely the same manner as GaAs. However, the reliability of GaN is greater than two orders of magnitude higher than GaAs, which revolutionises field performance and life cycle costs. So EEs shouldn’t constrain new products by thinking about GaN as just a GaAs replacement. Knowing the capabilities of GaN can enable a product you didn’t conceive of before.”—Reep
“While both GaN-on-SiC and GaN-on-Si have demonstrated impressive performance levels at microwave and mmW frequencies, EE designers should be aware that this technology is still maturing. Using both silicon and especially GaAs as examples, it shouldn’t be surprising that process, design, and even basic structure improvements are still evolving. For the short term, designers should take a more conservative approach to designing for applications that are based on the existing accepted GaN topographies, device structures, and processes. In addition, longer term projects might well require the designer to have the flexibility to employ more aggressive R&D concepts, i.e. GaN-on-Diamond, more exotic band gap solutions to the Schottky barrier layer configuration, etc., to meet the final system specifications.”—Boles
Looking ahead
According to a report by MarketsandMarkets, “The entire GaN semiconductors market is expected to grow robustly at a CAGR [compound annual growth rate] of 22.2% over the next eight years, i.e. from 2014 to 2022, at a CAGR in power semiconductors being more than 60.5%.”
Clearly, there is great deal of potential for GaN across a broad range of applications. Of course, as with any technology, technical hurdles remain. And, as with any new technology, perhaps the foremost hurdle is cost. GaN also would benefit from some thermal management improvements and increased integration. But, perhaps what it needs more than anything else is a bit more “time in the saddle,” proving its worth, reliability, and longevity, thus giving engineers the confidence to select it for their next design.
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