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What should engineers know more about GaN?

03 Dec 2014  | Janine Love

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Gallium nitride (GaN) found a strong foothold in RF/microwave applications, and several years ago was noted as a major trend at conferences such as the IEEE International Microwave Symposium. But where has it gone from there, and what are its future possibilities?

To answer these questions, I talked with teams at GaN Systems, Efficient Power Conversion (EPC),TriQuint Semiconductor, MACOM, and Element Six.

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 greater than 10W/mm 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. 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 per cent at 2.5GHz to 3.5GHz 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 2,000V in the lab. However, he points out that with current GaN on silicon technology, the breakdown voltage is limited by vertical breakdown through the silicon substrate.

"This gives us our current operating voltage limit of 650V," Patterson explains, "We anticipate increasing this to 900V or even 1,200V over the next couple of years with further substrate development." As for efficiency, Patterson says that users have demonstrated conversion efficiencies of 99 per cent with GaN versus silicon's less than 95 per cent.

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.

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