“Gallium nitride power transistors are poised to move out of the labs and into mains PSUs and motor drives,” was one message from the PCIM power show in Nuremberg this month.
“Only if the price drops and designers can bear the normally-on characteristic,” came the repost from the makers of the silicon super-junction mosfets and silicon IGBTs that currently dominate these markets.
“First-generation GaN on-resistance is substantially better than super-junction is now, and GaN should be able to come down one or two orders of magnitude,” Dr Mike Briere, CTO at International Rectifier, told Electronics Weekly – adding that GaN devices are far faster, which can be used to reduce switching energy loss.
IR was early into GaN power transistor research.
Briere has figures from two 400kHz 200W (300V in to 30V out) quasi-resonant ‘LLC’ converters, one using GaN and one using Si power devices.
“The GaN one was 3% better at 200W and 17% better at 20W,” he said. “In applications which are at 10% load most of the time, this is meaningful.”
microGaN is a German GaN transistor start-up.
“GaN makes for a fast switching device. You can go to several hundred kHz and there is no tail current. Dynamic loss is one tenth compared with silicon,” said microGaN business development manger Dr Ertugrul Sonmez.
The firm has an R&D line making transistors, and intends to have a pilot line in the next year.
“Right now, we are supplying project partners with devices,” added microGaN production director Dr Ulrich Heinle.
GaN Systems is another GaN power transistor start-up, this time from Canada.
“We are sampling key development partners now, and have thermal and electrical models,” said CEO Girvan Patterson. “We have done a lot of work at 600V, and at 1,200V we are probably nibbling at the IGBT market.”
Firm’s with experience in GaN RF transistors, including RFMD, are also showing interest in making GaN transistors for PSUs.
All the performance in the world would remain academic if GaN still had to be grown on expensive exotic wafers, but now GaN epi-layers can be grown on silicon wafers.
IR’s GaN transistors are made on an in-house GaN-on-Si process, and companies including EpiGaN and Azzurro Semiconductors are setting up to become independent GaN-on-Si wafer suppliers.
Having spun-out from Belgian research lab IMEC, “we are now shipping wafers from our own reactors in our own clean room”, said EpiGaN CTO Dr Joff Derluyn. “We are receiving orders continuously. All our customers are in R&D, and there are some indications that customers are moving to production, but it is difficult to say when.”
For various reasons, effective GaN power transistors have to be depletion-mode (normally-on) ‘high electron mobility transistors’ (HEMTs).
Normally-on devices need an inconvenient negative gate drive to turn them off, and do not fail-safe when control power fails.
Connecting the GaN HEMT in cascode with a low-voltage silicon mosfet is a simple way to make a normally-off three terminal device that retains much of the GaN benefit while accepting mosfet drive voltages.
Preserving as much GaN dynamic performance as possible requires careful selection of the mosfet, and controlled local parasitics.
IR, with its extended experience in both GaN HEMTs and Si mosfets, is
though to be developing co-packaged cascode pairs to ease acceptance amongst designers.
Although, “I think, in the end, people will direct drive the GaN depletion-mode devices directly”, said IR’s Briere.