GaN-on-SiC pushes RF and power performance
In a new headquarters in Linköping, Sweden, just down the road from the university, SweGan has reached a key milestone. Delivering 150mm (6in) wafers with its gallium nitride on silicon carbide (GaN-on-SiC) technology opens up high power, high frequency RF and power applications for chip makers.
GaN has been increasingly popular for chip designers with a layer of GaN on a silicon wafer but the current technology suffers from problems, says Jr-Tai ‘Ted’ Chen, chief technology officer and co-founder.
“In the power industry silicon has been the leading material for many years but now with high voltage devices people are convinced that wide bangap devices can do better,” said Chen. “In the market you can see SiC has a successful story. GaN fits in with medium power around 600 to 900V and the leaders in silicon have naturally moved to GaN on silicon as the substrates are cheap and that allows them to have vertical integration, so 99 percent of GaN devices are GaN-on-Si.”
“If you look at the quality of GaN on silicon there are still a lot of defects and the big question is reliability. This is where we can do a much better job with GaN on SiC – we use a different growth scheme. We have been developing this technology for ten years,” he said. “GaN on Silicon has to grow 5µm thick to get good quality but underneath you have the defects but a SiC layer is 2µm. Now for us it’s a 200 to 250nm layer as we can grow better quality with less defects.”
This thin epitaxial layer shows a much higher breakdown strength, four times higher than silicon. “It’s not the thickness, it’s about the quality,” he said. “In our structure we put the channel much closer to the SiC substrate and this helps to dissipate the heat.”
Next: Gan-on-SiC substrate technology
“At the same time you can see under the channel we have an aluminium nitride (AlN) layer and this has the highest bandgap. This back barrier helps confine the electrons inside the channel and this is another way to reduce the thickness,” said Chen.
SweGan takes SiC wafers from suppliers, and adds its own GaN technology on top. It sells these substrates to chip designers who then produce GaN devices in a foundry.
There are advantages to the SweGaN substrate technology that bring down the cost of devices. “With the higher breakdown you can make the device smaller so the cost per device might be cheaper. We have the high mobility of GaN which reduces the dimension of the device and the size of the passive components in the system.”
Because there is a thin epitaxial layer of GaN on the SiC substrate the devices have to be diffused laterally rather than vertical. “The [device] footprint will be larger than vertical but at the same time we have a chance to make it smaller from the quality of the material,” he said.
“The second thing is the yield – the GaN-on-Si yield is still around 60 percent and so this is still problem. With GaN-on-SiC we expect the yield to be much higher as the substrate is easier than silicon to handle – GaN-on-silicon is very fragile,” he added.
“Today we have many supplier of SiC wafers worldwide but I think in the long term we will try to have our own SiC production,” he said. “Cost has been a concern – we expect our material will be in qualification for RF, and the next step is for power.”
“We don’t want to compare ourselves to GaN on Si – the performance at 600V seems high but this voltage can be provided by superjunction and SiC power devices which is why GaN on Si has struggled,” he said. “We target 900 to 1200V for electric cars. Yes it costs more than Gan on Si but in the long term that’s not necessarily an issue as the cost of the substrate will reduce. The cost of a wafer has reduced significantly over the last three years and the substrate cost should be similar to SiC.
The first step is GaN-on-Si substrates for RF devices.
“Today we haven’t done mass production for power but for RF there’s a certain volume – the major cost is the substrate – with 4in wafers the cost reduced 30 to 40% from $2000 three years ago to $1000 today or even lower and we expect the same thing to happen with 6in. It was quite easy to scale from 4in to 6in.”
The next step of course is 200mm (8in) wafers that are the mainstream of foundries but 200mm SiC wafers are not yet widely available. “I think for SiC will be 8in standard in 2 years – its ongoing, but not a standard yet, 6in is the mainstream today. We will definitely look at it for power but we can make a significant impact with 6in wafers.”
SweGaN is looking at developing its own 200mm SiC wafers. “For the epi we can reduce a cost a lot for the volume production but the major cost is the substrate and this is where we are looking at development,” he said.
The next step is GaN-on-SiC wafers for power devices.
“There’s quite a lot of R&D we have to do for power and RF, there are fine adjustments and tweaks we have to do. For RF we will have to make it thinner and provide better confinement for very high frequency devices, For power the structure we have is enough but we have to work on the cost,” he said. “The volume orders for RF will help reduce the cost of the substrate and as we increase in volume that allows us to drive the cost down.”
Related GaN-on-SiC articles
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