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CREE - The Ins And Outs Of Silicon Carbide

16 - 04 - 2020

Cree’s CTO explains the different characteristics of silicon and SiC and where each one works best.

Semiconductor Engineering to talk about silicon carbide, how it compares to silicon, what’s different from a design and packaging standpoint, and where it’s being used. What follows are excerpts of that conversation.

SE: SiC is well-understood in power electronics and RF, but is the main advantage the ability to run devices hotter than silicon, or is it to save energy?

Palmour: The goal is to save energy and reduce system costs. Silicon carbide saves the OEM money.

SE: Right up front?

Palmour: Yes. For instance, if you say, ‘Okay, I can put in silicon carbide, which is more expensive than an IGBT but I can save three times that on battery cost, that’s what they do.’ More often than not being used for upfront cost.

SE: But that’s not necessarily a one-to-one saving on material. It’s more about the system cost, right?

Palmour: Yes, absolutely. Silicon carbide is more expensive than silicon IGBTs, and the places we get our wins is where they realize the savings at the system level. It’s almost always a system sell.

SE: Has that slowed the adoption of SiC?

Palmour: You have to find the applications where you save money at the system level. But as you do that and start shipping volume, the price comes down and you start opening up other applications. In the past, the limiting factor was the up-front cost, but people are starting to look a lot more at system costs and they realize the up-front cost from that perspective is better with silicon carbide.

SE: How about availability of SiC versus silicon?

Palmour: If you’re an automotive OEM, you do worry about capacity because the impact of these automotive designs will be to drive the market to become a lot larger than it is today. Assurance of supply is a concern. That’s why Cree announced numerous wafer supply agreements with other companies that make silicon carbide devices. We did an announcement with Delphi, where we sell chips to Delphi and they sell an inverter to a European OEM. Those things are getting looked at, and you have to lock in supply. On these long-term purchase agreements, we have to know that demand will be there before we invest a lot of capital for capacity. We announced last year we’re adding $1 billion of CapEx to greatly increase our capacity to meet this need. It’s required, and it’s just a start. If you run the numbers on the penetration of battery electric vehicles to the overall vehicle market, this is just beginning.

SE: Is this all 200mm, or is it older technology?

Palmour: The bulk of all production today is on 150mm 6-inch wafers. There is still some on 4-inch. We’re building a new fab in New York that will be 200mm-capable, but we’re not doing any 200mm today and aren’t expecting to be ready for that for several years. When 8-inch is ready, we can turn it on. The equipment is all going to be 200mm so that we can rapidly move it over to 8-inch when the time is right. There is no 8-inch in production today.

SE: Is the process radically different from silicon chip manufacturing? Does it utilize the same tools you would normally use?

Palmour: If you’re talking materials growth, it’s different. Crystal growth is radically different. Wafering, polishing, epitaxy are all quite different. But once you get into the fab, it’s fairly standard equipment with the exception of two or three processes, which are heavily tailored to silicon carbide. The fundamental fab processes are very silicon-like, and the bulk of the clean-room equipment is typical silicon equipment.

SE: How about on the test and inspection side?

Palmour: Those are quite similar to silicon.

SE: Because SiC is run at higher temperatures, is defectivity more of a problem?

Palmour: The reason silicon can’t go to very high temperatures is because intrinsically it starts to conduct. It really stops being a semiconductor around 175°C, and by 200°C it becomes a conductor. For silicon carbide that temperature is much higher — about 1,000°C — so it can operate at much higher temperatures. But we’re not targeting much higher temperatures than silicon because of the packaging. The higher the temperature at which you rate your package, the larger the delta T between low temp and high temp and the faster your package can degrade. We’re not going for radically higher temperature. And in fact, because we’re efficient, we actually don’t get that hot on a per-square-centimeter basis. Our chips are typically going for about 175°C, which is not all that much higher than silicon.

SE: That puts SiC into the ASIL D category for automotive or industrial applications, right?

Palmour: Yes, absolutely.

SE: What’s different on a physics level?

Palmour: Silicon has a bandgap of 1.1 electronvolts, and that is basically the definition of how much energy it takes to rip an electron out of the bond between two silicon atoms. So it takes 1.1 electronvolts to yank an electron out of that bond. Silicon carbide as a band gap of 3.2 electronvolts, and so it takes 3 times more energy. But it’s actually an exponential function. A lot of the characteristics of semiconductors bandgap are actually up in the exponent. We’ve got three times wider bandgap, but when it comes to electric breakdown we actually have 10 times higher electric breakdown field.

SE: What does that mean in terms of real-world applications?

Palmour: It means that if you make the exact same structure in silicon and silicon carbide — the same epi thickness, the same doping level — the silicon carbide version will block 10 times more voltage than the silicon version. You can make a MOSFET in silicon and you can make a MOSFET in silicon carbide. MOSFETs in silicon are very common in the low-voltage region, from 10 volts up to about 300 volts. Above 300 volts, the resistance of a silicon MOSFET gets very very high and it makes the MOSFET unattractive. It’s too expensive. So what they do is they switch over to a bipolar device. A MOSFET is a unipolar device, meaning there’s no minority carriers. There are only electrons flowing in the device. And when it’s a unipolar device, it can switch very, very fast. If you look at a 60-volt MOSFET, it switches very fast, and that’s, that’s why you can make gigahertz processors in silicon. They’re very low voltage MOSFETs — maybe 5 volts. But when you get up higher in voltage you have to go to a bipolar device, meaning that both electrons and electron holes are flowing in the device at the same time. And every time you switch, you have to dissipate all those electrons and holes recombining and generating energy. The bipolar device gives you much lower resistance and a much smaller, more affordable chip, but you’ve got to dissipate that excess heat every time you switch. That’s the tradeoff you’re making. You can make an affordable power switch, but it’s not very efficient.