The Ins And Outs Of Silicon Carbide

John Palmour, CTO at Cree, sat down with 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.

Fig. 1: SiC MOSFET. Source: Cree

SE: How about with SiC?

Palmour: Silicon carbide has a 10 times higher breakdown field. Our 600-volt MOSFET is going to be as fast as a 60-volt silicon MOSFET. The other way to look at it is if you say 600 volts is the voltage at which you switch from MOSFETs and silicon over to IGBTs, we would be at 10 times higher voltage. So you would use a MOSFET in silicon carbide up to 6,000 volts before you had to switch to an IGBT. The high electric breakdown field that we get from this wide bandgap allows us to use the device type that you would want to use in silicon, but you can’t because it’s too resistive to make it practical. So you can make the device in silicon carbide that you really wanted in silicon, but due to the physics of silicon it isn’t practical in that voltage range.

SE: Does the silicon carbide age the same as silicon due to the higher voltage?

Palmour: It’s the same. Voltage doesn’t matter. It’s the electric field, which is the same regardless of the voltage. Silicon carbide is very rugged, and it doesn’t age any differently than any other semiconductor.

SE: Will there be economies of scale as SiC gets used in more places?

Palmour: Yes. It will be a little more asymptotic than Moore’s Law because of the thermal considerations, but we are definitely early in the cost-down curve. From 2017 to 2024, we expect volume to increase by 30X. That will have an impact.

SE: Any constraints that could disrupt that increase in volume?

Palmour: Silicon carbide is sand and coal. Silicon and carbon are two of the most abundant elements on earth. It’s not like indium phosphide or hafnium. I worry more about whether battery electric vehicles can get enough lithium, and whether there are enough rare earths to do the permanent magnet motors. We can make the semiconductors.

SE: We’re now seeing much more attention focused on multiple chips in a package. How does SiC behave in those types of packages? Would it necessarily even be in the same package?

Palmour: In terms of silicon carbide power devices, we have three product lines. One is discrete power devices. So it’s a single MOSFET in a TO-247, or a diode in a TO-220 package — just a typical standard discrete package. And then we sell chips to other companies that are going to do their own package, but by and large those are module manufacturers. And then we have our own modules. A module includes multiple silicon carbide MOSFET chips in parallel, to get more power, in a very simple circuit. In the most common cases, it’s other identical silicon carbide chips in that power module. Let’s say you have a 100-amp chip, but you need a power module and an H-bridge configuration that gives you 600 amps. So you’d put six 100-amp devices on one side, six 100-amp devices on the other to give you that H-bridge, and then maybe some capacitors or some resistors. That is in the market today. The big issue — and what we do a lot of work on and what a lot of the guys working on automotive are working on — is if you were to drop our chips into a standard silicon power module package, you’d only be getting about half of the performance that the chips could give you because of the built-in inductances. I would equate it to dropping a Ferrari engine into a VW bug chassis.

SE: That sounds like a mismatch.

Palmour: What we and others are working on is how to optimize that module to take full advantage of silicon carbide. We have to build a Ferrari chassis for that engine, and that’s what’s being worked on in power modules. As for whether it would work with other chips in a package, the answer is yes. Typically today, the drivers and other chips that make up this power module are on a board. Usually it’s on a separate board placed right beside that module, but it could be in the same module. It’s called an intelligent power module. But you definitely can do the same in silicon carbide.

SE: How about things like noise and drift, which are growing problems in many designs? Is it any different with SiC?

Palmour: There are two parts to that question. In terms of stability of the oxides, there is some drift in silicon carbide. We spend a lot of time working on that minimizing it. It’s not a problem once you get it right. It’s really mostly time of operation. It will basically shift in the first 10 or 20 hours, and then it will stabilize. And if you turned everything off it would happen again, so the solution is to make that as minimal as possible. In terms of noise, we’re not so susceptible to noise like other chips. But because silicon carbide can be operated at such high frequencies, and can switch at really high dv/dt and di/dt, we actually create noise. You have to do your circuit design very carefully to minimize how much noise you generate.

SE: Does shielding help?

Palmour: It’s really not shielding as much as it is getting your design right. In silicon, you could put the driver a foot away and pipe a cable and it’s no big deal. In silicon carbide you’d have so much inductance it would ring like a like a banshee. You have to put the driver up very close to the module to minimize that inductive ringing and reduce noise. You need to keep those inductances minimal.

SE: So this heads into the big problem RF designers are dealing with today, right?

Palmour: Right, and we do both RF and power. When you use silicon carbide, it’s pushing you more towards the RF realm than a lot of people in power are used to thinking. RF is a different world. Capacitors become resistors, resistors become capacitors, and everything turns upside down.

SE: But SiC has been used extensively in the RF world, right?

Palmour: Yes, and RF is the other part of our business. There we use SiC as a substrate. We used to sell SiC MESFETs (metal-semiconductor FETs) for RF devices. For Gan RF, 99% of the Gan RF devices out there are done on a silicon carbide substrate.

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