Semiconductor Rivalry Rages on in High-Temperature Chips

This article is part of our exclusive IEEE Journal Watch series in partnership with IEEE Xplore.

Two semiconductors—silicon carbide and gallium nitride—are the rivals in a (quite literally) heated competition to make circuits capable of performing at the highest temperatures. Silicon carbide chips had taken the lead, operating at 600° C. But gallium nitride, which possesses unique features that make it more functional at high temperatures, has now surpassed SiC. Researchers at Pennsylvania State University led by Rongming Chu, a professor of electrical engineering, have designed a gallium nitride chip capable of operating at 800°C —hot enough to melt table salt.

The development could be critical to future space probes, jet engines, pharmaceutical processes, and a host of other applications that need circuits for extreme conditions. Silicon carbide high-temperature chips have allowed scientists to put sensors in places they weren’t able to before, says Alan Mantooth, a distinguished professor of electrical engineering and computer science at the University of Arkansas, who was not involved in the new gallium nitride result. He explains that the gallium nitride chip could do the same in monitoring the health of natural gas turbines, energy-intensive manufacturing processes in chemical plants and refineries, and systems no-one has even thought of yet.

“We can put this kind of electronics in places silicon simply can’t even imagine going,” he says.

Both silicon carbide and gallium nitride’s potential to perform under such extreme conditions comes from their wide bandgaps. Those are the energy gaps between the materials’ valence bands, where electrons are bound to the molecule, and the conduction band, where they are free to contribute to the flow of electricity. At high temperatures, electrons in materials with a narrower bandgap are always excited enough to reach the conduction band. This presents a problem for transistors, because they will then be unable to switch off. The wide bandgaps of silicon carbide and gallium nitride require more energy to excite electrons to the conduction band, so that the transistors aren’t unintentionally always switched on in high temperature environments.

Gallium nitride also has unique features compared to silicon carbide which allow its chips to perform better under high heat conditions. Chu’s group’s IC, which they described this month in IEEE Electron Device Letters, is composed of what are called gallium nitride high electron mobility transistors (HEMT). The structure of GaN HEMTs involves an aluminum gallium nitride film on top of a layer of gallium nitride. The structure draws electrons to the interface between the two materials.

This layer of electrons—called a two-dimensional electron gas (2DEG)—is highly concentrated and moves with little resistance. This means charge moves much faster in the 2DEG, leading the transistor to be able to respond to changes in voltage and switch between its on- and off-states more quickly. Faster electron movement also allows the transistor to carry more current in response to a given voltage. The 2DEG is harder to produce using silicon carbide, making it more difficult for its chips to match the performance of gallium nitride devices.

To coax a GaN HEMT into operating at 800° C took some alterations to its structure, explains Yixin Xiong, Chu’s graduate student. Some of those measures involved minimizing leakage current, charge that sneaks across even when the transistor is supposed to be off. They did this by using a tantalum silicide barrier to protect the device’s components from the environment and by preventing the outer layer of the metal on the sides of the device from touching the 2DEG, which would have further increased leakage current and instability in the transistor.

Penn State engineers tested high electron mobility transistors at 800°C.Rongming Chu/Pennsylvania State University

Chu says that the research and fabrication process of the chip went much faster than he had anticipated. The team had been confident that the experiment would work, he says. But it was “faster than my best guess,” he says.

Despite the notable benefits it presents, Mantooth is concerned about gallium nitride’s long-term reliability compared to silicon carbide. “One of the things that people have been concerned about with GaN at those extreme temperatures, 500℃ and above, is microfractures or microcracking [which is] not something that we’re necessarily seeing in silicon carbide, so there may be reliability issues” with GaN, he explains.

Chu agrees that long-term reliability is an area for improvement, saying “there are a few technical improvements we can make: one is making it more reliable at a high temperature. Right now, I think we can hold at 800 ℃ for probably one hour.”

Gallium Nitride vs. Silicon Carbide

There is still a lot of work to be done to improve the device, says Xiong. He explains that other than minimizing leakage current, one function of the tantalum silicide barrier is to prevent titanium in the device from potentially reacting with the AlGaN film, which could destroy the 2DEG. Eventually, Xiong wants to remove titanium from the device altogether. “The ultimate goal, I would say, is to not rely on titanium,” he concludes.

Despite its potential longevity challenges, the group’s chip is pushing the limits of where electronics can operate, such as on the surface of Venus. “If you can hold it for one hour at 800 ℃, that means that at 600 or 700 ℃, you can hold it for much longer,” Chu explains. Venus’ ambient temperature is 470 ℃, so GaN’s new temperature record could be useful for electronics in a Venus probe.

The 800 ℃ figure is also important for hypersonic aircraft and weapons, explains Mantooth. Their extreme speeds generate friction that can heat up the surface to 1500 ℃ or more. “One of the things a lot of people don’t realize is that when you’re flying at Mach 2, or Mach 3, the air friction creates an extreme environment on the leading edge of the wing…And guess what? That’s where your radar is located. That’s where other processing equipment is located. These applications are why the U.S. Defense Department is interested in electronics for extreme temperatures,” says Mantooth.

As far as plans for the future, Chu says the next steps are to “scale the device to make it run faster.” He also thinks that the chip may be ready for commercialization not too far down the line, because there are so few suppliers for chips capable of operating at such extreme temperatures. “I think it’s quite ready. It requires some improvements, but the nice thing about high-temperature electronics is there’s nothing else there,” he says.

The gallium nitride circuit’s victory against its silicon carbide companions may not last long, however. Mantooth’s lab also fabricates high-temperature chips, and is working on getting silicon carbide to hit the heat levels that Chu’s chips have. “We’ll be fabricating circuitry to try to attack the same temperatures with silicon carbide,” says Mantooth. Though it’s unclear who will eventually finish on top, at least one thing is certain: the competition is still heating up.