Two semiconductors - silicon carbide and gallium nitride - are locked in a (quite literally) fierce competition for circuits capable of operating at the highest temperatures. 

Silicon carbide chips once held the lead, operating at temperatures of up to 600°C. 

But gallium nitride, with its unique properties that allow it to perform better at high temperatures, has now surpassed silicon carbide. Researchers led by Rongming Chu, professor of electrical engineering at Pennsylvania State University, have designed a gallium nitride chip that can operate at 800°C - hot enough to melt table salt.

This advance could be crucial for future space probes, jet engines, pharmaceutical processes, and many other applications requiring circuits to operate under extreme conditions. Alan Mantooth, a professor of electrical engineering and computer science at the University of Arkansas, said that high-temperature silicon carbide chips allow scientists to place sensors in previously inaccessible locations. Mantooth, who was not involved in the new gallium nitride research, explained that gallium nitride chips could play a similar role in monitoring energy-intensive manufacturing processes in natural gas turbines, chemical plants, and refineries, as well as in systems no one has yet imagined.

"We can put these electronics in places that are simply unimaginable with silicon," he said.

The potential for silicon carbide and gallium nitride to perform under such extreme conditions stems from their wide band gaps. A wide band gap is the energy gap between a material's valence band (where electrons are bound to molecules) and its conduction band (where electrons are free to participate in the flow of current). At high temperatures, electrons in materials with narrower band gaps are always excited enough to reach the conduction band. This creates a problem for transistors, as they cannot be turned off. The wide band gaps of silicon carbide and gallium nitride require more energy to excite electrons to the conduction band, preventing transistors from accidentally becoming permanently on at high temperatures. Compared to silicon carbide, gallium nitride also has some unique properties that make its chips perform better at high temperatures. Chu's team described their integrated circuit, composed of so-called gallium nitride high electron mobility transistors (HEMTs), in the IEEE Electron Devices Letters this month. The GaN HEMT's structure consists of a thin film of aluminum gallium nitride and a layer of gallium nitride. This structure attracts electrons to the interface between the two materials.

This layer of electrons, known as the two-dimensional electron gas (2DEG), is extremely concentrated and offers minimal resistance to movement. This means that charges move faster within the 2DEG, enabling the transistor to respond to voltage changes and switch between on and off states more quickly. This faster electron movement also allows the transistor to carry a higher current for a given voltage. Fabricating a 2DEG using silicon carbide is more difficult, making it more difficult for such chips to match the performance of gallium nitride devices.

Professor Chu's graduate student, Yixin Xiong, explained that making the GaN HEMT capable of operating at 800°C requires several structural adjustments. These include minimizing leakage current, which is charge that can sneak in even when the transistor is supposed to be off. They used a tantalum silicide barrier layer to protect the device components from environmental influences and prevent the metal outer layers on the device's sides from contacting the two-dimensional electron gas (2DEG), which would further increase leakage current and transistor instability.

Chu said the chip development and manufacturing process was much faster than he had expected. The team had been confident in the success of the experiment, he said. But the results "came even faster than my best guess," he said.

Despite GaN's significant advantages, Mantooth remains skeptical about its long-term reliability compared to SiC. He explained, "There's been a concern about GaN experiencing microcracks at extreme temperatures of 500°C and above, which doesn't necessarily occur with SiC, so there might be reliability issues with GaN."

Professor Chu also sees long-term reliability as an area for improvement. "We can make some technical improvements," he said. "One is to improve its reliability at high temperatures. Currently, I think we can hold it at 800°C for about an hour." 

Gallium nitride and silicon carbide

Xiong stated that much work remains to improve the device. He explained that, in addition to minimizing leakage current, another function of the tantalum silicide barrier is to prevent the titanium in the device from potentially reacting with the AlGaN film, thereby destroying the two-dimensional electron gas (2DEG). Xiong ultimately hopes to remove titanium from the device entirely. "I would say the ultimate goal is to no longer rely on titanium," he concluded.

Despite potential lifetime challenges, the team's chip is pushing the limits of electronic device operation, such as on the surface of Venus. "If you can sustain it at 800°C for an hour, that means you can sustain it at 600°C or 700°C for even longer," Chu explained. Venus's ambient temperature is 470°C, so GaN's new temperature record could be useful for electronics on Venus probes.

Mantooth explained that the 800°C figure is also important for hypersonic aircraft and weapons. The friction generated by their extreme speeds can raise surface temperatures to 1,500°C or higher. "What a lot of people don't realize is that when you're flying at Mach 2 or 3, air friction creates an extreme environment at the leading edge of the wing... And guess what? That's where your radar is. And so are all the other processing equipment. These applications are exactly why the Department of Defense is interested in extreme-temperature electronics," Mantooth said.

Speaking of future plans, Chu said the next step is "scaling the equipment up and making it run even faster." He also believes that since there are so few chip suppliers capable of operating at such extreme temperatures, the chip could be commercialized soon. "I think it's pretty mature. It still needs some refinement, but the advantage of high-temperature electronics is that there's no other alternative," he said.

However, the dominance of gallium nitride circuits over silicon carbide may not last long. Mantooth's lab also makes high-temperature chips and is working to get silicon carbide to the same high temperatures as Chu's chip. "We're going to make circuits and try to reach the same temperatures with silicon carbide," Mantooth said. While it's unclear who will ultimately win, one thing is certain: the competition is heating up.

Source: Content compiled from IEEE

Reference link https://spectrum.ieee.org/broadband-internet-in-nigeria

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