Have the researchers found the best semiconductor of all?
A research team says cubic boron arsenide is the best semiconductor material ever discovered, and possibly the best. Credit: Christine Daniloff/MIT
Silicon is one of the most abundant elements on Earth, and in its pure form, it has been the basis of many modern technologies, from solar cells to computer chips. But silicon’s properties as a semiconductor are far from ideal.
For one thing, while silicon allows electrons to penetrate its structure easily, it is far less accommodating of “holes”—the positively charged counterparts of electrons—and making use of both is important for some types of chips. What’s more, silicon is not very good at conducting heat, which is why overheating problems and expensive cooling systems are common in computers.
Now, a team of researchers at MIT, the University of Houston, and other institutions has conducted experiments showing that a material known as cubic boron arsenide overcomes both of these limitations. It provides high mobility for electrons and holes, and has excellent thermal conductivity. This, the researchers say, is the best semiconductor material ever discovered, and perhaps the best.
So far, cubic boron arsenide has only been manufactured and tested in small, non-uniform, laboratory-scale batches. The researchers had to use a special method originally developed by former MIT postdoc Bai Song to test small regions within the material. More work will be needed to determine whether cubic boron arsenide can be made in a practical and economical form, let alone replace the ubiquitous silicon. But even in the near future, the material could find some uses where its unique properties will make a significant difference, the researchers said.
The findings are reported in the journal Science, in a paper by MIT postdoc Jungwoo Shin and MIT mechanical engineering professor Gang Chen; Zhifeng Ren at the University of Houston; and 14 others at MIT, University of Houston, University of Texas at Austin, and Boston College.
Previous research, including the work of David Broido, who is a co-author of the new paper, has theoretically predicted that the material will have a high thermal conductivity; Subsequent work proved that prediction experimentally. This latest work complements the analysis by experimentally confirming a prediction made by Chen’s group in 2018: that cubic boron arsenide will also have very high mobility for electrons and holes, “which makes this material truly unique,” Chen said.
Previous experiments showed that the thermal conductivity of cubic boron arsenide is almost 10 times greater than that of silicon. “So, it’s very interesting just for heat dissipation,” Chen said. They also demonstrated that the material has an excellent band gap, a property that gives it great potential as a semiconductor material.
Now, new work fills the picture, showing that with its high mobility for electrons and holes, boron arsenide has all the key qualities needed for an ideal semiconductor. “That’s important because of course in semiconductors we have equal positive and negative charges. So if you’re making a device, you want to have a material in which electrons and holes move with less resistance,” Chen said.
Silicon has good electron mobility but poor hole mobility, and other materials such as gallium arsenide, which is widely used for lasers, also have good mobility for electrons but not for holes.
“Heat is now a major bottleneck for many electronics,” said Shin, the paper’s lead author. “Silicon carbide is replacing silicon for power electronics in major EV industries including Tesla, as it has three times higher thermal conductivity than silicon despite its lower electrical mobility. Imagine what boron arsenide can achieve, with 10 times higher thermal conductivity and greater mobility. much higher than silicon. silicon. It could be a gamechanger.”
Shin added, “An important milestone that made this discovery possible was the advancement in the ultrafast laser grating system at MIT,” which was originally developed by Song. Without the technique, he said, it would be impossible to demonstrate the material’s high mobility for electrons and holes.
The electronic properties of cubic boron arsenide were originally predicted based on quantum mechanical density function calculations made by Chen’s group, he said, and those predictions have now been validated through experiments conducted at MIT, using an optical detection method on samples created by Ren and team members at the University of Houston.
Not only does the material have the best thermal conductivity of any semiconductor, the researchers say, it has the third best thermal conductivity of any material — next to isotope-enriched diamond and cubic boron nitride. “And now, we estimate the quantum mechanical behavior of electrons and holes, also from first principles, and that also proves to be true,” Chen said.
“This is impressive, because I don’t actually know of any other material, apart from graphene, that has all these properties,” he said. “And this is a bulk material that has these properties.”
The challenge now, he says, is finding a practical way to make this material in usable quantities. Its current manufacturing methods produce highly non-uniform material, so the team had to find a way to test only small local patches of material that was uniform enough to provide reliable data. While they have demonstrated the great potential of this material, “whether or where it will actually be used, we don’t know,” Chen said.
“Silicon is the workhorse of the entire industry,” Chen said. “So, okay, we have better material, but will it really keep pace with the industry? We don’t know.” While the material seems almost an ideal semiconductor, “whether it can actually get into devices and replace some of the current market, I think it’s still unproven.”
And while the thermal and electrical properties have proven excellent, there are many other properties of the material that have not been tested, such as its long-term stability, Chen said. “To make the device, there are many other factors that we don’t know yet.”
He added, “This has the potential to be very important, and people haven’t even really paid attention to this material yet.” Now that the desirable properties of boron arsenide have become clearer, indicating that the material is “in many ways the best semiconductor,” he said, “there will probably be more attention paid to this material.”
For commercial use, Ren says, “One big challenge is how to produce and purify cubic boron arsenide as effectively as silicon.… Silicon takes decades to win the crown, has a purity of over 99.99999999 percent, or ’10 nines’ for mass production today. this.”
In order to be practical in the market, Chen said, “it really needs more people to develop different ways to make materials better and characterize them.” Whether the funds needed for the development will be available remains to be seen, he said.
New heat management material keeps computer cool
Jungwoo Shin et al, High ambipolar mobility in cubic boron arsenide, Science (2022). DOI: 10.1126/science.abn4290
Provided by the Massachusetts Institute of Technology
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