The best semiconductor ever? | MIT News

Silicon is one of the most common elements on Earth, and in its pure form, this material has become the basis of many modern technologies, from solar cells to computer chips. But the properties of silicon as a semiconductor are far from ideal.

First, while silicon allows electrons to pass easily through its structure, it is much less amenable to “holes”—the positively charged counterparts of electrons—and the use of both is essential for some types of chips. Moreover, silicon does not conduct heat very well, so overheating problems and expensive cooling systems are common for computers.

Now, a team of researchers from MIT, the University of Houston, and other institutions have conducted experiments that show that a material known as cubic boron arsenide overcomes both of these limitations. It provides high mobility for both electrons and holes and has excellent thermal conductivity. Researchers say it’s the best semiconductor material ever found, and possibly the best ever.

So far, cubic boron arsenide has only been produced and tested in small, non-homogeneous laboratory batches. The researchers had to use special techniques originally developed by former MIT postdoc Bai Song to probe small areas in the material. More work is needed to determine whether cubic boron arsenide can be produced in a practical, economical form, much less replace the ubiquitous silicon. But even in the near future, this material may find applications where its unique properties will be of significant importance, researchers say.

The results are reported today in the journal Science, in a paper by MIT postdoc Chong Shin and MIT mechanical engineering professor Gang Chen; Zhifeng Ren at the University of Houston; and 14 others at MIT, the University of Houston, the University of Texas at Austin, and Boston College.

Previous research, including work by David Broido, who co-authored the new paper, theoretically predicted that the material would have high thermal conductivity; Further work confirmed this prediction experimentally. This latest work completes the analysis by experimentally confirming a prediction made by Chen’s group back in 2018: cubic boron arsenide will also have very high mobility for both electrons and holes, “which makes this material really unique,” says Chen.

Earlier experiments showed that the thermal conductivity of cubic boron arsenide is almost 10 times higher than the thermal conductivity of silicon. “So it’s very attractive just for heat dissipation,” Chen says. They also showed that the material has a very good band gap, a property that gives it great potential as a semiconductor material.

Now new work completes the picture, showing that, thanks to its high mobility of electrons and holes, boron arsenide has all the essential qualities needed for an ideal semiconductor. “This is important because, of course, in semiconductors we have both positive and negative charges equivalently. So if you’re building a device, you want to have a material where both electrons and holes move with less resistance,” Chen says.

Silicon has good electron mobility but poor hole mobility, and other materials such as gallium arsenide, widely used in lasers, also have good electron but not hole mobility.

“Heat is now a major bottleneck for many electronic devices,” says Shin, lead author of the paper. “Silicon carbide is replacing silicon for power electronics in mainstream electric vehicles, including Tesla, because it has three times the thermal conductivity of silicon despite its lower electrical mobility. Imagine what boron arsenides, which have 10 times the thermal conductivity and much higher mobility than silicon, can achieve. It can change the rules of the game.”

Shin adds, “The critical milestone that makes this discovery possible is the advances in ultrafast laser grating systems at MIT,” originally developed by Song. Without this technique, he says, it would be impossible to demonstrate the material’s high mobility for electrons and holes.

The electronic properties of cubic boron arsenide were first predicted based on quantum mechanical calculations of the density function by Chen’s group, he said, and now those predictions have been confirmed by experiments conducted at MIT using optical detection techniques on samples made by Ren and his team. University of Houston.

The researchers say that the thermal conductivity of this material is not only the best of all semiconductors, but also the third highest thermal conductivity of all materials, after diamond and isotopically enriched cubic boron nitride. “And now we have predicted the quantum mechanical behavior of electrons and holes, also from first principles, and that has also been confirmed,” says Chen.

“It’s amazing because I don’t really know of any material other than graphene that has all these properties,” he says. “And it’s a bulk material that has these properties.”

Now, he says, the challenge is to find practical ways to make this material in usable quantities. Current manufacturing methods produce highly heterogeneous material, so the team had to find ways to test only small, local areas of the material that were homogeneous enough to obtain reliable data. Although they have demonstrated the material’s great potential, “we don’t know if it will actually be used and where,” Chen says.

“Silicon is the workhorse of the entire industry,” says Chen. “So, okay, we have material that’s better, but does it really make up for the industry? We dont know”. Although the material appears to be a near-perfect semiconductor, “I think it remains to be seen if it can get into a device and replace part of the current market.”

And while the thermal and electrical properties have been shown to be excellent, there are many other properties of the material that have yet to be tested, such as its long-term stability, Chen says. “There are many other factors that go into making devices that we don’t know about yet.”

He adds, “This could potentially be very important, and people haven’t really even paid attention to this stuff.” Now that the desirable properties of boron arsenide have become clearer, suggesting that the material is “in many ways the best semiconductor,” he says, “maybe this material will get more attention.”

For commercial use, Shin says, “the big challenge would be to produce and purify cubic boron arsenide as efficiently as silicon. … Silicon took decades to achieve the crown, and its purity is over 99.99999999 percent, or ’10 nines’ for mass production today.”

For this to become practical on the market, Chen said, “we need more people to develop different ways to make better materials and characterize them.” According to him, whether funding will be necessary for such development remains to be seen.

The research was supported by the US Office of Naval Research and used equipment at MIT’s MRSEC Joint Experiment Facility with support from the National Science Foundation.

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