The properties of silicon as a semiconductor are far from ideal. While silicon lets electrons whiz through its structure easily, it’s much less suited to “holes” — the positively charged counterparts of electrons — and harnessing both is important for some types of chips. In addition, 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 from MIT, the University of Houston and other institutions has… shown that cubic boron arsenide overcomes these two limitations of silicon as a semiconductor material. Cubic boron arsenide provides high mobility of both holes and electrons, and has excellent thermal conductivity. It is, the researchers say, the best semiconductor material ever found, and perhaps the best possible.
MIT researchers say cubic boron arsenide is the best semiconductor material ever found, and perhaps the best possible. Credits: Image: Christine Daniloff, MIT
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, the University of Houston, the University of Texas at Austin, and Boston College.
So far, cubic boron arsenide has only been made and tested in small lab-scale batches that are not uniform. The researchers had to use special methods originally developed by former MIT postdoc Bai Song to test small regions in the material.
More work will be needed to determine whether cubic boron arsenide can be made into a practical, economical form, let alone replace the ubiquitous silicon. But even in the near future, the material could find a number of applications where its unique properties would make a significant difference, the researchers say.
Previous research, including work by David Broido, who is a co-author of the new paper, had theoretically predicted that the material would have high thermal conductivity; later work proved that prediction experimentally. This latest work completes the analysis by experimentally confirming a prediction made by Chen’s group in 2018: that cubic boron arsenide would also have very high mobility for both electrons and holes, “which makes this material truly unique,” Chen says.
The previous experiments showed that cubic boron arsenide’s thermal conductivity is nearly 10 times greater than silicon — very attractive only for heat dissipation, Chen says. They also showed that the material has a very good bandgap, a property that gives it great potential as a semiconductor material.
The new work shows that boron arsenide, with its high mobility for both electrons and holes, has all the important properties needed for an ideal semiconductor.
That’s important because in semiconductors, of course, we have both positive and negative charges. So if you’re building a device, you want to have a material where both electrons and holes travel with less resistance.
— Gang Chen
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 electron mobility, but not holes.
Heat is now a major bottleneck for many electronics. Silicon carbide is replacing silicon for power electronics in major EV industries, including Tesla, because, despite its lower electrical mobility, it has three times higher thermal conductivity than silicon. Imagine what boron arsenides can achieve, with a 10 times higher thermal conductivity and much greater mobility than silicon. It can be a game changer.
—Shin, lead author
Shin said the critical milestone that makes this discovery possible is advances in ultra-fast laser grating systems at MIT, originally developed by Song. Without that technique, Shin says, it would not have been possible to demonstrate the material’s high mobility for electrons and holes.
The electronic properties of cubic boron arsenide were initially predicted from quantum mechanical density function calculations made by Chen’s group, he says, and those predictions have now been validated through experiments conducted at MIT, using optical detection methods on samples created by Ren and members. from the University of Houston team.
Not only is the material’s thermal conductivity the best of all semiconductors, the researchers say, it has the third-best thermal conductivity of any material — next to diamond and isotopically enriched cubic boron nitride.
The challenge now, Chen says, is to come up with practical ways to make this material in usable quantities. The current methods of producing the material very unevenly, so the team had to find ways to test only small local pieces of material that were uniform enough to provide reliable data. Although they have demonstrated the great potential of this material, they do not know if and where it will actually be used.
While the thermal and electrical properties have proven to be excellent, there are many other properties of a material that have yet to be tested, such as long-term stability/
To make devices, there are many other factors that we don’t know yet.
— Gang Chen
The research was supported by the US Office of Naval Research and made use of facilities at MIT’s MRSEC Shared Experimental Facilities, supported by the National Science Foundation.
Shin, Jungwoo et al. (2022) “High ambipolar mobility in cubic boron arsenide” Science bye: 10.1126/science.abn4290