Silicon’s properties as a semiconductor are far from ideal. Athough silicon lets electrons whizz through its structure easily, it is much less accommodating to “holes”—electrons’ positively charged counterparts—and harnessing both is important for some kinds of chips. Furthermore, silicon is not very good at conducting heat, which is why overheating issues and expensive cooling systems are common in computers.

Now, a team of researchers at MIT, the University of Houston, and other institutions have shown that cubic boron arsenide overcomes these two limitations of silicon as a semiconductor material. Cubic boron arsenide provides high mobility to both holes as well as electrons, and has excellent thermal conductivity. It is, the researchers say, the best semiconductor material ever found, and perhaps the best possible one.

MIT researchers say cubic boron arsenide is the best semiconductor material ever found, and maybe the best possible one. Credits: Image: Christine Daniloff, MIT

The findings are reported in the journal Science, in a paper by MIT postdoc Jungwoo Shin and MIT professor of mechanical engineering 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 within the material.

More work will be needed to determine whether cubic boron arsenide can be made in a practical, economical form, much less replace the ubiquitous silicon. But even in the near future, the material could find some uses where its unique properties would make a significant difference, the researchers say.

Earlier 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; subsequent work proved that prediction experimentally. This latest work completes the analysis by confirming experimentally a prediction made by Chen’s group back in 2018: that cubic boron arsenide would also have very high mobility for both electrons and holes, “which makes this material really unique,” says Chen.

The earlier experiments showed that the thermal conductivity of cubic boron arsenide is almost 10 times greater than that of silicon—very attractive just 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, with its high mobility for both electrons and holes, boron arsenide has all the main qualities needed for an ideal semiconductor.

That’s important because of course in semiconductors we have both positive and negative charges equivalently. So, if you build 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, widely used for lasers, similarly have good mobility for electrons but not for holes.

Heat is now a major bottleneck for many electronics. Silicon carbide is replacing silicon for power electronics in major EV industries including Tesla, since it has three times higher thermal conductivity than silicon despite its lower electrical mobilities. Imagine what boron arsenides can achieve, with 10 times higher thermal conductivity and much higher mobility than silicon. It can be a gamechanger.

—Shin, lead author

Shin said that the critical milestone that makes this discovery possible is advances in ultrafast laser grating systems at MIT, initially 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 based on 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 made by Ren and members of the team at the University of Houston.

Not only is the material’s thermal conductivity the best of any semiconductor, 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 figure out practical ways of making this material in usable quantities. The current methods of making it produce very nonuniform material, so the team had to find ways to test just small local patches of the material that were uniform enough to provide reliable data. While they have demonstrated the great potential of this material, whether or where it’s going to actually be used, they do not know.

Although the thermal and electrical properties have been shown to be excellent, there are many other properties of a material that have yet to be tested, such as its 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 used facilities of MIT’s MRSEC Shared Experimental Facilities, supported by the National Science Foundation.

Resources

• Shin, Jungwoo et al. (2022) “High ambipolar mobility in cubic boron arsenide” Science doi: 10.1126/science.abn4290