Thermal Transistors Handle Heat With No Moving Parts

Electronic transistors are central to modern electronics. These devices precisely control the flow of electricity, but in doing so, they generate heat. Now, researchers at the University of California, Los Angeles have developed a solid-state thermal transistor-the first device of its kind that can use an electric field to control the flow of heat through electronics. Their study, which was recently published in Science, demonstrates the capabilities of the new technology.

There has been a strong desire from engineers and scientists to control heat transfer the same way we control electronics, but it has been very challenging," says study lead author Yongjie Hu, a professor of mechanical and aerospace engineering at UCLA.

Historically, electronics have been cooled down with heat sinks that passively draw the excess heat away. More active approaches to thermal management have also been proposed, but these often rely on moving parts or fluids and can take a long time-typically minutes to hours-to ramp up or ramp down the material's thermal conductivity. With thermal transistors, the researchers can actively modulate the flow of heat faster and with more precision. This speed makes them a promising option for managing heat in electronic devices.

I think we are living in a kind of thermal renaissance." -Miguel Munoz Rojo, Materials Science Institute of Madrid

Analogous to an electronic transistor, the UCLA group's thermal transistor also uses electric fields to modulate the conductance of a channel, in this case thermal conductance rather than electrical. This is done with a thin film of cagelike molecules engineered by the researchers that acts as the channel of the transistor; applying an electric field makes the molecular bonds in the film stronger, which increases its thermal conductance. Our contribution was literally only one molecule thin," says Paul Weiss, a professor of chemistry, bioengineering, and materials science at UCLA and the study's coauthor.

With that single-molecule layer, the researchers were able to reach the maximum change in conductivity at a frequency of more than 1 megahertz, several orders of magnitude faster than other heat-management systems. Molecular motion typically controls heat flow in other types of thermal switches. But molecular motion is quite slow compared with the motion of electrons, explains Weiss. By leveraging electric fields, the researchers are able to speed up the switch from millihertz to megahertz frequencies.

Molecular motion also can't achieve as large a difference in thermal conductance between the on state and the off state. The UCLA device, by comparison, achieves a 13-fold difference. It really is an enormous difference, both in terms of magnitude and speed," Weiss says.

With these improvements, the device could be important for cooling processors. The transistors are especially promising for semiconductors because they use a small amount of power to control the heat flow, compared with other routes of active energy dissipation. Many thermal transistors could also be integrated on the same chip in the same way that electronic transistors are, Hu says.

In particular, thermal transistors could effectively manage heat in new semiconductor designs, such as in 3D-stacked chiplets, where they would reduce hot spots, thereby allowing for more freedom in the design of the chiplets. They may also help cool power electronics made from wide-bandgap semiconductors like gallium nitride and silicon carbide, Hu says.

Our contribution was literally only one molecule thin." -Paul Weiss, UCLA

Beyond these applications in electronics, the UCLA researchers' work on thermal transistors could also provide insights into molecular-level mechanisms of how living cells regulate temperature. Hu thinks there may be a similar effect connecting heat flow and electric potential at work in our cells. In a separate ongoing project, he is studying the mechanisms of ion channels-the proteins that act as gates to control the flow of ions across a cell membrane. When it comes to heat flow in the human body, the macroscopic picture has been established in physiology; however, the molecular-level mechanism still remains largely unknown," Hu says.

I think we are living in a kind of thermal renaissance," says Miguel Munoz Rojo, a senior researcher at the Materials Science Institute of Madrid. Munoz Rojo is excited about the possibility of thermal transistors adding to the stock of heat--management technologies, and is interested in the possibility of using them for a wide array of large-scale applications, like refrigeration, in addition to the nanoscale cooling of electronics. He and his colleague Andrej Kitanovski, a thermal-engineering professor at the University of Ljubljana, in Slovenia, are working together to develop these thermal-management technologies. For Munoz Rojo, that range of potential uses makes thermal transistors the pinnacle of heat-management technology.

The demonstration of this technology is an exciting advance and will likely motivate more fundamental research, says Geoff Wehmeyer, an assistant professor of mechanical engineering at Rice University, in Houston. It will be interesting to see if thermal engineers can find ways to integrate these molecular thermal switches into switchable thermal-management systems for electronics or batteries."

While this proof of concept is promising, the UCLA researchers acknowledge that the technology is still early in its development. Going forward, Hu says they aim to further improve the device's performance.