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.

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.

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