Professor Yongjie Hu and Team Invent a First-of-its-Kind Thermal Transistor to Transform the Paradigm of Dynamic Heat Management

MAE Professor Yongjie Hu led a research team that has unveiled a first-of-its-kind solid-state thermal transistor that uses an electric field to control a semiconductor device’s heat movement with unprecedented performance. Their novel design, based on molecular engineering, could revolutionize further technologies for 3D IC packaging of power electronics and enhance our understanding of mechano-biology interactions.

Science magazine published the group’s study, detailing how the device works and its potential applications. With top speed and performance, the transistor could open new frontiers in heat management of computer chips through an atomic-level design and molecular engineering.

“The precision control of how heat flows through materials has been a long-held but elusive dream for scientists and engineers,” said study leader Yongjie Hu, a professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering. “This new design principle takes a big leap toward that, as it manages the heat movement with the on-off switching of an electric field, just like how it has been done with electrical transistors for decades.”

Electrical transistors are the foundational building blocks of modern information technology. They were first developed by Bell Labs in the 1940s and have three terminals — a gate, a source and a sink. When an electrical field is applied through the gate, it regulates how electricity (in the form of electrons) moves through the chip. These semiconductor devices can amplify or switch electrical signals and power. But as they continue to shrink in size over the years, billions of transistors can fit on one chip, resulting in more heat generated from the movement of electrons, which affects chip performance. Conventional heat sinks passively draw heat away from hotspots, but it has remained a challenge to find a more dynamic control to actively regulate heat.


Illustration of a UCLA-developed solid-state thermal transistor using an electric field to control heat movement (Image by Prof Hu’s group (H-Lab):

While there have been efforts in tuning thermal conductivity, their performances have suffered due to reliance on moving parts, ionic motions, or liquid solution components. This has resulted in slow switching speeds for heat movement on the order of minutes or far slower, creating issues in performance reliability as well as incompatibility with semiconductor manufacturing.

The new thermal transistor, which boasts a field effect (the modulation of the thermal conductivity of a material by the application of an external electric field) and a full solid state (no moving parts), offers high performance and compatibility with integrated circuits in semiconductor manufacturing processes. Their design incorporates the field effect on charge dynamics at an atomic interface to allow unprecedented performance using a negligible power to continuously switch and amplify a heat flux.

The UCLA team demonstrated electrically gated thermal transistors that achieved record-high performance with switching speed of more than 1 megahertz, or 1 million cycles per second. They also offered a 1,300% tunability in thermal conductance and reliable performance for more than 1 million switching cycles. According to the research, this represents the highest values for solid-state thermal devices at several orders of magnitude over the previously reported best results.

“This work is the result of a terrific collaboration in which we are able to leverage our detailed understanding of molecules and interfaces to make a major step forward in the control of important materials properties with the potential for real-world impact,” said co-author Paul Weiss, a professor of chemistry and biochemistry. “We have been able to improve both the speed and size of the thermal switching effect by orders of magnitude over what was previously possible.”

Figure caption: From left, UCLA researchers Man Li, Zihao Qin, Yongjie Hu, Paul Weiss, Huu Nguyen, and Huan Wu were part of the team that developed the first-of-its-kind solid-state thermal transistor with unparalleled performance, based on molecular engineering.

“Thanks to our wonderful team and congratulations to my excellent students for their work in making this breakthrough,” Prof. Hu said. The authors on the paper include MAE postdocs and PhD students, Man Li, Huan Wu, Zihao Qin, and Huu Duy Nguyen, from Prof. Hu’s research group, the H-Lab. Other authors include Erin Avery, Dominic Goronzy, and Tianhan Liu from UCLA Chemistry & Biochemistry.

The paper’s first author, Man Li, a recent PhD graduate from the MAE program and current postdoc in Hu’s group, stated, “The journey to develop the thermal transistor has been both challenging and gratifying. It was made possible only by our truly interdisciplinary research environments involving engineering, physics, chemistry, and materials science through the integration of thermal science, nanotechnology, ultrafast optics, and first-principles theory. I am very excited that our study enables a novel and powerful approach for controlling heat for many applications.”

In their proof-of-concept design, a self-assembled molecular interface is fabricated and acts as a conduit for heat movement. Switching an electrical field on and off through a third-terminal gate controls the thermal resistance across the atomic interfaces and thus heat moves through the material with precision. The researchers validated the transistor’s performance with spectroscopy experiments and conducted first-principles theory computations that accounted for the field effects on the characteristics of atoms and molecules.

The study presents a scalable technology innovation for wide applications in computer chips’ performance and sustainable energy. In addition, it pioneers a platform to support MAE’s emerging directions like mechanobiology. The development offers a new approach to precisely control and investigate nanoscale vibrational interactions with living cells and provide insights for molecular-level mechanisms under such stimuli.

Reference: Li et al. “Electrically gated molecular thermal switch,” Science 382, 585-589 (2023).