Heat Transistor May Make for Cooler Computers

Everyday computing generates a lot of waste heat. The high density of electrical transistors within the average laptop—literally billions of these transistors can now fit on a single chip—offers users greater working speed, but also results in higher temperatures, said Yongjie Hu, a professor of mechanical and aerospace engineering at the UCLA Samueli School of Engineering.

“There’s a lot of heat generated, which can affect electronic performance and stability, and also sustainability,” Hu explained. “Cooling these processors down wastes a lot of energy. In fact, if you look at statistics from U.S. data centers, you’ll find that more than half of the total electricity used isn’t for computing, data processing, or memory. Instead, it’s used to cool the devices. This is a huge waste of energy.”

While engineers have historically used copper or fluids to manage the thermal output of electrical transistors, engineers and physicists have been looking to develop a dynamic heat management technology that can control heat pathways in three dimensions to more efficiently dissipate heat.

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Traditional electrical transistors, made up of a gate, a source, and a sink, regulate how electrons move through the chip when an electrical field is applied through the gate, resulting in enhanced power. But with great power comes even greater thermal issues. Hu said it’s been a dream of physicists, material scientists, and electrical engineers to find a way to manage the resulting heat without compromising other properties of the transistor. While many have relied on heat sinks, a type of substance placed on top of a chip or other device, to draw the heat away, this approach does not offer any kind of dynamic control.

Now, Hu’s laboratory, in collaboration with Paul Weiss, professor of chemistry and biochemistry, has created a new solid-state thermal transistor, the first demonstration of a device that can control heat with high precision and performance down to the nano scale level. This new electronic switch is a self-assembled molecular interface. When an electrical field is switched onto the device, it allows heat to move through the material with incredible efficiency.

“This material has two atomic layers. We know, fundamentally, that atom to atom bonding happens through electron waves. It’s a kind of glue that can attach atoms together,” Hu said. “We have two parallel layers of atoms. There’s then a third terminal that works as a gate. It applies an electrical field which controls the electron distribution, effectively controlling the bonding strengths between the layers.”

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Surprisingly, this also manages the heat flow through a field effect. It’s all done with the on-off switching of an electrical field, just like what controls power in transistors.

When the group tested the thermal transistor using spectroscopy experiments, they found the novel switch achieved a whopping switching speed of more than 1 megahertz—a record high performance. What’s more, since this self-assembled molecular interface is solid-state, with no moving parts, it should be quite easy to scale up so that it can be used in future computer chip manufacturing applications.

“It’s very compatible with semiconducting industry fabrications and easy to integrate with solid-state chips. And that’s something we’re trying to do now,” said Hu. “There is a great deal of benefit of integrating this thermal transistor with computer chips, especially in next generation computer chips with triplet technology where memory and logic are integrated into three-dimensional architectures. Managing heat becomes an even more important issue. But by integrating this small transistor, we can direct the heat flow in this 3-D space so it can effectively dissipate the heat.”

But Hu added that there are likely other applications where such a transistor would have value. There are many industrial processes where heat control is essential. But he is especially excited about potential biomedical applications for this novel switch.

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“You could perhaps use this kind of transistor in cancer therapies where heat or hypothermia is used to kill off cancer cells,” he said. “These kinds of therapies show a lot of promise, but it is very difficult to control the heating profile, especially down to a small scale, because we just want to kill the cancer cells, not damage the healthy tissue around those cells. But there is the potential we could use this switch at nanoscale resolution to make it easier to control these types of therapies.”

Hu said he is excited by the possibilities and appreciates that the development of this solid-state thermal transistor demonstrates why it’s so important for scientists and engineers to think outside the box.

“There are many problems that we are trying to solve in engineering, but we are often confined by the existing technology frameworks that are already in use,” he said. “This is a great example of something disruptive, a new concept which can open up new and different operation mechanisms so we can tackle big problems from transformative angles.”

Kayt Sukel is a technology writer in Houston.