Thermal Transistor: The World's Tiniest Refrigerator

Thermal transistors refrigerate one electron at a time and physicists plan to compute with heat

1 January 2008—Traditionally, heat and electronics don’t agree. But physicists in Europe and Asia are beginning to see some signs of cooperation. A Finnish-Italian team has demonstrated that electrons in a specially designed transistor can carry away heat, making the device they built the smallest known refrigerator. Another team, from Singapore, has shown that heat can carry information in a transistor-like device, just like electrons do in conventional computers.

Researchers from the Helsinki University of Technology, in Finland, and the Scuola Normale Superiore in Pisa, Italy, have created a tiny transistor—resembling in structure if not in composition the field-effect transistors in ICs—that they call a single-electron refrigerator. Two superconducting electrodes are connected to a small micrometer-sized copper slab, about 2 mm long and 1/5 mm wide. These electrodes are analogous to the source and drain of a conventional transistor, except that they are electrically isolated from the copper by a thin layer of aluminum oxide. (Two extra electrodes are attached on both sides of the source and drain for measurement purposes.) Along the copper island is placed the ”gate,” an electrode that controls the flow of electrons through the copper slab.

At temperatures below 1 K, electrons can pass from the copper slab, which is not superconducting, to the superconducting aluminum probes via a quantum mechanical trick called tunneling.

Then, by applying a voltage to the source and drain, electrons are pushed through the copper island. Whenever an electron is admitted to the copper island, another leaves. But the device also acts like a filter, allowing only the ”hottest” electrons, those with higher energy, to leave the island. So when a current flows from the source to the drain, the copper island becomes depleted of hot electrons, thus lowering its overall temperature. The researchers found that applying a certain voltage to the gate electrode adjusts the electrostatic repulsion between electrons, called the ”Coulomb blockade.” Tuning the gate voltage properly leads to a point where only one electron goes through the device at a time. The turnstile-filter combination improves not only the cooling efficiency but also lets you switch on and off the flow of heat from the transistor.

Although the cooling power of the device is very small and only effective at very low temperatures to start with, the fact that the heat flux can be controlled at all makes the device interesting for future electronics, says team leader Jukka Pekola, of Helsinki University of Technology. The technology could find a use in cooling devices that already operate at low temperatures, such as superconducting transistors or supercooled magnetometers, known as superconducting quantum interference devices, or SQUIDs, says team member Francesco Giazotto, of Scuola Normale Superiore.

In a separate development, a team of physicists at the National University of Singapore have come up with a plan for a device that could use phonons, vibrations in a crystal lattice that carry heat, as bits of information. Phonons are usually viewed as a nuisance because they produce noise in electronic circuits and cause other problems. But the Singaporean physicists report that they have modeled what they call ”thermal logic gates,” based on the concept of thermal transistors, that compute with phonons. A thermal transistor would consist of two terminals, called source and drain in an analogy with the conventional transistor, while a third terminal that is linked to the source functions like a gate, controlling the flow of phonons from the drain to the source. 

Just as electrons flow because of a difference in voltage, phonons flow because of a difference in temperature. But if the lattice vibration spectra—the spectrum of energies of phonons—in the two terminals don’t match, very little heat can flow between them. The Singapore team says that by injecting lattice vibrations from the gate into the source, it becomes possible to match the source’s vibrational spectrum to that of the drain segment, allowing the phonons to flow.

The team produced several models of logic gates, in which phonons have taken over the role of electrons. This research should ultimately lead to the construction of thermal computers, says Li Bao Wen, a professor of physics at the National University of Singapore. ”If one can build a logic gate, one should be able to build a computer,” he says. Research groups, such as a group at the University of California at Berkeley led by Alex Zettl and Arunava Majumdar, are already trying to build thermal devices. The Berkeley group has constructed a thermal rectifier consisting of a nanotube that conducts heat better in one direction than the other, and they are now experimenting with thermal transistors. ”We do have our own plans to do this,” says Bao Wen.

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