Researchers at the University of Wisconsin, Madison, have made ultrathin silicon transistors that operate more than 50 times faster than previous flexible-silicon devices. The advance could help make possible flexible high-end electronics that would be useful in a variety of applications, from computers to communication. Zhenqiang (Jack) Ma, professor of electrical and computer engineering and lead researcher on the project, is interested in using flexible electronics to redesign large-scale antennas that could be molded in the shape of, say, an airplane. For instance, radar antennas could be made to cover a large area on an airplane, he says, increasing sensitivity and area of coverage.
Most flexible electronics, such as those used in e-paper and roll-up displays for mobile devices, rely on transistors made of either organic polymers, printed directly on a plastic substrate, or amorphous, or noncrystalline, silicon. However, transistors made of these materials can’t perform at the gigahertz speeds needed for complex circuitry or antennas. “People have for some time been able to make slow flexible electronics,” but the speed of the transistors has been limited, says Max Lagally, professor of materials science and physics at the University of Wisconsin and collaborator of Ma. The next step, he says, has been to make the transistors out of high-quality, single-crystal silicon instead of organic polymers and amorphous silicon because electrons simply move faster in single-crystal silicon.
Ma says his research, published in Applied Physics Letters, is an extension of the previous work done to put high-quality, single-crystal silicon on a flexible plastic substrate. While single-crystal silicon is normally stiff, it can bend if made thin enough. Previously, researchers at the University of Illinois, Urbana-Champaign, have shown that nanometer-thin films of single-crystal silicon transistors can be fabricated and successfully transferred to flexible and stretchable substrates. (See “Stretchable Silicon.”)
Work by Ma and Lagally at Wisconsin further increased the performance of the silicon by adding strain to its crystalline structure, a technique used by Intel and other chip makers to increase the electron mobility of the material. (See “High-Quality Flexible Silicon.”) But, says Ma, “high electron mobility is not equal to high device speed.” Speed of a device is also dependent on its engineering, he says, specifically the resistivity of the contact connections–the points on the transistor where electrons flow in and out of the device.
But here’s the problem: the resistivity of the contact connection is usually modified at temperatures of more than 800 ºC. Plastic can withstand no higher than 200 ºC.
The contact resistivity is due to the chemical makeup of two transistor components, called the source and the drain, between which electric current flows. To make low-resistivity sources and drains, the researchers blasted phosphorous ions at the silicon while it was still on its original, rigid substrate and let the material bake at 850 ºC.
The researchers next peeled off the thin film of silicon and stuck it to a plastic with a layer of epoxy to help it adhere. Then, Ma says, the gate–the part of the transistor that turns the current on and off–was added at room temperature. Usually, silicon dioxide is employed as the gate material, but the researchers used silicon monoxide. The advantage here, says Ma, is that silicon monoxide can be made thinner than silicon dioxide.
The researchers’ approach is a “clever way of mounting the circuit on a flexible substrate without having to deal with high temperatures,” says Ed Croke, researcher at HRL Laboratories, an electronics and information-sciences lab in Malibu, CA. “They do all their processing before they undercut the silicon [from its original substrate],” he says.
John Rogers, professor of materials science, engineering, and chemistry at the University of Illinois, says that the recent advance is “a nice piece of device engineering work that exploits the previously demonstrated approach of using thin-film single-crystal silicon on plastic.” Ma’s research is important, he says, because it helps show that the same sort of performance, previously only possible in a rigid silicon chip, is possible with flexible electronics.
In the current paper, the Wisconsin researchers report transistor speeds of 3.1 gigahertz; in an upcoming paper, the group will report a speed of 7.8 gigahertz. Both are significant gains over the previous 0.5-gigahertz speed demonstrated in Rogers’s lab, says Ma. With further fine-tuning of the fabrication process, including reducing the size of the transistors’ gates, he expects to achieve at least 20-gigahertz speed.
In order to be used in complex circuits such as the microprocessors found in computers, these transistors would still need to operate about twice as fast. However, transistors that operate from 2.4 to 20 gigahertz could be used for antennas that send and receive a range of signals, from radar to Wi-Fi.