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.