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.