29 August 2008—Researchers from Universidade Nova de Lisboa, in Portugal, say they’ve made a transistor in which paper acts as a functional component. Elvira Fortunato, who led the research, says that paper-based transistors will be orders of magnitude cheaper and could be used for low-cost, flexible, and disposable microelectronics, such as biosensors, intelligent packaging, and lightbulbs.
Compared with equivalent transistors using the same class of semiconductor but built on silicon dioxide, ”the performances are similar,” Fortunato says.
To build the transistor, the team used an inorganic oxide as a semiconductor, because it can be processed at room temperature instead of at the hundreds of degrees silicon usually requires. The Lisbon researchers reported their work in the September issue of IEEE Electron Device Letters .
Transistors have been built on paper before, but it is unprecedented for the paper to make up a functional part of the device. ”What’s neat about it is that the paper is not only the substrate—meaning that the paper is a passive component—but, in this case, the paper acts as the dielectric layer,” says Lynn Loo, associate professor of chemical engineering at Princeton University. The dielectric layer insulates the part of the transistor through which current flows (the channel) from the electrode that controls that flow (the gate).
The work was also unique because of the inorganic oxide semiconductor, says Ananth Dodabalapur, professor of engineering at the University of Texas. ”This is very important because until recently it was considered the prerogative of organic semiconductors to be compatible with paper substrates,” he says. And since inorganic semiconductors often perform better than organics, this is a significant achievement, he adds.
However, there are still some kinks to work out before this becomes a practical application. Building transistors on paper is tricky because of the roughness and porosity of the paper, says Loo. And despite the fact that the transistor worked, Loo says there is too much current when the transistor is off.
The paper’s porosity is the cause of the high off current, Loo says, because it creates paths for current to seep through. To fix the problem, the researchers would have to use a more compact paper, she adds.
Fortunato says her group is looking at using laminated paper, which would be smoother and less porous. She says the goal is to eventually make integrated circuits on paper. ”The general idea is not to replace silicon transistors,” Fortunato says, ”but for some specific applications—low-cost, disposable things—maybe this can be a solution.”