Once the arrays have been grown, Rogers makes a transistor using existing patterning techniques to lay source and drain electrodes over both of the ends of all the nanotubes, and by placing a gate across their collective width. “From that point on, the process is just like making silicon on insulator devices,” says Rogers.
“As silicon transistors become smaller, inherent limitations become more critical,” says Alex Zettl, a physicist at the University of California, Berkeley. “Nanotubes as a material are an exciting alternative material for forming extremely small, stable transistors,” he says.
But while the original interest in nanotubes for electronics lay in their nanometer size, there has been an increasing amount of interest in their use for analog electronic devices, says Burke. There are now predictions that nanotubes will actually outperform conventional analog transistors, he says.
Rogers’s radio is relatively big, with each transistor consisting of thousands of carbon nanotubes. But he says that there is plenty of room to scale them down, not least because there are relatively large gaps between some of the nanotubes. So it should be possible to pack them in more densely. “Ideally, you would want the nanotubes to be sitting right next to each other,” Rogers says. He and his colleagues are now working on creating integrated circuits containing up to 100 of these transistors.