The researchers made and tested hundreds of nanotube transistors, and they found that the devices have consistent electrical properties, even though the property of each nanotube in a device may vary slightly. “There’s such a large number of tubes operational in each device that there’s a statistical averaging effect,” Rogers says.
Moreover, the nanotubes’ properties do not change even if they are transferred to plastics or other substrates. “[The] tubes are physically lifted off quartz and then printed down on target substrate so that it doesn’t disturb the position and orientation of the nanotubes,” Rogers says. Because of this transfer process, he says that the arrays could be integrated with silicon fabrication to make circuits with interconnected nanotube and silicon devices–the nanotube devices could handle the circuit’s high-speed operations. To make such a chip, one would only need to transfer the nanotube arrays to the silicon wafer at the beginning of fabrication. Once that is done, one could add silicon devices. “You don’t even think about them as tubes,” says Roger. “In effect, it’s a thin-film uniform substrate, and you just do your processing.”
For now, the new transistors will be useful for larger electronics circuits such as those in flexible displays and RF chips, but to be used in high-performance electronics like computer chips, the devices need a much better structure and geometry, Javey says. For instance, the devices would need to be much smaller than they are now: the transistors are currently tens of micrometers long and wide.
To make smaller devices, the UIUC team is working on making the arrays denser. Right now, the distance between adjacent tubes is 100 nanometers, but theoretically, this separation could go down to only one nanometer without affecting electrical properties, Martel says.
Another key area that needs work is finding an effective way to make devices with only semiconducting nanotubes, Rogers says. Typically, a third of the nanotubes in any grown batch are metallic, which causes a small current to flow through a transistor even when it is turned off. The researchers use a common trick to get rid of metallic tubes: turn a transistor off and apply a high voltage that blows out the metallic tubes. But to make good-quality transistors on a larger scale, they would need to find a better way to get rid of the metallic tubes or selectively grow semiconducting tubes. That, according to Javey, is the “last big key” for making nanotube electronics.