A team led by Charles Lieber, a professor of chemistry at Harvard, and Shamik Das, lead engineer in MITRE’s nanosystems group, has designed and built a reprogrammable circuit out of nanowire transistors. Several tiles wired together would make the first scalable nanowire computer, says Lieber. Such a device could run inside microscopic, implantable biosensors, and ultra-low-power environmental or structural sensors, say the researchers.
For more than a decade, nanowires and nanotubes have promised to shrink computing to scales impossible to achieve with traditional semiconductor materials. But there have been doubts about the practicality of nanowires and nanotubes as actual computing systems. “There had been little progress in terms of increasing the complexity of circuits,” says Lieber.
One big problem has been reproducing structures made from nanowires and nanotubes reliably. Each structure needs to be virtually identical to ensure that a circuit operates as intended. But now, says Lieber, some of those problems are being solved. His group, in particular, has developed ways to produce identical nanowires in bulk. Because of this, he and colleagues at MITRE have been able to design a nanowire circuit architecture that has the potential to scale up. The details are published in the current issue of Nature.
Traditional chips are made using a so-called top-down approach in which a design is essentially exposed like a photograph onto a semiconductor wafer, and excess material is etched away. In contrast, a bottom-up approach is used to make the nanowire circuits. This means they can be deposited on various types of surfaces, and can be made more compact. “You want [sensor] systems that are physically small,” says James Klemic, nanotechnology laboratory director at MITRE. “Right now, your only option is to use a chip that dwarfs the sensor.”
To make the new nanowire circuit, researchers deposited lines of nanowires, made of a germanium core and silicon shell, on a substrate and crossed them with lines of metal electrodes to create a grid. The points where the nanowires and electrodes intersect act as a transistor that can be turned on and off independently. The researchers made a single tile, with an area of 960 square microns containing 496 functional transistors. It is designed to wire to other tiles so that the transistors, in aggregate, could act as complex logic gates for processing or memory.