The other challenge has been packing these two different types of nanowire transistors together on the same surface–and in a way that lends itself to mass production. The Caltech team created a checkerboard pattern of alternating p- and n-type silicon squares. Then they carved these squares into densely packed lines of nanowires, yielding p- and n-type nanowires that could easily be linked up to form transistors and circuits. Although the experimental circuits involved the use of relatively slow e-beam lithography to connect the wires, the researchers say that for mass production, similarly dense circuits could be made using much faster photolithography.
The first applications for the devices, sensors, are possible for the same reason nanowires are difficult to make reliably–their electronic properties change dramatically in response to slight changes to their surfaces. For example, a single strand of DNA could be attached to the surface of a nanowire. When a complementary strand of DNA (say, from a pathogen) in a blood sample linked to this DNA, a marked change in the resistance of the nanowire would register a hit. Hundreds of such sensors, each set to measure a different target, could easily be packed into a small chip in a handheld device. Testing each target with both types of transistors provides an automatic check against false positives, Heath says.
The Caltech work is a significant step forward for nanoelectronics, says Hongjie Dai, professor of chemistry at Stanford University, who has made similar circuits using carbon nanotubes. The move to CMOS with nanowires, he says, is “important if nanowires are to be used for future electronics applications.”