The technology might also be useful for extremely compact devices, since it would be possible to layer memory, logic, and even sensing circuitry on top of each other, rather than side by side or on separate chips. The nanowires are applied to chips and connected to the source, drain, and gate using room-temperature processes, allowing consecutive layers to be applied without damaging previous layers. “If you can put ultra-high-performance materials into 3-D structures, through layer by layer assembly, it allows you to put a lot more stuff into an area,” says Lieber. The proximity of the layers, a mere 100 nanometers apart, could also speed performance, he says.
One of the qualities that distinguishes this current work from earlier nanoscale electronics research, including his own, Lieber says, is that the measurements used are industry standards, which makes it possible to compare how nanowires would perform in real devices.
The key to the improved performance is a “core-shell” structure of the nanowires, which confines electrons, or their counterparts, electron holes, in a small space. That allows electrons to zip through the wires quickly, which is key to the speed improvements. In a recent paper in the journal Nature, Lieber made nanowires with a germanium center surrounded by a thin coating of crystalline silicon. And in work described in Nano Letters, the researchers showed the versatility of nanowires by using gallium nitride, which could be useful for high-power, high-temperature applications.
“These two papers come up with very interesting ideas for using this core-shell structure to enhance the performance of these transistors and basically make them much more robust and reliable,” says Berkeley’s Yang. With nanowires, he says, “you get very small features and different compositions, and you also have access to all these nonconventional heterostructures, like these core-shell structures, that enable you to engineer the electronic structure. These are not things you can do easily with conventional technology.”