High-dielectric-constant materials are useful as gate insulators as they can provide efficient charge injection into transistor channels and reduce direct-tunnelling leakage currents.

"Using high-K dielectrics as gate insulators for molecular electronics is an obvious step to take to realize the full potential of molecule-based devices," Stanford scientist Hongjie Dai told nanotechweb.org. "We achieved the highest performance carbon nanotube field-effect transistors made to date by integrating zirconia gate insulators. For instance, 70 mV/decade subthreshold swings are obtained - approaching the theoretical limit for transistors."

The scientists used semiconducting single-walled nanotubes (SWNTs) to make p-type field-effect-transistors (FETs). They formed the zirconia gate insulators by atomic layer deposition, creating zirconia films of about 8 nm thick. Fortunately, the process did not significantly degrade key transistor performance parameters of the nanotubes, such as mobility.

The team converted p-type ZrO2/SWNT-FETs to n-type transistors by heating them in molecular hydrogen at 400°C for one hour. The properties of the n-type transistors, although good, were not as ideal as the p-type FETs. The researchers also made a NOT logic gate, i.e. an inverter, by connecting a p- and n-type ZrO2/SWNT-FET. The device had a high voltage gain.

Now Dai says that the researchers, who reported their work in Nature Materials, are working to improve device performance and exploring the fundamental limit.