Nov 19, 2002
Carbon nanotube transistor is 'better than silicon'
Researchers at Stanford University, Cornell University and Purdue University in the US have produced a carbon nanotube transistor that they claim has better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constant, as the gate insulator.
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.
About the author
Liz Kalaugher is editor of nanotechweb.org.