Nanotube FETs Go Ballistic
Normal metal contacts with nanotubes create a Schottky barrier. This inhibits current flow. Palladium is a noble metal with a large work function and good wetting interactions with nanotubes.
With Pd contacts, the "on" states of semiconducting nanotubes can behave as ohmically contacted ballistic metallic tubes. In ballistic conduction, the charge carriers travel from one contact to another without scattering. Normal conduction involves a large number of scattering events that creates a large resistance to current flow.
Room temperature conductances for the palladium-contacted tubes are near the ballistic limit of 4e2/h. Currents can reach up to 25microA per tube. At high voltages, current saturation appears to be set by backscattering of the charge carriers by optical phonons.
Previous conductances for such FET arrangements have been at most 0.30 of the ballistic value. Some conductances can be as low as 0.001. In the Stanford/Purdue experiments, conductances up to 0.65 of 4e2/h are seen at room temperature.
In terms of modelling, a diffusive model of the short (300nm) tube FETs completely fails to reproduce the device characteristics. Preliminary analysis using a ballistic MOSFET model leads to a good but not ideal fit. The researchers are not quite sure why palladium has such good properties, while other noble metals like platinum do not.