Mobility in semiconducting carbon nanotubes at finite carrier density

Carbon nanotube field-effect transistors operate over a wide range of electron or hole density, controlled by the gate voltage. Here we calculate the mobility in semiconducting nanotubes as a function of carrier density and electric field, for different tube diameters and temperatures. The low-field mobility is a nonmonotonic function of carrier density and varies by as much as a factor of 4 at room temperature. At low density, with increasing field the drift velocity reaches a maximum and then exhibits negative differential mobility, due to the nonparabolicity of the bandstructure. At a critical density, rho(c) similar to 0.35-0.5 electrons/nm, the drift velocity saturates at around one-third of the Fermi velocity. Above rho(c), the velocity increases with field strength with no apparent saturation.