Effect of negative differential conductance in carbon nanotubes.

Measurements of transport at high electric fields in metallic single-walled carbon nanotubes (CNTs) have shown either saturation of the current or a region of negative differential conductance (NDC) characterized by the current, after reaching a maximum, decreasing with further increase in voltage. We point out that both types of behavior are characteristic of NDC, but the NDC is masked in samples showing current saturation due to generation of space charge, leading to a nonuniform electric field. We derive the relation between the carrier concentration, the electric field at which the drift velocity peaks, and the length of the sample that is required for the NDC to be manifest as saturation.