  We have fabricated carbon nanotube (CN) field-effect transistors with
multiple, individually addressable gate segments. The devices exhibit markedly
different transistor characteristics when switched using gate segments
controlling the device interior versus those near the source and drain. We
ascribe this difference to a change from Schottky barrier modulation at the
contacts to bulk switching. We also find that the current through the bulk
portion is independent of gate length for any gate voltage, offering direct
evidence for ballistic transport in semiconducting CNs over at least a few
hundred nanometers, even for relatively small carrier velocities.
