  Most carbon nanotube field-effect transistors (CNTFETs) directly attach metal
source/drain contacts to an intrinsic nanotube channel. When the gate oxide
thickness is reduced, such transistors display strong ambipolar conduction,
even when the Schottky barrier for electrons (or for holes) is zero. The
resulting leakage current, which increases exponentially with the drain
voltage, constrains the potential applications of such devices. In this paper,
we use numerical simulations to show that if CNT based
metal-oxide-semiconductor (MOS) FETs can be achieved by using heavily doped CNT
sections as source and drain, ambipolar conduction will be suppressed, leakage
current will be reduced, and the scaling limit imposed by source-drain
tunneling will be extended. By eliminating the Schottky barrier between the
source and channel, the transistor will be capable of delivering more
on-current. The leakage current of such devices will be controlled by the full
bandgap of CNTs (instead of half of the bandgap for SB CNTFETs) and
band-to-band tunneling. These factors will depend on the diameter of nanotubes
and the power supply voltage.
