  Carbon Nanotube (CNT) is one of the most significant materials for the
development of faster and improved performance of nano-scaled transistors. This
paper aims at analyzing a trade-off between device performance and device size
of CNT based transistor. Acoustic and Optical Phonon scattering along with the
elastic scattering lead to the non-ballistic performances of those transistors.
The main focus of this work is mainly on finding an optimum diameter to obtain
the highest degree of ballisticity from both single-walled and double-walled
Carbon Nanotube Field Effect Transistors (CNT-FETs). At first, an n-type
single-walled Carbon Nanotube Field Effect Transistor has been considered and
the diameter dependence on degree of ballisticity has been simulated. The
effects of drain voltage, gate voltage and channel length have been
investigated for such characteristics followed by a comparison with
double-walled CNT-FET structure. First of all, it has been found that degree of
ballisticity along with the optimum diameter increases with the increase of
(VDS VGS). The increase of channel length, however, degrades the ballistic
performance demanding a higher diameter to reach the optimum point. Finally, it
can be concluded that optimum diameter for DWCNT-FET reaches earlier than
SWCNT-FET but at lower degree of ballisticity.
