  We present the detailed treatment of dissipative quantum transport in carbon
nanotube field-effect transistors (CNTFETs) using the non-equilibrium Green's
function formalism. The effect of phonon scattering on the device
characteristics of CNTFETs is explored using extensive numerical simulation.
Both intra-valley and inter-valley scattering mediated by acoustic (AP),
optical (OP), and radial breathing mode (RBM) phonons are treated. Realistic
phonon dispersion calculations are performed using force-constant methods, and
electron-phonon coupling is determined through microscopic theory. Specific
simulation results are presented for (16,0), (19,0), and (22,0) zigzag CNTFETs
that are in the experimentally useful diameter range. We find that the effect
of phonon scattering on device performance has a distinct bias dependence. Up
to moderate gate biases the influence of high-energy OP scattering is
suppressed, and the device current is reduced due to elastic back-scattering by
AP and low-energy RBM phonons. At large gate biases the current degradation is
mainly due to high-energy OP scattering. The influence of both AP and
high-energy OP scattering is reduced for larger diameter tubes. The effect of
RBM mode, however, is nearly independent of the diameter for the tubes studied
here.
