Nonequilibrium green's function treatment of phonon scattering in carbon-nanotube transistors

We present a detailed treatment of dissipative quantum transport in carbon-nanotube field-effect transistors (CNT-FETs) using the nonequilibrium Green's function formalism. The effect of phonon scattering on the device characteristics of CNT-FETs is explored using extensive numerical simulation. Both intra-and intervalley 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 CNT-FETs, which 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 backscattering 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.