Ballisticity of nanotube field-effect transistors: Role of phonon energy and gate bias

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube field-effect transistors (FETs). Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. For comparable electron-phonon coupling, a lower phonon energy leads to a larger degradation of drive current. Thus in semiconductor nanowire FETs, the drive current will be more sensitive than in carbon nanotube FETs because of the smaller phonon energies in semiconductors. Acoustic phonons and other elastic scattering mechanisms are most detrimental to nanotube FETs irrespective of biasing conditions.