The impact of acoustic and optical phonon scattering on the performance of CNT-FETs is investigated using a full-quantum transport model within the NEGF formalism. Different gate lengths, dielectric materials and chiralities are considered. It is shown that the use of a high-kappa dielectric lowers the off-current dominated by phonon-assisted hand-to-hand tunneling. The device scalability, is demonstrated: with the oxide thickness fixed to 1.5 nm, good performance is obtained with 15 nm and 10 nm gate lengths with SiO2 and HfO2 gate dielectrics, respectively. The role of phonon scattering in CNT-FETs of different chiralities is investigated for the HfO2 devices. A similar analysis has also been carried out for source/drain underlap geometries. The results confirm that the calculation of the off-currents and delay times is strongly influenced by phonon scattering. (C) 2008 Elsevier Ltd. All rights reserved.