As discussed in Section 5.5.2, high energy phonons such as and -point phonons reduce the on-current only weakly, but can increase the gate-delay time considerably due to charge pileup in the channel. Low energy phonons such as the RBM phonon can reduce the on-current more effectively, but have a weaker effect on the gate-delay time. However, due to strong coupling, scattering processes are mostly due to electron-phonon interaction with high energy phonons. Therefore, the on-current of short CNT-FETs can be close to the ballistic limit  (see Fig. 5.26), whereas the gate-delay time can be significantly below that limit [280,89]. The intrinsic (without parasitic capacitances) gate-delay time for the ballistic case can be approximated as , or equivalently . The highest reported cutoff frequency for a device with a length of less than is , which is far below the ballistic limit. Apart from parasitic capacitances, inelastic electron-phonon interaction with high energy phonon has to be considered to explain the results.
M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors