As discussed in Section 5.5.2, high energy phonons such as and point phonons reduce the oncurrent only weakly, but can increase the gatedelay time considerably due to charge pileup in the channel. Low energy phonons such as the RBM phonon can reduce the oncurrent more effectively, but have a weaker effect on the gatedelay time. However, due to strong coupling, scattering processes are mostly due to electronphonon interaction with high energy phonons. Therefore, the oncurrent of short CNTFETs can be close to the ballistic limit [279] (see Fig. 5.26), whereas the gatedelay time can be significantly below that limit [280,89]. The intrinsic (without parasitic capacitances) gatedelay time for the ballistic case can be approximated as , or equivalently [281]. The highest reported cutoff frequency for a device with a length of less than is [90], which is far below the ballistic limit. Apart from parasitic capacitances, inelastic electronphonon interaction with high energy phonon has to be considered to explain the results.

M. Pourfath: Numerical Study of Quantum Transport in Carbon NanotubeBased Transistors