  Carbon nanotubes (CNTs) have large intrinsic carrier mobility due to weak
acoustic phonon scattering. However, unlike two-dimensional
metal-oxide-semiconductor field effect transistors (MOSFETs), substrate surface
polar phonon (SPP) scattering has a dramatic effect on the CNTFET mobility, due
to the reduced vertical dimensions of the latter. We find that for the Van der
Waals distance between CNT and an SiO$_2$ substrate, the low-field mobility at
room temperature is reduced by almost an order of magnitude depending on the
tube diameter. We predict additional experimental signatures of the SPP
mechanism in dependence of the mobility on density, temperature, tube diameter,
and CNT - substrate separation.
