  We report a numerical study of the tunnel conductance through the Schottky
barrier at the contact between a semiconducting carbon nanotube and a metal
electrode. In a planar gate model the asymmetry between the p--doped and the
n--doped region is shown to depend mainly on the difference between the
electrode Fermi level and the band gap of carbon nanotubes. We quantitatively
show how the gate/nanotube distance is important to get large on--off ratios.
We explain the bend of the current versus gate voltage as the transition from a
thermal--activation region to a tunneling region. A good agreement is obtained
with experimental results for carbon nanotubes field--effect transistors.
