  We investigate electronic transport properties of Schottky-barrier
field-effect transistors (FET) based on double-walled carbon nanotubes (DWNT)
with a semiconducting outer shell and a metallic inner one. These kind of
DWNT-FET show asymmetries of the $I$-$V$ characteristics and threshold voltages
due to the electron-hole asymmetry of the Schottky barrier. The presence of the
metallic inner shell induces a large effective band gap, which is one order of
magnitude larger than that due to the semiconducting shell alone of a
single-walled carbon nanotube FET.
