Carbon nanotube Schottky diodes were fabricated using asymmetric metal-nanotube contacts. These devices were prepared from semiconducting single-walled carbon nanotubes contacted by one Al or Ti electrode and one Au electrode. Nanotubes formed a low resistance contact with the Au electrode and a Schottky contact with the Al or Ti electrode. Electronic transport through the Schottky barriers was determined by the competition between tunneling and thermionic emission, which could be tuned by a back gate voltage. © 2006 American Institute of Physics.