The contact resistance (R_(cont)) between nanotube and metal electrodes was directly measured in a four-terminal configuration of field-effect transistors for individual single-walled carbon nanotube (SWNT) bundles and a multiwalled carbon nanotube (MWNT). Both R_(cont) and the nanotube resistance (R_(NT)) in a semiconducting SWNT device drastically changed with gate voltage, while R_(cont), being more than one-order smaller than R_(cont) in metallic SWNTs and MWNTs, was almost constant against the gate voltage. Carriers introduced either by gate voltage or chemical doping induced a rapid decrease in R_(cont) compared with the resistance of semiconducting SWNTs.
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