The authors fabricated single- and multi-wall carbon nanotube field-effect transistors (FETs) and analyzed their performance. Single-wall carbon nanotubes (SWNTs) were found to dominate transport through holes, which appear to be diffusive at room temperature. The conductance of a SWNT FET could be modulated by more than 5 orders of magnitude using the gate voltage. Multi-wall carbon nanotubes (MWNTs) typically showed no gate effect, but structural deformations, such as a collapsed tube, could make them operate as FETs. The study also explored the carrier density and transport mechanism in these devices, suggesting that the SWNTs have a higher carrier density than graphite and a hole mobility comparable to heavily p-doped silicon. The MWNTs, despite their large diameter, exhibited FET behavior due to structural deformations that modified their electronic structure.The authors fabricated single- and multi-wall carbon nanotube field-effect transistors (FETs) and analyzed their performance. Single-wall carbon nanotubes (SWNTs) were found to dominate transport through holes, which appear to be diffusive at room temperature. The conductance of a SWNT FET could be modulated by more than 5 orders of magnitude using the gate voltage. Multi-wall carbon nanotubes (MWNTs) typically showed no gate effect, but structural deformations, such as a collapsed tube, could make them operate as FETs. The study also explored the carrier density and transport mechanism in these devices, suggesting that the SWNTs have a higher carrier density than graphite and a hole mobility comparable to heavily p-doped silicon. The MWNTs, despite their large diameter, exhibited FET behavior due to structural deformations that modified their electronic structure.