We report on the fabrication and electrical characterization of two uniquely structured carbon nanotube field-effect transistors (CNFETs). These devices have been designed to emphasize different aspects of their electrical switching behavior, so that we may learn more about their transport properties. In one structure, back-gate CNFETs were built to study the effects of vertical scaling by varying the thickness of an underlying high-quality gate dielectric. An unexpected dependence, of the subthreshold slope and transconductance on the gate dielectric thickness was observed, and was attributed to Schottky barrier switching at the metal/nanotube contacts. A second structure incorporates a top gate electrode with multiple, individually addressable segments of different width. This design decouples the interior of the device from the contacts, thereby enabling the study of lateral scaling within the same physical device structure. Clear indications of bulk switching were observed in this device, with strong evidence of ballistic transport, suggesting that CNFETs do not follow conventional gate length scaling rules. (C) 2003 American Vacuum Society.