We describe the fabrication of single-wall carbon nanotube field-effect transistors in a conventional metal-oxide-semiconductor field-effect transistor structure, with gate electrodes above the conduction channel separated from the channel by a thin dielectric. We use these devices to study the performance improvements achieved by reducing the gate-to-channel separation. The top gate structure offers certain structural advantages over earlier, back gated carbon nanotube devices. In addition, these devices exhibit excellent electrical characteristics, including steep subthreshold slope and high transconductance, at gate voltages close to 1 V. The measured device characteristics are significantly better than previously reported carbon nanotube devices, providing further motivation to explore the use of carbon nanotubes for future nanoelectronic applications.