The switching times and on/off-current ratios are computed for Schottky-barrier carbon nanotube field-effect transistors with different tube diameters and insulator thicknesses. It is indicated that it may be difficult to obtain a device exhibiting both high speed and low leakage current. A small-diameter nanotube with a thin insulator may offer the best compromise. It is also demonstrated that interelectrode capacitances can be large, thereby calling into question the usefulness of the intrinsic switching time as a figure of merit for transistors intended for digital-logic applications. The extrinsic switching time is a more appropriate metric and it is shown here that considerable optimization of the carbon nanotube field-effect transistor will be required to achieve figures better than for modem Si complementary metal-oxide-semiconductor transistors over a reasonable range of on/off-current ratio. (c) 2006 American Vacuum Society.