In this Letter, we present a simulation study of the electrical characteristics of ultimately scaled carbon nanotube field-effect transistors. Devices with Schottky contacts and doped source/drain contacts are compared. We show that for small bias devices with doped source/drain contacts exhibit a better on- as well as off-state compared to devices with Schottky contacts. Both device types, however, show a poor off-state for larger bias. We will discuss the relevant transport mechanisms involved and explain our observations. (C) 2004 Elsevier Ltd. All rights reserved.