Single-walled carbon nanotube field effect transistors (CNTFETs) are used as NH3 gas sensors and their sensing performances are studied in terms of gate biasing effect. By applying a positive gate bias, a high sensitivity of 178.5% per ppm, is achieved. The reversibility of the CNTFET sensors is also found to be significantly improved under the appropriate positive gate voltages. In this study, an in-depth understanding of how the electrical properties of the CNTFETs are affected by NH3 gas is gained. (C) 2008 Elsevier B.V. All rights reserved.