This paper presents an explicit surface-potential-based analytic model for the undoped long-channel symmetric double-gate MOSFET. The analytic model is derived from the rigorous solution of Poisson's equation. Unlike the exiting analytic models using the numerical iterative method, this analytic model is based on an explicit analytic approximation. The accuracy of the proposed analytic model is justified by extensive comparisons with the numerical calculations. The relative error in nV range has been achieved. The resulting current-voltage and the derivative curves are in complete agreement with the numerical iterative results. (c) 2007 Elsevier Ltd. All rights reserved.