The subthreshold conduction regime in short-channel recessed source/drain (ReS/D) ultra-thin body (UTB) silicon-on-insulator (SOI) MOSFETs is studied. A physics-based model for the subthreshold slope of ReS/D UTB SOI MOSFETs is developed, based on an analytical solution of 2-D Poisson's equation for the front-gate and back-gate potential distributions. In order to verify the accuracy of the model, the calculated subthreshold slope values are compared with the results obtained by Medici 2-D numerical device simulator over a wide range of different device structures, and very good agreement is obtained down to channel lengths of sub-30 nm. The model is given in explicit form without any fitting parameters and requires no iterative calculation, thus making it ideally suitable for fast prediction and evaluation of device design criteria for optimal scaling of the ReS/D UTB SOI MOSFETs. (C) 2010 Elsevier Ltd. All rights reserved.