We present an analytic, explicit and continuous charge model for a long-channel UTB (ultra-thin body) SO] (silicon-on-insulator) MOSFET, from which analytical expressions of the total capacitances are obtained. Our model is valid from below to well above threshold, without suffering from discontinuities between the regimes. It is based on a unified charge control model derived from Poisson's equation. The drain-current, charge and capacitances expressions result in continuous explicit functions of the applied bias. The calculated capacitance characteristics are validated by 2D numerical simulations showing a very good agreement for different silicon film thicknesses. (C) 2008 Elsevier Ltd. All rights reserved.