An analytical closed-form model to calculate the electrostatics in undoped or lightly doped Double-Gate MOSFETs or Schottky barrier Double-Gate MOSFETs (SB-DG-MOSFETs) in subthreshold region is presented. The model does not introduce any kind of fitting parameters, all parameters depend on geometry and boundary conditions. 2D Poisson's equation is solved in an analytical closed-form with the conformal mapping technique. The extended approach for the potential is compared with a previously presented approach of the same authors. Furthermore, a framework to apply the 2D analytical closed-form calculation of the electrostatic potential and a 2D analytical closed-form solution of the electric field from a previous work is presented. Solutions for constant and linear boundary conditions are used to model the potential and the electric field of these Double-Gate MOSFET structures step by step. (c) 2011 Elsevier Ltd. All rights reserved.