In this paper we propose a new method to calculate the inversion channel charge in the mobility region of fully depleted (FD) SOI-MOSFETs. The proposed calculation is based on our DC model [Technische Fakultdt der CAU Kiel, Ph.D. Thesis, 1999 [1]], which includes both the partially depleted (PD) and the FD operation modes as well as the FD-PD transition. A new approach for the description of integrated minority concentration relying on piece-wise linear dependencies on the quasi-Fermi potential of electrons is used. This approach allows a very accurate description of the total intrinsic inversion channel charge of FD double gate devices, accounting for all possible conditions at both silicon film surfaces. We introduce the piece-wise linear description of the integrated concentration first. Then we explain the proposed calculation in detail and give a comparison between the gained analytical expressions and results of numerical simulations using MEDICI [MEDICI. Avant! Corporation, Freemont, CA 94538, 1996-2000 [2]]. (C) 2002 Elsevier Science Ltd. All rights reserved.