The short-channel threshold voltage model for Fully depleted (FD) silicon-on-insulator (SOI) MOSFETs, based on a quasi-2D approach, has been investigated and extended. The effect of the source/drain fringing fields, which penetrate into the buried oxide, has been taken into account by using an extra 2D analytical equation which is based on conformal mapping. This equation models the enhanced short-channel behavior of the FD SOT MOSFET in the buried oxide. The extended quasi-2D analytical threshold voltage model has been compared to 2D Medici simulation data, down to channel lengths of 0.1 mum The obtained results show good agreement between the analytical model and 2D Medici simulation data.