New analytical expressions for the subthreshold swing in FDSOI structures operated in both front gate and bottom gate modes are developed. These new equations (9)-(10) provide an accurate description of the subthreshold swing SW as a function of the main FDSOI stack parameters (film thickness, front gate thickness, back gate oxide thickness) and front/back interface trap density. They can be very useful for architecture design as well as electrical characterization of FDSOI devices operated either in front gate or free top surface bottom gate configurations.