Characterizations of Silicon-on-Insulator (SOI) Metal-Oxide-Semiconductor Field-Effect transistors (MOSFET``s) have been performed. These include general characteristics of SOI MOSFET``s, the required surface silicon thickness, the flat-band voltage, and the interface trap density at the gate oxide/silicon interface. Especially, basic device parameters affecting the subthreshold and threshold characteristics have been deeply analyzed. The criterion for the surface silicon thickness has been determined in consideration of the elimination of substrate floating effect and the controllability of the inversion charge in the surface silicon. The required surface silicon thickness resulting from these considerations is lower than the one from the conventional consideration. The flat-band voltages of the SOI structures have been determined by measuring the dependence of the back channel threshold voltage on the front gate bias, considering the charge coupling effect between the front gate and the back gate of the fully depleted SOI MOSFET. The interface trap density in SOI MOSFET can also be determined by means of the charge coupling effect. Many interface traps in SOI MOSFET may cause anomalous latches on its drain current curves. The flat-band voltages and the interface trap densities of the SOI structures, extracted by this analysis, have been compared with those of the bulk silicon MOSFET``s.