Properties of ESFI MOS Transistors due to the floating substrate and the finite volume

The specific current voltage characteristics of ESFI (or SOS) MOS Transistors are shown and explained. The ESFI MOSTs are produced on silicon islands: in most applications the electrical substrate is at floating potential. This results two effects: At first a threshold voltage change occurs with increasing drain voltage producing a kink in the current curve; if the drain voltage further increases a parasitic bipolar transistor begins to work and effects another kink or bend in the curve. The finite volume and charge below the gate causes a reduced influence of the substrate on the charge in the channel; therefore at higher substrate doping ESFI MOSTs show a higher transconductance than the corresponding bulk transistors. All these effects are described theoretically; the I_D-U_D-characteristics could be simulated by a computer model based on the physical effects.