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

Abstract

The specific current-voltage characteristics of epitaxial silicon films on insulator (ESFI®) SOS MOS transistors are shown, discussed in comparison to bulk silicon MOST's, and explained by the differences in geometrical considerations, charge distribution, and operation mode, The ESFI MOST's are produced on silicon islands, in most applications, the electrical substrate is at floating potential. This results in 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. On the other hand, the finite vo|ume effects a strong dependence of the base width of the parasitic bipolar transistor on the drain voltage and causes a rise of the current amplification with the drain voltage. The finite volume below the gate oxide also limits the bulk-charge magnitudes with subsequent increase in mobile carrier charge, thereby increasing the transconductance. All these effects are also described theoretically; the ID-VDcharacteristics could be simulated by computer model based on the physical effects.