Jenö Dr. Tihanyi

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

The specific current-voltage characteristics of epitaxial silicon films on insulator (ESFI®) SOS MOS transistors are shown, discussed in comparison to bulk silicon MOSTs, and explained by the differences in geometrical considerations, charge distribution, and operation mode, The ESFI MOSTs 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.

Influence of the floating substrate potential on the characteristics of ESFI MOS transistors

Abstract A model is proposed to explain the anomalous current-voltage characteristics of ESFI MOS transistors. Due to the floating state the substrate potential of the ESFI transistor is increasing with increasing majority carrier current flowing through the substrate to source. In the region of multiplication by avalanche that effect will get quite pronounced. The change of substrate potential yields a change of the threshold voltage hereby increasing the drain current and resulting a bend in the I D (U D ) curves. The assumptions of the model have been justified by additional experiments as with illumination of light or increased temperatures. Based on the physical model a computer program was developed to simulate the I D (U D ) characteristics of ESFI MOS transistors of the enhancement type, resulting good agreement between measured and simulated characteristics.

Functional integration of power MOS and bipolar devices

The development of vertical power MOSFETs has made it possible for MOS and bipolar functions to be combined advantageously. The SIPMOS (Siemens Power MOS) technique is eminently suitable for this purpose. Vertical MOSFET-triggered thyristors, optically coupled lateral thyristors with MOS input and optically coupled MOS triacs have been implemented and investigated. The common advantages of functionally integrated MOS-bipolar structures are the high input sensitivity, high dV/dt immunity and excellent di/dt capability.

Integrated power devices

One of the two main trends in the power integration is the combination of logic functions with high voltage output stages resulting in complex HVIC chips which are mainly used for display driving and telecommunication applications. The other trend is to build high current high voltage structures with integrated additional functions making the power devices better compatible to normal ICs and microcomputers.