Subthreshold conduction in silicon-on-sapphire transistors

The subthreshold conduction in silicon-on-sapphire MOS transistors has been studied both theoretically and experimentally. A simple model to describe the subthreshold conduction current for both thick films and thin films is derived in terms of charges in the silicon and charges at the silicon-silicon dioxide and silicon-sapphire interfaces. The model has been extended to cover short-channel transistors by application of charge conservation under the channel region. It is shown that the subthreshold conduction current for a SOS-MOS transistor has a form similar to that found in bulk transistors, but with modification of the terms due to the finite silicon film thickness and the unique geometry of the SOS-MOS transistor. The general form of the model has been confirmed by measurement of the subthreshold current on several hundred SOS-MOS transistors of different geometries manufactured by various companies.