Threshold voltage instability at low temperatures in partially depleted thin film SOI MOSFETs

The threshold voltage instability at low temperatures due to the floating Si film in partially depleted SIMOX was examined at low temperatures under normal operating conditions. Floating-film SOI MOS transistors suffer an accumulation of holes generated by impact ionization near the drain, at the lower Si film interface. As the potential at this interface increases due to hole accumulation, the source junction becomes forward biased, limiting the amount of charge which can accumulate. This causes the saturation kink effect. The increase in potential at the lower interface acts analogously to a positive bias in bulk devices and effectively decreases the threshold voltage of the device. The use of the channel contact alleviates the hole accumulation effect by providing a conducting path for the generated holes. Hence, the grounded film exhibits a higher threshold voltage than the floating film