ANALYSIS OF THE DRAIN BREAKDOWN MECHANISM IN ULTRA-THIN-FILM SOI MOSFETS

The drain breakdown phenomenon in ultra-thin-film SOI MOSFET’s has been studied. Two-dimensional simulation revealed that the thinning of the SOI film brings about an increase in the drain electric field due to the two-dimensional effect, causing a significant lowering in the drain breakdown voltage, as has been commonly seen in fabricated ultra-thin-film SOI MOSFET’s. The simulation also showed that the lowered drain breakdown voltage recovered almost to its original value by restoring the drain SOI thickness, suggesting that the drain structure, rather than the source, plays a major role in determining the drain breakdown voltage. Experiments using an asymmetric device structure supported this hypothesis, showing that the breakdown voltage was mostly dependent on the drain structure, the initial potential barrier height at the source/SOI-body junction being only a minor factor. Transient simulation was also carried out to investigate the detailed breakdown process, showing that holes accumulate near the source/SOI-body junction at a high drain bias, eventually forward-biasing the junction. These results indicate that a careful drain design, and/or proper choices of the SOI thickness as well as the supply voltage are quite important to realize high performance of ultra-thin-film SOI MOSFET’s. © 1990 IEEE