Analysis of short‐channel effect in thin‐film SOI MOS transistor

The short‐channel effect in thin‐film SOI MOS‐FETs was analyzed by two‐dimensional, two‐carrier simulation. In thin‐film SOI MOSFETs, the channel length dependence of the threshold voltage is influenced strongly by the avalanche effect caused by impact ionization. It was found that the short‐channel effect is enhanced by the avalanche effect. The holes generated by impact ionization raises the substrate potential, causing a decrease in the potential barrier between the source and substrate. In bulk MOSFETs, the potential barrier is not affected by the holes generated by the avalanche effect because the holes flow into the substrate electrode. However, in thin‐film SOI MOSFETs, the potential distribution near the source is influenced strongly by the holes because the holes flow into the source. The holes generated by the avalanche effect degrade the punch‐through breakdown voltage. The punch‐through breakdown voltage is highest when the SOI layer has a certain thickness. When the layer thickness is lower than a certain thickness, the punch‐through breakdown voltage decreases. The punch‐through breakdown voltage is lowest when the underlying oxide film is a certain thickness.