Measurement of substrate current in SOI MOSFETs

A new "Quasi-SOI" MOSFET structure is shown to allow direct measurement of substrate current in a fully-depleted SOI device. The holes generated by impact ionization near the drain are collected at the substrate terminal after they have traversed the source-body barrier and caused bipolar multiplication. By monitoring this hole current, direct characterization of the impact-ionization multiplication factor, M, and the parasitic bipolar gain, /spl beta/, was performed. It was found that M/spl minus/1 increases exponentially with V/sub DS/ and decreases with V/sub GS/, exhibiting a drain field dependence. The bipolar gain /spl beta/ was found to be as high as 1000 for V/sub GS//spl minus/V/sub T/=0 V and V/sub DS/=/spl minus/2.5 V, but decreases exponentially as V/sub DS/ increases. Finally, it was found that /spl beta/ also decreases as V/sub GS/ increases.<
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