MEASUREMENT OF SUBSTRATE CURRENT IN SOI MOSFETS

A new ''Quasi-SOI'' MOSFET structure is shown to allow direct measurem ent of substrate current in a fully-depleted SOI device. The holes gen erated by impact ionization near the drain are collected at the substr ate terminal after they have traversed the source-body barrier and cau sed bipolar multiplication. By monitoring this hole current, direct ch aracterization of the impact-ionization multiplication factor, M, and the parasitic bipolar gain, beta, was performed. It was found that M - 1 increases exponentially with V(DS) and decreases with V(GS), exhibi ting a drain field dependence. The bipolar gain beta was found to be a s high as 1000 for V(GS) - V(T) = 0 V and V(DS) = -2.5 V, but decrease s exponentially as V(DS) increases. Finally, it was found that beta al so decreases as V(GS) increases.