Radiation-induced changes in floating-body phenomena in SOI MOSFET´s

A model is proposed which enables a quantitative assessment of the floating body effects in SOI (silicon on insulator) devices. It is shown that an SOI MOSFET may simultaneously exhibit a negative conductance and a negative transconductance. The evolution of these effects under X-ray exposure is studied and described in detail. The study of floating body effects as a function of dose reveals that the spacer hardening is a key factor when floating body phenomena prevail. Thus, it is essential to evaluate it quantitatively, and a tradeoff must be found for the implanted dose of the LDD (lightly doped drain). A low dose initially attenuates the impact ionization, but the device becomes more sensitive to a charge trapping into the spacer oxide. Despite this limitation, a properly optimized SOI technology is still suitable for space applications without any specific hardening process. For very hardened applications, specific structures can totally avoid these undesirable phenomena. The extraction of the series resistance as a function of dose is proposed as a practical tool for evaluating the hardening level of the LDD