Abnormal behaviour of low frequency noise in irradiated LDD MOSFETs

In this paper we report on noise measurements from LDD MOS transistors through irradiations performed in a Co-60 gamma-cell up to a total dose of 300 krad(Si). Conducting parameters and 1/f noise have been studied to correlate noise levels and radiation effects. We have found that the noise in MOS transistors can be associated with channel and series resistance due to the LDD structure. No change in 1/f noise is observed when measurements are performed under constant effective gate voltage after irradiation. Nevertheless abnormal increases in noise level reveal the occurrence of particular failures induced by ionizing radiations which cannot be detected by classical analysis. These defects have been localized. They concern the source-substrate junction or the gate-drain insulation. Such results must be taken into account in hardness evaluation