Softening of breakdown in ultra-thin gate oxide nMOSFETs at low inversion layer density

The post breakdown I-V characteristics of ultra-thin gate oxides subjected to constant voltage Fowler-Nordheim stress in nMOSFETs were investigated. It is shown that by varying the electron density through the application of a substrate bias under the same stress field conditions, the oxide degradation does not change while the oxide I-V characteristics after the breakdown event are strongly modified. We have also studied the dependence of the post-breakdown I-V curve on the current compliance used during the constant voltage stress. Finally we speculate on the physical structure of the breakdown spot on the basis of the experimental observations.