Fundamental Mechanism Analyses of NBTI-Induced Effects on Single-Event Upset Hardness for SRAM Cells

Negative bias temperature instability (NBTI)-induced effects on the single-event upset (SEU) hardness of static random access memories (SRAM) cells have been investigated by SPICE simulations, for fully understanding the role of NBTI in changing the SEU response of SRAM cells, based on a modified double-exponential current pulse model. Both threshold voltage shift and mobility degradation due to NBTI can lead to a reduction in the SEU hardness in most cases. In particular, the parameter shifts of the two p-MOS transistors of a cell produce the opposite effects on the cell SEU hardness, i.e. the one is to reduce this SEU hardness, and the other is to enhance it, leading to a possible increase in cell SEU hardness due to asymmetry parameter shifts induced by NBTI between the two p-MOS transistors in some cases. In addition, there is an approximately linear correlation between this threshold shift, mobility degradation and the SEU hardness reduction. No obvious synergy exists between the threshold shift and mobility degradation for reducing the cell SEU hardness, showing that there seems to be a superposition principle for the changes in cell SEU hardness caused by the two degraded p-MOS transistors.