BTI variability of SRAM cells under periodically changing stress profiles

We present a BTI compact model that is able to account for the complex BTI stress patterns encountered in complex electronic circuits. Such stress patterns are composed of various blocks corresponding to different circuit operation states, protocol modes or input conditions, and the blocks repeat within a composite, hierarchical structure. The present work extends a previously introduced physics-based accurate NBTI modeling while preserving its numerical efficiency. We provide insight into some principal characteristics of BTI degradation under hierarchical stress patterns, such as a non-trivial dependence on multiple duty cycles. In particular, the NBTI degradation can sensitively depend on the temporal sequence of NBTI stress blocks, and building a model on just the average stress or on stress histograms can be misleading. An SRAM cell example demonstrates this method and compares the cell's BTI failure statistics for two different hierarchic-periods stress patterns.