Towards Understanding Negative Bias Temperature Instability

Modeling efforts of negative bias temperature instability date back to the work of Jeppson and Svensson in 1977, who proposed the basic form of the popular reaction-diffusion model. This model is still at the heart of many modeling attempts today. However, recent research indicates that even refined variants of this model, while getting some features of NBTI right, cannot capture some crucial aspects of the phenomenon, most notably its ubiquitous logarithmically-decaying recovery phase. Consequently, alternative models have been developed. Some of these models, like the extensions based on dispersive transport of the released hydrogen species, predict like the underlying reaction-diffusion model, that the overall degradation is controlled by (dispersive) diffusion of hydrogen. Alternatively, some models assume that the actual depassivation reaction is the rate limiting step. On top of the creation of interface states, some authors have argued that trapped holes form a considerable part of the overall degradation. Despite all the efforts, however, no universally accepted theory of NBTI is available today, with published models covering only some aspects of the phenomenon and giving contradictory predictions of other aspects. This tutorial attempted to give a broad review of published modeling attempts, comparing their strengths and weaknesses, and eventually listing the requirements for a more complete model of NBTI.