Revision with unchanged content. Semiconductor process and device simulators are well established tools for the reduction of the development time for semiconductor devices. Nowadays simulation efforts go beyond solving the basic semiconductor device equations. Especially the modeling and simulation of aging processes has tremendously gained in importance. This book gives insight into the topic of semiconductor device simulation and focuses on the modeling of degradation mechanisms. Negative bias temperature instability (NBTI) causes degradation of MOS structures at elevated temperatures and negative gate voltages. An elaborate investigation of literature from the first report to the recent understanding of this degradation mechanism is presented. A comprehensive model is derived, combining research results from different groups and the coupling to the basic semiconductor device equations. The new NBTI model is compared to measurement data and gives excellent results. This book is addressed to researchers in the field of semiconductor process development but also recommended to engineers in IC design to strengthen their understanding for device degradation.