Influence of MOSFET geometry on the statistical distribution of NBTI induced parameter degradation

NBTI parameter degradation of MOSFETs shows a statistical variation. The distribution of the threshold voltage Vth after NBTI stress originates from a convolution of the distribution of the virgin devices together with the additional distribution of the NBTI degradation itself. The variability of the Vth (and other electrical parameters) of the virgin devices bases on process induced fluctuations of dopant atoms, oxide thickness, channel length, etc. The dependence on the transistor size is proven by several publications [e.g. 1,2]. The variability of the NBTI parameter degradation itself and the convolution is not fully understood yet and need further investigation. In this paper we investigate the dependency of the NBTI variability of Vth on the transistor size and geometry. For the necessary statistical relevance we perform NBTI stress experiments with the help of a smart array test-structure at a large amount of pMOS devices with various geometries. We show that the variability of pMOSFETs after NBTI depends not only on the size of the active area (w×l) but also on its geometry (w/l).