Negative Bias Temperature Instability in Low-Temperature Polycrystalline Silicon Thin-Film Transistors

The authors have proved that negative bias temperature instability (NBTI) is an important reliability issue in low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs). The measurements revealed that the threshold-voltage shift is highly correlated to the generation of grain-boundary trap states. Both these two physical quantities follow almost the same power law dependence on the stress time; that is, the same exponential dependence on the stress voltage and the reciprocal of the ambient temperature. In addition, the threshold-voltage shift is closely associated with the subthreshold-swing degradation, which originates from dangling bond formation. By expanding the model proposed for bulk-Si MOSFETs, a new model to explain the NBTI-degradation mechanism for LTPS TFTs is introduced.