Negative Bias Temperature Stress (NBTI) designates a degradation phenomenon in MOSFETs, which results in a decrease of drain current when subjected to strong-inversion biases at elevated temperatures. NBTI was discovered more than 30 years ago, but its physical origin is still unknown. This is a property that NBTI shares with flicker noise, also termed 1/f-noise after the shape of its power spectral density. Flicker noise has been observed in a variety of processes from different scientific disciplines, suggesting that it is a universal property. Another striking fact is the absence of roll-off frequencies from any measurement data obtained up to now: Flicker noise follows the 1/f-shape from the micro-hertz to the kilo-hertz regime, where it is usually overtaken by thermal noise.
Despite the fact that the spectral shape of flicker noise is always 1/f^c with c close to unity, the magnitude of the noise, however, depends on the physics of the process. For electronic devices exhibiting ohmic conductance, like a MOSFET in the ohmic region, flicker noise manifests itself as fluctuations of the conductance of the channel. In a popular model published by McWhorter, the conductance fluctuations are modeled as fluctuations in the number of carriers, induced by carrier trapping into the oxide. On the other hand, conductance fluctuations could also be due to fluctuations of the individual carrier's mobility. The former model predicts the noise power to be inversely proportional to the gate overdrive, while the latter gives a noise power independent of the gate overdrive. Since oxide traps are believed to play a major role also in NBTI, we conducted a series of flicker noise measurements on wafers with different known NBTI performance. The unstressed devices showed very low, nearly bias independent flicker noise levels but after subjecting them to NBTI the flicker noise level increased up to tenfold and additionally became bias dependent.