Non-poissonian behavior of hot carrier degradation induced variability in MOSFETs

Abstract

This paper reports new experimental findings about the variability of the MOSFET threshold voltage (V T ) introduced by hot carrier (HC) degradation. Previously, it was reported that the V t shift due to HC stress (ΔV t ) follows a Poissonian behavior, i.e. the standard deviation of ΔV t distribution is proportional to the square root of its mean value. Here, we show data coming from very different devices over a wide range of stress levels that deviates from the poissonian behavior under two particular conditions. First, in the initial stage of the HC stress ΔV t may exhibit a super-poissonian behavior, i.e. its standard deviation is related to the mean value yet by a power law but with an exponent larger than 0.5, that will then tend to the square root dependence as the stress continues. Second, and most notably, for very high stress levels ΔV t standard deviation tends to saturate to a maximum value, or even decrease in some cases, although HC degradation keeps increasing. A new physical model able to explain these phenomena is proposed and validated with numerical simulations. Quantitative agreement of statistical simulation with experimental data, hence predictive capability of the model, could be attained only by considering a realistic 3D geometry and the atomistic nature of both channel doping and HC induced trapped charge.