This paper investigates the saturation of threshold-voltage shift ΔV th of HfSiON/SiO 2 n-channel MOSFETs (nMOFSETs) under positive bias temperature instability (PBTI) and proposes an empirical PBTI degradation model that can predict operational lifetime t L accurately. Experimental results indicate that secondary-hole trapping occurred in the bulk dielectric due to hole injection at the anode after electron trapping in the initial bulk trap. This secondary-hole trapping causes a decrease in the time exponent n of the power law ΔV th / t n as the gate stress voltage V g,str increases. This dependency of n on V g,str results in overestimation of t L when it was estimated using the conventional method which assumes a constant value of n. An empirical model that considers the effect of V g,str on n is proposed; this model predicted operational t L = 2.6 ' 10 5 s, which agreed well with experimentally-measured t L = 3.9 ' 10 5 s.
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