A simple method to estimate lifetime of NMOSFET´s in the circuits using DC stress data

It was experimentally found that a 10% frequency degradation of a CMOS ring oscillator operated at V_dd=6.8 volts took approximately 400 times longer than a 10% degradation of the current drive under the conventional peak substrate DC stress of an identically drawn NMOSFET at V_ds=6.8 volts. In order to correlate degradation rate of the DC and AC stress, a simple analytical expression to estimate lifetime of NMOSFET´s in the circuits has been developed based upon the results of accelerated DC stress on NMOSFET´s and quasi-static DC stress on CMOS inverters using a HP4145A DC parametric tester. The ring oscillator lifetime is in good agreement with the estimate using this method