Effective threshold voltage shift: A measure for NBTI removing uncertainty in mobility degradation

The conventional threshold voltage shift measured by extrapolating transfer characteristics, ΔVth(ex), underestimates the NBTI-induced degradation of drain current, ΔId. Mobility degradation, Δμ, has been proposed as a potential contributor to ΔId. Evaluating Δμ, however, can be problematic and controversial. For test engineers, it is desirable to include all degradations in one parameter and we propose the effective threshold voltage shift, ΔVeff, as such a parameter. A method for evaluating ΔVeff is given, which only requires standard NBTI tests and samples and is easy to implement. It will be shown that good agreement can be achieved between the measured ΔId and that predicted from ΔVeff without evaluating Δμ. The advantage of using ΔVeff over ΔId/Id is that it does not depend on device size and source/drain series resistance.