A Weibull distribution-based new approach to represent hot carrier degradation in threshold voltage of MOS transistors

Abstract

The importance of long term reliability in MOS VLSI circuits is becoming an important subject because of the increasing densities of VLSI chips. Hot carrier effects cause noncatastrophic failures which develop gradually over time and change the circuit performance. The Weibull distribution is often used to describe the life times of parts and widely applied to characterize the breakdown statistics of solid dielectric samples. In this study the degradation in the threshold voltage is observed experimentally by operating the MOSFET under different voltage stress conditions. Using Weibull distribution, a new approach is proposed to represent the hot carrier degradation in threshold voltage of MOS transistors. The linear regression method is used to estimate the Weibull parameters and the correlation coefficient is used to confirm the results. The observed and the estimated values of the degradation are compared and found to be in good agreement with each other. The results obtained can be easily applied to SPICE NMOS and PMOS device models, therefore the method proposed will be helpful to estimate the influence of hot carrier degradation to the performance of any analog circuit topology.