AC VERSUS DC HOT-CARRIER DEGRADATION IN N-CHANNEL MOSFETS

The origins of the enhanced ac hot-carrier stress damage are examined in this paper. The enhancement in hot-carrier stress damage under ac stress conditions observed with respect to damage under dc stress conditions can fully be explained by the presence of three different damage mechanisms occurring during both dc and ac operation: interface states created at low and mid-gate voltages, oxide electron traps created under conditions of hole injection into the oxide, and oxide electron traps created under conditions of hot-electron injection. It is shown that the quasi-static contributions of these three mechanisms fully account for hot-carrier degradation under ac stress. No transient effects during gate voltage edges are observed, and these effects are not required to explain ac stress results. Our ac stress model is applied to devices from several different technologies and to several different ac stress wave-forms. Excellent agreement is shown to be obtained in each case. Our results demonstrate the validity of the model for frequencies up to 1 MHz. The absence of any transient effect indicates, however, that our model could be applicable at much higher frequencies.