A simple method to qualify the LDD structure against the early mode of hot-carrier degradation

Abstract

A simple combination of the heating gate technique and measurements of the forward and reverse (source and drain interchanged) saturation I/sub DS/ versus V/sub GS/ characteristics for an LDD NMOSFET is shown to reveal more information on the nature of an early hot-carrier degradation. Any susceptibility of the LDD structure to this type of degradation leads to an early evolution of the floating gate drain current, and a corresponding evolution in the I/sub DS/ versus V/sub GS/ curves mentioned above, without affecting the threshold voltage. Our method reveals that the early mode affects both forward and reverse saturation I/sub DS/ versus V/sub GS/ characteristics. While the effect on the reverse characteristic can be attributed to an increase in the drain parasitic resistance, the effect on the forward characteristic, apparently indicating source side activity, may be actually due to an increase in the effective channel length, as suggested by simulations. An additional new observation of the decrease of the peak substrate current with the floating gate cycles, when coupled with simulations, allow us to locate and quantify the damage at the edge of the gate. We expect our observations to be useful in qualifying the LDD structure.