Electrical stability in self-aligned p-channel polysilicon thin film transistors

In this work we present a study of the electrical stability of self-aligned p-channel thin film transitor fabricated using excimer laser annealing. The electrical stability was tested performing different bias-temperature stress experiments and we found an increased degradation in devices with large channel width and also for increasing temperatures in the bias-temperature stress performed at zero drain voltage. These results clearly point out to instabilities related to self-heating effects of the devices, showing a substantial increase of the threshold voltage and degradation of the subthreshold region, as well as a transconductance (Gm) increase. From extensive analysis of the phenomenon through numerical simulations, we found that the bias-temperature-stress effects, including Gm overshoot, could be perfectly reproduced assuming that degradation is confined in a narrow channel region near the source and/or drain contacts. From the present results we conclude that self-heating triggers some degradation of a spatially limited region of the channel, presumably related to residual damage of the ion-implantation process.