Subthreshold behavior of thin-film LPCVD PolySilicon MOSFET's

Abstract

A physical model that characterizes the subthreshold drain current (gate-voltage swing) and the threshold voltage of thin-film LPCVD polysilicon MOSFET's is developed and supported experimentally. The model describes the influence of the grain boundaries and of the charge coupling between the front and back gates on the subthreshold behavior. Main predictions are that the gate-voltage swing depends strongly on grain-boundary properties but weakly on the charge-coupling effects, that the threshold voltage depends strongly on grain-boundary properties and charge-coupling effects, and that the charge-coupling effects diminish as the grain-boundary trap density, the thickness of the film, or the doping density in the film increases. Comparisons of model predictions and measured data for passivated (hydrogenated) and unpassivated devices indicate quantitatively how hydrogenation reduces the trap density and increases the carrier mobility in the channel.