Analysis of transition region and accumulation layer effect in the subthreshold slope in SOI nMOSFETs and their influences on the interface trap density extraction

Abstract

Measurements were performed in thin film silicon on insulator (SOI) nMOSFETs and it was observed a transition region in the subthreshold slope, larger than the theoretically expected, when the back interface (silicon film/buried oxide) changes from accumulation to depletion. Also, it was observed a non-constant plateau in the subthreshold slope when the back interface is accumulated. MEDICI numerical bidimensional simulations were performed in order to analyze this transition region. It was verified that there is a back gate voltage range where a part of the back interface is not depleted over the whole subthreshold region, depending on the front gate voltage, which influences strongly the determination of the subthreshold slope, resulting in a non-abrupt transient region. It is proposed a method for extracting the interface trap density in gate oxide/silicon film and silicon film/buried oxide interfaces minimizing the influence of the back accumulation layer in the subthreshold slope with the back interface accumulated. This method was also applied experimentally.