Investigation of the threshold voltage turn-around effect in long-channel n-MOSFETs due to hot-carrier stress

Abstract

In this paper, we have investigated the turn-around effect of the threshold voltage (Vth) shift in the case of an n-type long channel MOSFET during hot-carrier stress. This effect is explained by the interplay between interface states and oxide traps, i.e. by the partial compensation of the rapidly created oxide charges by the more slowly created interface states. Significant hole trapping is observed from the negative shift of the threshold voltage during the first seconds of stress. Afterwards, Vth has switched to the positive voltage direction due to the negative charging of interface traps after relatively long stress time. To analyze this phenomenon in detail, a refined extraction technique for the defect distribution from charge-pumping measurements has been employed. Additionally, the obtained results have been explained by our physics-based model of hot-carrier degradation which considers not only channel electrons but also secondary holes generated by impact ionization. In spite of the small hole contribution (compared to that of electrons) to the total defect creation, its impact on the threshold voltage shift is comparable with the electronic one. The reason behind this trend is that hole-induced traps are shifted towards the source, thereby more severely affecting the device behavior.