Peculiar characteristics of nanocrystal memory cells programming window

In this work the authors study nanocrystal memory cells, focusing on a peculiar characteristic of these devices: the programming window measured in subthreshold region is larger than that measured in linear region. For converse, floating gate flash memory cells with a similar structure feature the same programming window in linear and subthreshold regions. To understand the cause of the difference observed in the nanocrystal memory cells, the authors perform two dimensional technology computer aided design simulations. They well reproduce the experimental results, indicating that the difference observed in nanocrystal memories is due to the localization of the charge into specific regions of the oxide above the channel, i.e., the nanocrystals. On the other hand, when the charge is uniformly stored in the oxide above the channel (as in the case of floating gate flash memory cells), this difference disappears.