Understanding memory window of ferroelectric field-effect transistor under coexistence of charge trapping and ferroelectric polarization: violation of linear superposition

Ferroelectric FETs (FeFETs) are nonvolatile memory devices in which the threshold voltage is programmed by the polarization state of the ferroelectric gate insulator. In this study, we theoretically investigate the impact of charge trapping on the ferroelectric memory window by establishing an accurate picture of mechanism determining the threshold voltage. Under the co-existence of ferroelectric polarization and charge trapping, we show that the threshold voltage shift cannot be described by the linear superposition of the ferroelectric-polarization component and the charge-trapping component. We point out that the misuse of the linear superposition can significantly underestimate the trapped charge density in FeFETs, sometimes even by an order of magnitude. The concept of effective dielectric constant determined by both the original dielectric constant and the polarization state is discussed to describe the relation between the threshold voltage shift and trapped charge density in FeFETs.