Collective tunneling model in charge-trap-type nonvolatile memory cell

A new tunneling model between an inversion layer and the trap sites for the charge-trap-type (CT-) nonvolatile memory (NVM) cell is proposed. By considering the geometrical mismatch between the inversion layer and the trap site of the CT-NVM cell, we can conclude that electron tunneling is induced by a rare event, which causes the localization of electrons in the inversion layer near the trap sites. In addition, we also reveal that the successive tunneling of electrons is triggered by this rare event tunneling by focusing on the temporal fluctuation of the electronic state in the inversion layer. On the basis of these phenomena, we propose the collective tunneling model in the charge injection of the CT-NVM cell, where the electrons tunnel to the trap sites collectively with a long waiting time. This insight is important in designing the CT-NVM cell. By using collective tunneling, the amount of injection charge can be controlled discretely by adjusting the charge injection time. This enables us to realize a multilevel charge trap cell.