Three-Dimensional Numerical Analysis of Electron Devices

The goal of this research activity is the study of the main mechanisms related to the ``erase'' and ``write'' operations of non-volatile memories, aimed at developing of a three-dimensional numerical simulation code to be used in the design phase of the memory cells. In the last period, extensive simulations of realistic devices have been carried out; the geometries have been defined in cooperation with SGS-Thomson, and the simulation results compared with experimental data provided by the same Company. The analysis of such a comparison brought to the following conclusions.

Concerning the electron injection through the gate oxide due to Fowler-Nordheim tunneling, i.e., the physical phenomenon used to remove electrons from the floating gate in order to erase the non-volatile memory cell, the experimental characteristics as function of the applied voltages have been reproduced with good agreement using only one fitting parameter. It has been noticed that a small localized variation in the thin-oxide thickness strongly influences the erasing process. Also, the injected current density is uniformly distributed in the thin-oxide region, but its value strongly dependens on the cell geometry, as the capacitive coupling between the control gate and the floating gate changes. Hence, the possibility of using a three-dimensional simulator is essential because it allows for a realistic description of the device geometry. Finally, in the observed devices the band-to-band tunneling phenomenon showed a negligible influence on the erasing curves, due to the high doping values in the source region.

The hot-electron injection phenomenon, that occurs when electrons are able to overcome the potential energy barrier at the Si-SiO2 interface, produces a carrier injection in the floating gate, hence the writing of the non-volatile memory cells. The hot-carrier injection simulations have pointed out the difficulty of a good physical description of this model, and therefore further investigations will be devoted to this issue.


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