Meaning
Solid-state storage cells utilize a non-conductive layer to hold electrons in place, creating a permanent shift in the threshold voltage of a transistor. In nor flash charge trapping, the dielectric material is typically a silicon nitride layer sandwiched between two oxides. This structure is more resistant to defects than the traditional floating-gate design because a single puncture in the oxide does not drain the entire stored charge.
Electron Injection
Programming the cell involves moving electrons into the trapping layer using a process called hot-carrier injection. A high voltage applied to the gate pulls the charge through the oxide barrier where it becomes stuck in the nitride. This state remains stable for years even without an external power source.
Because the traps are distributed throughout the nitride layer, the cell is less sensitive to localized physical damage.
Voltage Stress
Erasing the data requires a reverse bias that pulls the electrons back out of the trap or neutralizes them with holes. This operation subjects the thin oxide layers to electrical pressure.
Cell Degradation
Repeated program and erase cycles eventually wear out the physical structure of the memory. The insulating properties of the oxides diminish, making it harder to distinguish between a programmed and an erased state.
