Non-linear coupling voltage of split-gate flash memory cells with additional top coupling gate

The study presents the dependence of floating gate (FG) coupling potential, V_FG on the source line (SL) programming voltage, V_SL of the split-gate flash memory cells with an additional top coupling gate above FG, called the `SG-TCG` cells. The mathematical analysis shows non-linear V_FG against V_SL behaviour of SG-TCG cells depending on the operation region of FG-MOSFETs. It is found that as the value of V_SL increases, the value of V_FG initially increases steeply, then gradually and finally, linearly with a lower slope. This anomalous V_FG against V_SL behaviour is because of the potential drop in the bulk of FG-MOSFETs by the applied V_SL. The mathematical analysis, also, shows SL coupling factor (κ_SL) roll-off because of the increase in the FG-MOSFETs body potential with the increase in V_SL. In addition, κ_SL is shown to approach a constant value in the saturation region of FG-MOSFETs where V_FG is less susceptible to supply voltage fluctuation. The mathematical analysis agrees very well with the numerical device simulation. The study, clearly, shows that in order to achieve higher shift in programme cell threshold voltage and reduce performance variability owing to supply voltage fluctuation, the target programming bias V_SL of nanoscale SG-TCG cells must be higher than the saturation voltage of FG-MOSFETs.