EEPROM cell design: paradoxical choice of the coupling ratio

Generally the design of the memory cell, as an electrically erasable programmable read-only memory, is based on the calculation of coupling ratio. This step is fundamental because it determines cell performances. Practically designers use the maximum coupling ratio to decrease the necessary supply voltage. Despite lower voltage, we show in this paper that this approach induces a paradoxical increase of electric field across tunnel oxide which is directly related to the lifetime of the cell. Using simulations, we demonstrate that the optimum coupling ratio depends on the best compromise between a smaller supply voltage and the cell lifetime.