Bias and W/L dependence of capacitive coupling coefficients in floating gate memory cells

In this paper, a complete study of capacitive coupling coefficients dependence on bias and W/L will be presented, including a review on classic methods to extract their value from electrical characterization. Capacitive coupling ratios have been usually adopted to model floating gate (FG) memory cells, in particular to deduce the value of FG potential. Now they are determined by means of a new model (recently proposed in the literature), starting from a new procedure to evaluate the FG potential. Results obtained with this new model will be compared to classic values. Particularly, their bias dependence (during write/read/erase of Flash memory cells) will be deeply investigated, thus demonstrating the limits of considering them constants, as is usually done. By analyzing their W/L dependence, we will deduce useful information on the effects of scaling and the role played by fringing capacitances. The most important methods reported in the literature to estimate the control gate and drain capacitive coupling ratios will be accurately reviewed, thus showing that such procedures are often cumbersome and inaccurate. It is worth noting that for the first time, alpha (B) and alpha (S) will be studied in detail.