MODELING OF THE CHARGE BALANCE CONDITION ON FLOATING GATES AND SIMULATION OF EEPROMS

A robust numerical model for the charge balance condition (CBC) on flo ating gates (FG) is presented, in which the quasi-Fermi level, instead of the potential (as in conventional models), is assumed to be a cons tant in the FG. By solving Poisson's equation in the FG region with an extra equation for the CBC, the accurate distribution of both potenti al and charge can be obtained. A quasi-stationary scheme is developed to simulate the writing or erasing procedure of EEPROM cells, which ma y serve as a useful tool for accurate simulation of devices, such as E EPROM's or EPROM's. Simulation results reveal a significant degradatio n of tunnelling efficiency when N(FG) < 10(20) cm-3, which is improper ly ignored in the flat-potential model.