Fowler-Nordheim formula derivation), but composed of the series of a trapezoidal barrier in the oxide, and a triangular barrier in the nitride. The Poole-Frenkel process is the limiting mechanism of conduction at the beginning of the memory programming regime due to the higher current provided by tunneling. As the trapped electron charge raises, beginning to screen the field, the modified Fowler-Nordheim tunneling becomes the limiting process. The trapped charge density at the oxide-nitride interface is proportional to the integral of the Poole-Frenkel current flowed across it. With an increasing number of memory write and erase cycles, retention characteristics worsen due to the increasing bulk conductivity in the nitride. See also Schottky effect Charge trap flash Electrical breakdown Dielectrics References Condensed matter physics Solid state engineering