Transport model in n/sup ++/-poly/SiO/sub x//SiO/sub 2//p-sub MOS capacitors for low-voltage nonvolatile memory applications

The origin of enhanced injection in n/sup ++/-poly/SiO/sub x//SiO/sub 2//p-sub MOS capacitors under accumulation is investigated. Starting from experimental evidences as the structural homogeneity of the off-stochiometric oxide and the temperature dependence of current in n/sup ++/-poly/SiO/sub x//p-sub capacitors, we developed a new transport model. In this picture, transport consists on the Poole-Frenkel and multistep tunneling of the SiO/sub x/ barrier and the Fowler-Nordheim (FN) tunnel of the SiO/sub 2/ barrier, the latter definitely limiting the current flowing through the MOS. The model explains how the presence of two barriers and an accelerating electric field in the SiO/sub x/ gives rise to the injection enhancement, respect to the case of a single conventional n/sup ++/-poly/SiO/sub 2/ barrier. In fact, after the trap-assisted tunnel of the first barrier, the electron arrives at the SiO/sub x//SiO/sub 2/ interface with an excess energy furnished by the electric field. There, it sees an FN barrier lower than in the conventional case. Experiment and model calculations are in excellent agreement.