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The edge encroachment of tunnel oxide is experimentally found to degrade the Fowler-Nordheim (FN) tunneling gate current of NAND-type Flash cells. This work elucidates the impact of edge encroachment on FN tunneling current for use in programming and erasing operations. The fringing field effect and tunnel oxide with trapezoidal edge are considered in the determination of physical gate current in which a conformal-mapping method is used to estimate the contribution of the fringing fields. These analytical results are confirmed using 2-D device simulations and experimental measurements. The results show that the overlapped encroachment causes an exponential degradation of intrinsic FN tunneling current. Preventing the encroachment of lateral edges resulting from overall tunnel-oxide enlargement is critical to ensuring normal programming and erasing speeds in future NAND-type Flash cells.