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String read current ( ${I}_{\textsf {read}}$ ) reduction with rising mold height and grain boundary traps is one of the major hurdle in the development of 3-D NAND flash memory. In this paper, we have investigated ${I}_{\textsf {read}}$ with variation in polysilicon channel grain size (GS), grain boundary trap density, and channel thickness ( ${T}_{\textsf {Si}}$ ), using TCAD. We find that under a critical value of GS, ${I}_{\textsf {read}}$ decreases with increase in ${T}_{\textsf {Si}}$. This is attributed to the fact that with smaller GS, the total number of grain boundaries and associated traps are significantly higher. Moreover, there exists a typical value of GS for which ${I}_{\textsf {read}}$ is independent of ${T}_{\textsf {Si}}$ , which is desirable to minimize the deviations in ${I}_{\textsf {read}}$ arising from ${T}_{\textsf {Si}}$ variations. The resulting tradeoff in the design of more efficient 3-D NAND flash is demonstrated and discussed. Further, it is found that ${I}_{\textsf {read}}$ increases significantly by limiting the polysilicon channel grain boundary trap concentration under 1012 cm−2. The results presented in this paper are crucial for optimizing ${I}_{\textsf {read}}$ and program/erase threshold voltage ( ${V}_{T}$ ) window, and serve as key guidelines in the design of 3-D NAND flash memory with better performance. [ABSTRACT FROM AUTHOR]