The stack structure tuning of the ferroelectric tunnel junction (FTJ) devices is reported based on the ferroelectric (FE) layer thickness and interface layer (IL) type/thickness optimization to maximize the FTJ I_on/I_offratio. A FE thickness scaling shows a low voltage FTJ operation, further challenged by a diminishing trend in the maximum I_on/I_offratio due to the thickness dependence of the FE polarization, independent of the IL thickness. The maximum I_on/I_offratio varies by tuning the IL type (SiO₂, Al₂O₃) and thickness (1 nm, 2 nm), indicating a maximum at the SiO₂(1 nm) IL condition. A stable endurance of 10⁴cycles is limited by the high field/cycles induced IL degradation, a stable FTJ at 10y extrapolated retention time is shown. The FTJ synaptic device operation is reported with insight on the stack structure tunmg impact on the synaptic LTP/LTD nonlmeanty and maximum dynamic range.