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This paper describes a comprehensive study on the threshold voltage (Vth) controllability of four-terminal-driven double-gate (DG) MOSFETs (4T-XMOSFETs) with independently switched DGs. Two types of 4T-XMOSFETs (fin and vertical) are experimentally demonstrated and their Vth controllability is thoroughly investigated in relation to the initial Vth in the DG-mode based on comprehensible modeling of the devices. Based on the investigation and simulated predictions, device design guidelines for 4T-XMOSFETs are proposed. Decreasing the workfunction of the DGs and increasing the oxide thickness of the second gate (Tox2) are preferable for improving the performance of the 4T-XMOSFET. The optimum workfunction of DGs for attaining low Ioff(stand-by) and high Ion(active) under the limited Vg2 condition is also proposed.