Using a dual spacer consisting of 50% SiO2 and 50% HfO2, the ratio of the on-state current/the off-state current in the order of 106 is achieved for the explored devices. Based on the experimentally validated simulation, the result indicates that the variation of gate-capacitance is significant owing to the sizeable parasitic capacitance resulting from the spacer. The role of the spacer acts as the parallel plate capacitor, the amount of gate capacitance will be increased largely with the HfO2 due to its high parasitic capacitance. For the devices without any spacers, induced by a donor-type single charge trap (SCT), the statistically calculated highest amplitude of random telegraph noise (RTN) is 2.32%. It occurs when the SCT locates in the middle of the channel due to the high occurrence of capture and emission for SCT under low gate voltage. Notably, for devices with considerable spacers, the RTN can be significantly suppressed (≤1%).