Metal-oxide-semiconductor field-effect transistors (MOSFETs) with a functional gate, which enables self-adjustment of threshold voltage (Vth), were proposed for ultralow power operation and fabricated with conventional complementary metal-oxide-semiconductor (CMOS) technology. In the on-current state of fabricated nMOSFETs, electron ejection from the charge trap layer by direct tunneling makes Vth low and increases on-current further. In the off-current state, electron injection into the charge trap layer makes Vth high and suppresses subthreshold leakage current. Although the characteristic time of electron transfer of the functional gate from on-current state to off-current state is fairly long, the logic mode operating principle has been verified with the experimental device. Reduction of tunnel oxide thickness (Tox) will reduce the time, which will lead to the practical use of the proposed device for CMOS logic application.