A functional gate metal-oxide-semiconductor field-effect transistor that enables self-adjustment of threshold voltage (Vth) was developed for the ultralow power operation. The operating principle enables the on-current to be increased without increasing the off-current. Prototype devices were fabricated with complementary metal-oxide-semiconductor (CMOS) fabrication technology using a silicon-on-insulator substrate, and the fundamental device characteristics necessary for ultralow power operation were demonstrated with an emphasis on the device reliability. A negative Vth shift was caused by electron ejection from the poly-Si charge trap layer, and a positive V th shift was caused by electron injection from the top gate electrode. A fabricated device endured 105 electron ejection-and-injection cycles when only a positive bias Vg was applied. Endurance characteristics of the fabricated devices showed that the number of cycles to oxide breakdown increased as the channel size decreased. The authors explained the SiO2 breakdown mechanism by using a percolation model. They consider that scaling down of the channel size and the thickness of the tunnel gate oxide will open the way to the development of CMOS logic applications for this device.