A semiconductor device includes a base dielectric layer, a semiconductor substrate layer disposed on the base dielectric layer, and a transistor disposed in the semiconductor substrate layer. The transistor includes a gate dielectric layer disposed on the semiconductor substrate layer, a gate electrode disposed on the gate dielectric layer, source and drain electrodes disposed within the semiconductor substrate layer on opposite sides of the gate electrode, an undoped channel region, a base dopant region, and a threshold voltage setting region. The undoped channel region, base dopant region, and threshold voltage setting region are disposed within the semiconductor substrate layer. The undoped channel region is disposed between the source electrode and the drain electrode, and the base dopant region and the threshold voltage setting region extend beneath the source electrode and the drain electrode. The threshold voltage setting region is disposed between the undoped channel region and the base dopant region.