A method includes providing a substrate including a channel region, the substrate comprising a two-stage structure having a first surface, a second surface higher than the first surface and a third surface connected between the first surface and the second surface; covering the substrate from a top thereof with an oxide layer; forming a ferroelectric material strip on a topmost surface of the oxide layer; and forming a gate strip covering the ferroelectric material strip and the oxide layer from a top of the gate strip.