A method includes forming a first transistor, a second transistor, a third transistor, and a fourth transistor over a substrate, wherein at least the second and third transistors include ferroelectric materials; forming an interlayer dielectric (ILD) layer over the first to fourth transistors; forming a first metal line over the ILD layer to interconnect drains of the second and third transistors and a gate of the fourth transistor; forming a second metal line over the ILD layer to interconnect a drain of the first transistor and gates of the second and third transistors; forming a write word line over the ILD layer and electrically connected to a gate of the first transistor but electrically isolated from the fourth transistor; forming a word line over the ILD layer and electrically connected to a source of the first transistor; and forming a bit line electrically connected to the fourth transistor.