A device includes a first channel; a second channel above the first channel; and a gate structure surrounding the first and second channels, wherein the gate structure includes a ferroelectric (FE) layer surrounding the first and second channels and a gate metal layer surrounding the FE layer. The device further includes two first electrodes connected to two sides of the first channel; two second electrodes connected to two sides of the second channel; a dielectric layer between the first and the second electrodes; and an inner spacer layer between the two first electrodes and the gate structure.