In an embodiment, a device includes: a dielectric fin on a substrate; a low-dimensional layer on the dielectric fin, the low-dimensional layer including a source/drain region and a channel region; a source/drain contact on the source/drain region; and a gate structure on the channel region adjacent the source/drain contact, the gate structure having a first width at a top of the gate structure, a second width at a middle of the gate structure, and a third width at a bottom of the gate structure, the second width being less than each of the first width and the third width.