Three-dimensional (3D) transistors and methods of manufacturing thereof include a first semiconductor fin extending over a substrate. The first semiconductor fin has a vertical recess extending from a first sidewall of the first semiconductor fin toward a second sidewall of the first semiconductor fin opposite the first sidewall. A distance between two opposing sidewalls of the vertical recess decreases as the vertical recess extends toward the second sidewall of the first semiconductor fin. The device further includes a vertically recessed channel region between the second sidewall of the first semiconductor fin and a bottom of the vertical recess, source/drain (S/D) regions at opposite ends of the vertically recessed channel region, and a gate stack over the vertically recessed channel region.