Semiconductor devices and methods of forming the same include forming spacers on respective sidewalls above a stack of alternating channel layers and sacrificial layers, leaving an opening between the spacers. The stack is etched, between the spacers, to form a central opening in the stack that separates the channel layers into respective pairs of channel structures. The sacrificial material is etched away to expose top and bottom surfaces of the channel structures. A gate stack is formed on, between, and around the channel structures, including in the central opening between pairs of channel structures.