Aspects of the disclosure provide a semiconductor device and a method to manufacture the semiconductor device. A trench is formed in a stack over a substrate of the semiconductor device where the stack includes alternating first layers and second layers. The trench has a first sidewall and a second sidewall opposite to the first sidewall. Channel materials are formed along the first and second sidewalls of the trench, respectively. The trench is further divided into multiple units by replacing portions of the channel materials with first dielectric structures. Remaining portions of the channel materials along the first and second sidewalls form first and second channel structures of first and second strings of transistors, respectively. The second layers are replaced with first and second gate structures of the first and second strings of transistors, respectively. Each of the first and second strings of transistors is vertically stacked over the substrate.