The present disclosure provides a method of manufacturing a semiconductor device. The method includes providing a structure having a frontside and a backside, the structure including a substrate and a stack of a first type and a second type epitaxial layers having different material compositions alternatively stacked above the substrate, wherein the stack is at the frontside of the structure and the substrate is at the backside of the structure; patterning the stack, thereby forming a fin above the substrate; implanting a first dopant into a first region of the fin, the first dopant having a first conductivity type; implanting a second dopant into a second region of the fin, the second dopant having a second conductivity type opposite the first conductivity type; and forming a first contact on the first region and a second contact on the second region.