A semiconductor device at least one first transistor of a first type disposed above a substrate and comprising a channel wider in one cross-section than tall, wherein the first type is a PFET transistor or an NFET transistor; and at least one second transistor of a second type disposed above the at least one first transistor and comprising a channel taller in the one cross-section than wide, wherein the second type is a PFET transistor or an NFET transistor, and the second type is different from the first type. Methods and systems for forming the semiconductor structure.