A method comprises growing an epitaxial layer on a first region of a first wafer while remaining a second region of the first wafer exposed; forming a first dielectric layer over the epitaxial layer and the second region; forming a first transistor on a second wafer; forming a second dielectric layer over the first transistor; bonding the first and second dielectric layers; and forming second and third transistors on the epitaxial layer and on the second region of the first wafer, respectively.