Abstract: The preferred embodiments of the present invention provide a three-dimensional (3D) semiconductor structure and a method of forming the same. The 3D semiconductor structure includes a first substrate bonded to a second substrate. The first substrate includes substantially all NMOS devices. The second substrate includes substantially all PMOS devices. The substrates can be bonded face-to-face, face-to-back, or back-to-back. The method includes providing a first substrate and a second substrate, forming a first circuit comprising at least one NMOS device on the first substrate, wherein the first substrate includes substantially no PMOS devices, forming a second circuit comprising at least one PMOS device on the second substrate, wherein the second substrate includes substantially no NMOS devices, and bonding the first and second substrates after forming the first and second circuits.