A method to process a 3D device, the method including: providing a first substrate including a first level including a first single crystal silicon layer and a plurality of first transistors; providing a second substrate including a second level including a second single crystal silicon layer; performing an epitaxial growth of a SiGe layer on top of the second single crystal silicon layer; performing an epitaxial growth of a third single crystal silicon layer on top of the SiGe layer; forming a plurality of third transistors including the third single crystal silicon layer; forming a plurality of metal layers interconnecting the plurality of third transistors; and then performing a hybrid bonding of the second level onto the first level.