US Patent 7579623 Stacked transistors and process

A method of horizontally stacking transistors on a common semiconductor substrate is initiated by providing a single crystal, generally silicon, semiconductor substrate. A plurality of transistors are formed on the single crystal semiconductor substrate and encapsulated in an insulating layer, such as silicon dioxide. One or more openings are formed through the insulating layer between the plurality of transistors so as to expose a surface of the single crystal semiconductor substrate. A layer of single crystal rare earth insulator material is epitaxially grown on the exposed surface of the single crystal semiconductor substrate. A layer of single crystal semiconductor material, generally silicon, is epitaxially grown on the layer of single crystal rare earth insulator material. An intermixed transistor is formed on the layer of single crystal semiconductor material.