A three-dimensional integration technology that electrically connects an independently optimized thin-film device layer to a Si circuitry layer is reported in this paper. An epitaxial liftoff GaAs thin-film optical detector is integrated directly on top of Si amplifier circuitry with a planarizing, insulating layer of polyimide between the detector and the circuitry. The detector is vertically connected to the circuitry below through an electrical via in the insulator. This integration technology enables monolithic, massively parallel vertical interconnection between two independently optimized device layers. Systems such as image processing arrays will significantly bene- fit from this massively parallel integration technology. © 1992 IEEE