Embodiments of three-dimensional (3D) memory devices formed by bonded semiconductor devices and methods for forming the same are disclosed. In an example, a method for forming a semiconductor device is disclosed. The method includes the following operations. First, an insulating material layer can be formed over a substrate. In an example, single-crystalline silicon is not essential to the substrate. The insulating material layer can be patterned to form an isolation structure and a plurality of trenches in the isolation structure. A semiconductor material can be deposited to fill up the plurality of trenches to form a plurality of array-base regions in the isolation structure, the isolation structure insulating the plurality of array-base regions from one another. Further, a plurality of memory arrays can be formed over the plurality of array-base regions, and an insulating structure can be formed to cover the plurality of memory arrays and the plurality of array-base regions.