A method includes forming a first low-dimensional layer over an isolation layer, forming a first insulator over the first low-dimensional layer, forming a second low-dimensional layer over the first insulator, forming a second insulator over the second low-dimensional layer, and patterning the first low-dimensional layer, the first insulator, the second low-dimensional layer, and the second insulator into a protruding fin. Remaining portions of the first low-dimensional layer, the first insulator, the second low-dimensional layer, and the second insulator form a first low-dimensional strip, a first insulator strip, a second low-dimensional strip, and a second insulator strip, respectively. A transistor is then formed based on the protruding fin.