FIG. 14 shows the one or more lithography and etching operations to fabricate the structures shown in FIGS. 13A-13F on the same substrate, according to some embodiments. To fabricate all of the structures shown in FIGS. 13A-13F, at most three lithography/etching operations can be performed. For example, the first, third and fifth S/D regions for a one-wire contact structure, a three-wire contact structure and a five-wire contact structure, respectively, are subjected to a first etching operation to a depth of D, which corresponds to an etching depth to cut and expose ends of the uppermost second semiconductor layer 25, but not reach the next second semiconductor layer 25. The second, fourth and sixth S/D regions for a two-wire contact structure, a four-wire contact structure and a six-wire contact structure, respectively, are covered by, for example, photoresist formed by a lithography operation. Then, the second, third and sixth S/D regions are subjected to a second etching operation to a depth of 2D, while the first, fourth and fifth S/D regions are covered. Further, the fourth, fifth and sixth S/D regions are subjected to a third etching operation to a depth of 3D, while the first, second and third S/D regions are covered. The order of the first to third etching operations can be any order.