Further, spatially relative terms, such as “beneath,” “below,” “l(fā)ower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. In addition, the term “being made of” may mean either “comprising” or “consisting of.” In the present disclosure, a phrase “one of A, B and C” means “A, B and/or C” (A, B, C, A and B, A and C, B and C, or A, B and C), and does not mean one element from A, one element from B and one element from C, unless otherwise described.

In a GAA FET, a driving current can be adjusted by changing dimensions (e.g., a cross sectional area) of each semiconductor wires constituting channel regions and/or by changing the number of semiconductor wires vertically arranged over a substrate. However, it is generally difficult to manufacture GAA FETs with different numbers of semiconductor wires within one semiconductor chip.

In the present disclosure, the driving current of GAA FETs is modulated by changing the number of semiconductor wires electrically connected to a source/drain epitaxial layer. In this disclosure, a source/drain refers to a source and/or a drain. It is noted that in the present disclosure, a source and a drain are interchangeably used and the structures thereof are substantially the same, unless otherwise described.