An integrated circuit (IC) device includes a fin-type active region extending longitudinally in a first lateral direction on a substrate. A nanosheet is apart from a fin top surface of the fin-type active region in a vertical direction. An inner insulating spacer is between the substrate and the nanosheet. A gate line includes a main gate portion and a sub-gate portion. The main gate portion extends longitudinally in a second lateral direction on the nanosheet. The sub-gate portion is integrally connected to the main gate portion and between the substrate and the nanosheet. A source/drain region is in contact with the inner insulating spacer and the nanosheet. The source/drain region includes a single crystalline semiconductor body and at least one lower stacking fault surface linearly extending from the inner insulating spacer through the single crystalline semiconductor body.