A semiconductor device, comprising: a string of transistors stacked along a vertical direction above a substrate of the semiconductor device, the string of transistors comprising: a first substring of transistors having a first channel structure that includes a first channel layer and a first gate dielectric structure that extend along the vertical direction; a channel connector disposed above the first substring; and a second substring of transistors stacked above the channel connector, the second substring having a second channel structure that includes a second channel layer and a second gate dielectric structure that extend along the vertical direction, the second channel layer being formed over the sidewalls of a channel opening, the channel connector electrically coupling the first channel layer and the second channel layer to form a channel layer of the string of transistors, and being disposed below the second gate dielectric structure, characterised in that the channel connector includes a first structure and a second structure, the first structure is in contact with the first channel layer and has a first upper surface with a first recess that is overfilled by the second structure, the second structure is disposed in the first recess and is in contact with the second channel layer and has a second upper surface with a second recess, the second upper surface is above the first upper surface; the second channel layer is disposed in the second recess and the second gate dielectric structure is disposed on the second upper surface. a string of transistors stacked along a vertical direction above a substrate of the semiconductor device, the string of transistors comprising: a first substring of transistors having a first channel structure that includes a first channel layer and a first gate dielectric structure that extend along the vertical direction; a channel connector disposed above the first substring; and a second substring of transistors stacked above the channel connector, the second substring having a second channel structure that includes a second channel layer and a second gate dielectric structure that extend along the vertical direction, the second channel layer being formed over the sidewalls of a channel opening, the channel connector electrically coupling the first channel layer and the second channel layer to form a channel layer of the string of transistors, and being disposed below the second gate dielectric structure, characterised in that the channel connector includes a first structure and a second structure, the first structure is in contact with the first channel layer and has a first upper surface with a first recess that is overfilled by the second structure, the second structure is disposed in the first recess and is in contact with the second channel layer and has a second upper surface with a second recess, the second upper surface is above the first upper surface; the second channel layer is disposed in the second recess and the second gate dielectric structure is disposed on the second upper surface. a first substring of transistors having a first channel structure that includes a first channel layer and a first gate dielectric structure that extend along the vertical direction; a channel connector disposed above the first substring; and a second substring of transistors stacked above the channel connector, the second substring having a second channel structure that includes a second channel layer and a second gate dielectric structure that extend along the vertical direction, the second channel layer being formed over the sidewalls of a channel opening, the channel connector electrically coupling the first channel layer and the second channel layer to form a channel layer of the string of transistors, and being disposed below the second gate dielectric structure, characterised in that the channel connector includes a first structure and a second structure, the first structure is in contact with the first channel layer and has a first upper surface with a first recess that is overfilled by the second structure, the second structure is disposed in the first recess and is in contact with the second channel layer and has a second upper surface with a second recess, the second upper surface is above the first upper surface; the second channel layer is disposed in the second recess and the second gate dielectric structure is disposed on the second upper surface.