A method of forming a semiconductor structure includes forming a nanosheet stack over a substrate, the nanosheet stack including alternating sacrificial and channel layers, the channel layers providing nanosheet channels for nanosheet field-effect transistors. The method also includes forming vertical fins in the nanosheet stack and a portion of the substrate, and forming indents in sidewalls of the sacrificial layers at sidewalls of the vertical fins. The method further includes forming nanosheet extension regions in portions of the channel layers which extend from the indented sidewalls of the sacrificial layers to the sidewalls of the vertical fins, the nanosheet extension regions increasing in thickness from the indented sidewalls of the sacrificial layers to the sidewalls of the vertical fins. The method further includes forming inner spacers using a conformal deposition process that forms air gaps in spaces between the nanosheet extension regions and the indented sidewalls of the sacrificial layers.