The present disclosure relates to semiconductor structures and, more particularly, to nanosheet transistor structures with tunable channels and inner sidewall spacers and methods of manufacture. The structure includes: a plurality of stacked semiconductor nanosheets over a semiconductor substrate; a plurality of gate structures surrounding individual nanosheets of the plurality of semiconductor nanosheets, with a lower gate structure comprising a length at least equal to a length of each remaining gate structure of the plurality of gate structures; an inner sidewall spacer adjacent each of the plurality of gate structures; and source/drain regions on opposing sides of the plurality of gate structures, separated therefrom by the inner sidewall spacer.