Semiconductor devices and methods of forming the same include partially etching sacrificial layers in a first stack of alternating sacrificial layers and channel layers to form first recesses. A first inner spacer sub-layer is formed in the first recesses from a first dielectric material. A second inner spacer sub-layer is formed in the first recesses from a second dielectric material, different from the first dielectric material. The sacrificial layers are etched away. The first inner spacer sub-layer is etched away. A gate stack is formed on and around the channel layers and in contact with the second inner spacer sub-layer.