A method is provided for forming a semiconductor device. The method includes: forming a device layer stack on a substrate, the device layer stack comprising a bottom sacrificial layer and an alternating sequence of upper sacrificial layers and channel layers; forming a sacrificial gate structure; etching through at least the upper sacrificial and channel layers of the device layer stack while using the sacrificial gate structure as an etch mask; forming a sacrificial spacer covering end surfaces of the upper sacrificial and channel layers; while the sacrificial spacer masks the end surfaces of the upper sacrificial and channel layers, further etching the device layer stack to remove the bottom sacrificial layer and thereby form a cavity in the device layer stack; forming a dielectric layer in the cavity, wherein forming the dielectric layer comprises depositing and then etching back a dielectric bottom material to a level below a bottom-most one of the channel layers; removing the sacrificial spacer; forming recesses and forming inner spacers in the recesses; and forming source and drain regions by epitaxially growing semiconductor material on the end surfaces of the channel layers.