Dielectric isolation layer between a nanowire transistor and a substrate

Gate all around semiconductor devices, such as nanowire or nanoribbon devices, are described that include a low dielectric constant ("low-k") material disposed between a first nanowire closest to the substrate and the substrate. This configuration enables gate control over all surfaces of the nanowires in a channel region of a semiconductor device via the high-k dielectric material, while also preventing leakage current from the first nanowire into the substrate.