Embodiments of the disclosure are in the field of advanced integrated circuit structure fabrication and, in particular, integrated circuit structures having channel structures with sub-fin dopant diffusion blocking layers are described. In an example, an integrated circuit structure includes a fin having a lower fin portion and an upper fin portion. The lower fin portion includes a dopant diffusion blocking layer on a first semiconductor layer doped to a first conductivity type. The upper fin portion includes a portion of a second semiconductor layer, the second semiconductor layer on the dopant diffusion blocking layer. An isolation structure is along sidewalls of the lower fin portion. A gate stack is over a top of and along sidewalls of the upper fin portion, the gate stack having a first side opposite a second side. A first source or drain structure at the first side of the gate stack.