Abstract: Photovoltaic semiconductor structures, photovoltaic base structures for forming tandem photovoltaic cells, and methods of fabrication thereof, are provided, in which at least one tunnel junction layer is integrally formed within a semiconductor substrate via a shallow diffusion doping process. In some embodiments, two tunnel junction layers are formed within a common semiconductor substrate having a photovoltaic homojunction therein, such as silicon or germanium, via a two-step shallow diffusion doping process. In other embodiments, a first tunnel junction layer is formed within a semiconductor substrate having a photovoltaic homojunction via a shallow diffusion doping process, while a second tunnel junction layer is formed by an epitaxial or other additive process. In other embodiments, photovoltaic semiconductor structures are provided having an emitter layer and a first tunnel junction layer formed as a composite layer having a graded profile within a semiconductor substrate.