Diodes and fabrication methods thereof are presented. The diodes include, for instance: a first semiconductor region disposed at least partially within a substrate, the first semiconductor region having a first conductivity type; and a second semiconductor region disposed at least partially within the first semiconductor region, the second semiconductor region having a second conductivity type, wherein the first semiconductor region separates the second semiconductor region from the substrate. In one embodiment, the substrate and the first semiconductor region have a sigma-shaped boundary. In another embodiment, the substrate and the first semiconductor region have U-shaped boundary. In a further embodiment, the first semiconductor region comprises an alloy of a first material and a second material, where the concentration of the second material varies from a maximum to a minimum, where the first semiconductor region adjacent to the second semiconductor region has the minimum of the concentration of the second material.