Transistor structures and methods of fabricating transistor structures are provided. The methods include: fabricating a transistor structure at least partially within a substrate, the fabricating including: providing a cavity within the substrate; and forming a first portion and a second portion of the transistor structure at least partially within the cavity, the first portion being disposed at least partially between the substrate and the second portion, where the first portion inhibits diffusion of material from the second portion into the substrate. In one embodiment, the transistor structure is a field-effect transistor structure, and the first portion and the second portion include one of a source region or a drain region of the field-effect transistor structure. In another embodiment, the transistor structure is a bipolar junction transistor structure.