The present invention includes methods for stressing transistor channels of semiconductor device structures. Such methods include the formation of so-called near-surface “nanocavities” adjacent to the source/drain regions, forming extensions of the source/drain regions adjacent to and including the nanocavities, and implanting matter of a type that will expand or contract the volume of the nanocavities, depending respectively upon whether compressive strain is desirable in transistor channels between the nanocavities, as in PMOS field effect transistors, or tensile strain is wanted in transistor channels, as in NMOS field effect transistors, to enhance carrier mobility and transistor speed. Semiconductor device structures and semiconductor devices including these features are also disclosed.