Techniques are provided herein to form semiconductor devices having strained channel regions. In an example, semiconductor nanoribbons of silicon germanium (SiGe) or germanium tin (GeSn) may be formed and subsequently annealed to drive the germanium or tin inwards along a portion of the semiconductor nanoribbons thus increasing the germanium or tin concentration through a central portion along the lengths of the one or more nanoribbons. Specifically, a nanoribbon may have a first region at one end of the nanoribbon having a first germanium concentration, a second region at the other end of the nanoribbon having substantially the same first germanium concentration (e.g., within 5%), and a third region between the first and second regions having a second germanium concentration higher than the first concentration. A similar material gradient may also be created using tin. The change in material composition (gradient) along the nanoribbon length imparts a compressive strain.