Transistor structures employing metal chalcogenide channel materials may be formed where a chalcogen is introduced into at least a portion of a precursor material that comprises reactive metal(s). The precursor material may be substantially metallic, or may be a metallic oxide (e.g., an oxide semiconductor). The metal(s) may be transition, Group II, Group III, Group V elements, or alloys thereof. An oxide of one or more such metals (e.g., IGZO) may be converted into a chalcogenide (e.g., IGZSx or IGZSex) having semiconducting properties. The chalcogenide formed in this manner may be only a few monolayers in thickness (and may be more thermally stable than many oxide semiconductors. Where not all of the precursor material is converted, a transistor structure may retain the precursor material, for example as part of a transistor channel or a gate dielectric. Backend transistors including metal chalcogenide channel materials may be fabricated over silicon CMOS circuitry.