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Because of the rapid diffusion of hydrogen in zinc oxide even at a relatively low temperature, zinc-oxide-based thin-film transistors (TFTs) with hydrogen-doped source/drain regions suffer from degraded thermal stability. The use of boron, which is a heavier and a more slowly diffusing dopant, is systematically investigated as a replacement of hydrogen. Its effectiveness as a dopant has been studied in terms of a range of process conditions, including its implantation dosage and the subsequent heat treatment temperature, time, and ambience. The lowest resistivity of 2 mΩ-cm has been obtained at a boron dose of 1016/cm2. Self-aligned top-gated zinc-oxide TFTs with source/drain regions doped with implanted boron are shown to be more stable than those doped with hydrogen, even when subjected to the relatively high temperature needed for the formation of a good-quality passivation layer.