In transistor structures such as thin film transistors (TFTs) in an array of cells, a layer of semiconducting oxide material that includes a channel is protected by a protective layer that includes low-temperature encapsulant material. The semiconducting oxide material can be a transition metal oxide material such as zinc oxide, and can be in an active layered substructure that also includes channel end electrodes. The low-temperature encapsulant can, for example, be an organic polymer such as poly(methyl methacrylate) or parylene, deposited on an exposed region of the oxide layer such as by spinning, spin-casting, evaporation, or vacuum deposition or an inorganic polymer deposited such as by spinning or liquid deposition. The protective layer can include a lower sublayer of low-temperature encapsulant on the exposed region and an upper sublayer of inorganic material on the lower sublayer. For roll-to-roll processing, a mechanically flexible, low-temperature substrate can be used.