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, spincasting, 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.