This letter presents amorphous TiO2-based thin-film phototransistors whose structures are designed for front-side and back-side illumination. The channel and gate dielectric layers are deposited by ultrasonic spray pyrolysis deposition, which is a non-vacuum, cost-effective, thin-film deposition technique similar to chemical vapor deposition. The material structure, chemical composition, and optical characteristics of the amorphous TiO2 thin film are analyzed. The phototransistor shows the typical current–voltage ( ${I}$ – ${V}$ ) characteristics of field-effect transistors. It demonstrates a UV-to-visible rejection ratio of approximately 70 –80, a high responsivity of 13.64 A/W, and a high detectivity of $1.64\times 10^{12}$ Jones. The phototransistor can be operated in the high-photo-responsivity and high-detectivity modes by biasing at different quiescent points.