We investigated the transient electrical response of deep UV or near UV-treated ZnO-based bottom gate transparent thin film transistors (TTFTs). In both cases, the turn-on voltage (V-ON) negatively shifts due to the treatment and subsequently relaxes in the positive direction. The subthreshold swing as well as the off-current also decreases with time following the initial increase. However, VON of the near UV-treated TTFT apparently recovers its initial value, whereas that of the deep UV-treated TTFT fails to fully return. Furthermore, the relaxation of the electrical properties with time appears much faster in case of the near UV-treated TTFT. We demonstrated that exposure to the near UV illumination does not affect SiNx, the gate insulator (GI), and that the reversible variation of electrical properties in this case is a consequence of the ionization/deionization of oxygen vacancies present in ZnO. In case of the deep UV-treated TTFT, on the other hand, GI also absorbs the UV energy to induce inter-diffusion of oxygen and hydrogen near the ZnO/SiNx interface. We stated that these spectroscopic changes near the channel region brought about irreversible modifications to the electrical properties of the deep UV-treated TTFT as well as formation of new trap centers in GI. The slower recovery in this case is a consequence of de-trapping of the charges trapped by the centers newly formed in SiNx near the interface. (C) 2012 Elsevier B. V. All rights reserved.