We have analyzed the time-temperature dependence of positive bias stress (PBS) and recovery in amorphous indium-gallium-zinc-oxide (a-IGZO) Thin-film-transistors (TFTs) incorporating SiO2 back channel passivation. The data are fitted to stretched exponentials, yielding the time constant τ and stretch parameter β as fitting parameters. As-fabricated samples and samples annealed in vacuum at 250 °C 200 h are compared. The time constant for room temperature stress increases fivefold with the 200 h anneal to the value τ=1.3×106 s. The dependence of τ from stress temperature is well described by an Arrhenius plot, with activation Eτ=0.95 eV. Stress and recovery show very similar activation energies, supporting the defect formation in the bulk or at the gate insulator/a-IGZO interface as the mechanism responsible for PBS.