ZnO TFTs with bottom gate top S/D contact architecture were fabricated by sputtering of ZnO with layer thicknesses from 30 nm to 100 nm. The effect of post deposition annealing in oxygen and forming gas atmospheres at 400°C to 500°C on the devices was investigated. The tendencies of a lower threshold voltage Vth
and a higher saturation mobility μsat
for higher annealing temperature can be observed for both oxygen and forming gas annealing. Reduction of trap density in oxygen annealing and additional hydrogen incorporation in forming gas annealing play an important role for these electrical parameters. Morphological changes of increased grain size and fewer grain boundaries in the channel also contribute to tendencies in electrical characteristics of ZnO TFTs.