Abnormal threshold voltage shift by the effect of H2O during negative bias stress in amorphous InGaZnO thin film transistors

• Two-phase V T shift occurred under negative gate bias stress after soaking TFTs in H 2 O. • Abnormal V T shift was caused by dissociation of H 2 O: H + and OH − . • V T decreased due to H + trapping, then increased because of neutralization between H + and OH − . • Recovery also occurred in two phase: it was recombination process. • TCAD simulation identified the mechanisms. We observed abnormal threshold voltage ( V T ) shift in amorphous InGaZnO (a-IGZO) thin-film transistors under negative gate bias stress (NBS) after soaking them in H 2 O (pH 8). Before NBS, we soaked a-IGZO TFTs in H 2 O. During application of NBS, V T decreased by −0.43 V, then increased to nearly the initial value. We hypothesize that the electrical field that was applied during NBS caused some dissociation of H 2 O to hydrogen ions (H + ) and hydroxide ions (OH − ); the effects between H + and OH − are responsible for the changes of Δ V T . The initial decrease was a result of trapping of H + at the front channel; the subsequent increase was caused by neutralization of the H + and the OH − ; the a-IGZO was very thin, so the front channel and the back channel could affect each other; therefore, mitigation of energy band bending was possible. Recovery after NBS also occurred in two-phases: V T first increased then decreased to its initial value. During the recovery process, accumulation of an OH − layer generated electric field that attracted H + so that the two species recombined. Increase in Δ V T occurred due to desorption of H + from the front-channel interface, and decrease in Δ V T occurred by recombination.