A comparison of behaviors between hydrogenated/unhydrogenated polysilicon thin film transistors under electric stress

The effects and kinetics of electric stress on hydrogenated/unhydrogenated polysilicon thin-film transistors (poly-TFTs) have been investigated. The p oststress characteristics of TFTs depend not only on the stress condition b ut also on the hydrogenation process. Under off-state stress conditions (V- gs = 0 V and V-ds = -15 V), the poststress subthreshold swing and on-curren t of unhydrogenated TFTs are improved due to the annealing effect of donor- like traps, which is caused by tunneling electrons/captured holes interacti on. While under on-state stress conditions (V-gs = -20V and V-ds = -15 V), these characteristics are first improved, then deteriorated because of elec tron trapping in the gate oxide and carrier-induced metastable defects in t he channel. The poststress characteristics will not be improved if the trap states have been previously removed by hydrogenation. In addition, we foun d that the poststress characteristics under off-state stress conditions are not changed after thermal annealing at 250 degrees C for 30 min, while the metastable defects and electron trapping under on-state stress conditions can be easily annealed after thermal treatment.