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 poststress characteristics of TFTs depend not only on the stress condition but 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-current of unhydrogenated TFTs are improved due to the annealing effect of donor-like traps, which is caused by tunneling electrons/captured holes interaction. While under on-state stress conditions ( V gs =-20 V and V ds =-15 V), these characteristics are first improved, then deteriorated because of electron trapping in the gate oxide and carrier-induced metastable defects in the channel. The poststress characteristics will not be improved if the trap states have been previously removed by hydrogenation. In addition, we found that the poststress characteristics under off-state stress conditions are not changed after thermal annealing at 250°C for 30 min, while the metastable defects and electron trapping under on-state stress conditions can be easily annealed after thermal treatment.