The devices of n-channel poly silicon thin film transistors(TFTs) hydrogenated by plasma, $H_2$ and $H_2$/plasma processes are fabricated. The carriers sensitivity characteristics are analyzed with voltage bias stress at the gate oxide. The parametric sensitivity characteristics caused by electrical stress conditions in hydrogenated devices are investigated by measuring the drain current, threshold voltage($V_{th}$), subthreshold slope(S) and maximum transconductance($G_m$) values. As a analyzed results, the degradation characteristics in hydrogenated n-channel polysilicon thin film transistors are mainly caused by the enhancement of dangling bonds at the poly-Si/$SiO_2$ interface and the poly-Si grain boundary due to dissolution of Si-H bonds. The generation of traps in gate oxide are mainly dued to hot electrons injection into the gate oxide from the channel region.