Current Enhancement Behavior Under Positive Bias Stress in a-InGaZnO Thin-Film Transistors

Enhancement behavior on saturation current of output characteristics in amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors under positive bias stress (PBS) is investigated. The threshold voltage (V_th) of the a-IGZO TFT demonstrates a typical positive shift during PBS. Notably, the output current at an identical overdrive voltage (V_ov) initially rises with increasing bias stress duration. The observed phenomena are elucidated by the detrapping of positively charged defects situated at the interface between the dielectric and active layer due to electrons induced by bias stress during PBS, which diminishes carrier scattering at the channel interface. Following the full release of positive interface charge, additional electron trapping states emerge. The presence of trapped electrons intensifies carrier scattering at the channel interface, leading to a degradation in drive current. Low-frequency noise (LFN) measurements are conducted to verify the suggested mechanism of PBS instability in a-IGZO TFTs. Moreover, the energy distribution of PBS-induced traps is shown by C-V characterization to be exponential, dominated by shallow traps. Passivation greatly enhanced the PBS stability, which is attributed to hydrogen doping-induced defect passivation and the shielding of the back-channel interface from the air atmosphere.