Investigation of Hump Behavior of Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistor Under Positive Bias Stress

We investigate positive bias stress (PBS)-induced hump behavior in the subthreshold current region of the transfer characteristics of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs). We analyze the origin of hump creation as parasitic edge conduction using both experiment and simulation. Based on the simulated results, we confirm that an additional narrow (~0.1 eV), Gaussian (~2.94 eV) trap states away from valance band maximum (VBM) is created within the parasitic channel after the PBS. A sharp decrease in the active layer thickness at the edge region of the active layer during the fabrication process increases the electric field during PBS which resulted in the creation of shallow oxygen vacancies at the edge region. As a result, a higher electron concentration at parasitic conduction path turned on the transistor earlier causing hump behavior in the subthreshold region of the transfer curve. Moreover, the increase of various distances between the main channel and the parasitic transistor is also simulated. It is identified that the extended width (
$W_{E}$
) of the active layer ~
$10.5 ~\mu \text{m}$
when compared to the main channel exhibits a significant difference in the conduction current than the parasitic channel and the main channel current is not affected anymore by parasitic conduction which efficiently eliminates the hump.