Field-induced carrier generation in amorphous-InGaZnO4 thin-film transistors

• Instability mechanism for Positive bias stress (PBS) in a-IGZO TFTs is studied.
• Donor concentration increases after application of low PBS (VGS (stress)≤30 V).
• Electron trapping dominant after application of high PBS (VGS (stress)>30 V).

The positive threshold voltage (VTH) shift induced by positive gate bias stress (PBS) in amorphous-indium–gallium–zinc-oxide (a-IGZO) thin-film transistors (TFTs) is commonly attributed to carrier trapping mechanism. Here we show that in addition to these trapping mechanisms, the concentration of donors also increases during PBS when the applied gate bias stress voltage VGS_Stress≤30 V. In the early stages of the PBS, this increase in donor concentration may manifest itself as a negative VTH shift. In the case of VGS_Stress≥40 V, however, the increase is not detectable because electron trapping at the semiconductor/gate-insulator interface is dominant – even in the early stages of the PBS.