The electrical instability in thin-film transistors (TFTs) based on amorphous indium-gallium-zinc oxide (a-IGZO) was investigated using constant bias and constant current stress. Stress conditions were chosen such that TFT is turned on, whereas the potential at the gate dielectric-a-IGZO interface is below the electron accumulation threshold. Drain current in this low bias regime is still sufficient for driving pixels in an organic light-emitting diode display (AMOLED). We found that the degradation is due to charge trapping near the a-IGZO-gate dielectric interface. The obtained results enabled a straightforward assessment of a-IGZO TFTs' instability for their application in AMOLEDs.