Study of Positive-Gate-Bias-Induced Hump Phenomenon in Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistors

A positive-gate-bias stress (PBS)-induced hump phenomenon in staggered bottom-gate amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) is observed, and the possible reason is discussed. It is found that the hump phenomenon is closely related to device layouts. In conventional metal-over-active (MOA) structure, the hump current is independent of the channel width. However, for the active-over-metal (AOM) structure, the hump current disappears when the width of active layer exceeds a critical value. These results indicate that the hump current caused by PBS comes from parasitic edge transistors rather than back-channel transistors. Furthermore, the possible origin of channel edge conduction mechanism is checked by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. From AFM image, we notice that there is a long gentle slope at edge of active layer, which will enhance the electrical filed at local area and accelerate the device performance degradation under PBS condition. XPS measurements before and after PBS on devices at channel edge confirm our assumption, that is, the oxygen vacancy content increases from 29.33% to 36.89% after PBS. This implies that the carrier concentration in channel edge is enhanced resulting in lower turn-on voltage of parasitic edge transistors, which, in turn, causes hump phenomenon.