Reduction of Negative Bias and Light Instability of a-IGZO TFTs by Dual-Gate Driving

Stability under negative-bias-illumination-stress (NBIS) of dual-gate (top- and bottom-gate) amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors is investigated. It is found that the negative threshold-voltage shift (ΔV_TH) induced by NBIS is much smaller under dual-gate driving (when the two gates are electrically tied together) compared with single-gate driving. For a 20 nm-thick a-IGZO active layer, this is attributed to bulk accumulation, where electrons are accumulated across the entire depth of the active layer, which is responsible for the small negative ΔV_TH after NBIS. Due to bulk accumulation, the Fermi level can be easily shifted by dual-gate driving as compared with the conventional single-gate driving, even after NBIS.