Positive gate bias instability alleviated by self-passivation effect in amorphous InGaZnO thin-film transistors

Abstract

The threshold voltage shift (ΔVth) under positive gate bias stress (PGBS), generally found in amorphous InGaZnO thin-film transistors (a-IGZO TFTs), has usually been suppressed by external passivation layers. We report it can also be alleviated by the self-passivation effect of the active layer, where moderately increasing the active layer thickness (ds) reduces ΔVth by 82% in SiOx-passivated a-IGZO TFTs. Our experiments in conjunction with simulations show that the instability of Vth comes from ambient factors at the back channel. Larger ds results in lower carrier concentrations at the back channel (Nback), fewer diffusive ions affecting the front channel, and much more stable operations under PGBS. The optimal thickness of an IGZO film simultaneously obtaining a small ΔVth, near-zero Vth, and sharp sub-threshold swing is about 80–90 nm, thicker than those usually adopted. The self-passivation effect combined with the externally deposited passivation layer can improve the overall device reliability.