Threshold Voltage Instability in Flexible Bottom-Gate Al₂O₃/IGZO TFTs

This paper investigates the threshold voltage instability in flexible n-type thin film transistors (TFTs) featuring a bottom-gate <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Ti}/\text{Al}_{2}\mathrm{O}_{3}/\text{IGZO}$</tex> MOS structure. Hysteresis, positive gate bias stress and negative gate bias recovery measurements were performed at room temperature on a set of 20 devices with the same channel width (W = 280 µm) and different channel length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$L$</tex> ranging from 100 µm down to 5 uµm. Both hysteresis (applying <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{Gs}$</tex> from -1 V to 5 V and vice versa) and stress (applying <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{Gs}=5\mathrm{V}$</tex> for 1000 s) data show a positive threshold voltage shift, which can be attributed to negative charge trapping at the channel/oxide interface and/or in the bulk oxide. Moreover, the trapping rate parameter exhibits a general decreasing trend over time with values ranging from 0.28 to 0.46 in the initial stress phase and a reduction up to 88% after 1000 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$s$</tex> of stress. Measurements also show that in most devices the stress-induced shift can be fully recovered by applying a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$VGs{=}-2\mathrm{V}$</tex> for 1000 s as a result of releasing charges previously trapped during stress.