Zinc-tin-oxide (ZTO) thin film transistors (TFTs) with atomic layer deposition (ALD) Al₂O₃ gate dielectrics were fabricated and compared to devices with PECVD SiO₂ gate dielectrics. The Al₂O₃ devices showed acceptable mobility (~14 cm2/VÂ·s), subthreshold slope (~0.4 V/dec), and ION/IOFF (~107). However, a pronounced positive VT shift was observed during initial gate voltage sweeps, consistent with electron trapping. In an effort to stabilize operation, devices were made with varying Al₂O₃ deposition temperature, O₂ plasma exposure, and a plasma-enhanced chemical vapor deposited (PECVD) SiO₂ capping layer. It was found that capping of the Al₂O₃ with a thin (2-3 nm) layer of PECVD SiO₂ altered the trapping behavior and reduced the bias instability significantly, suggesting that the Al₂O₃/ZTO interface is the source of the trapping.