In this study, we reported the employment of atomic-layer-deposited (ALD) ultrathin (~1.8 nm) amorphous InSnZnO (a-ITZO) as an innovative channel material to develop the back-end-of-line (BEOL) compatible thin film transistor (TFT) for monolithic 3D integration for the first time. By carefully adjusting the In/Sn/Zn ratio through ALD cycles, the bottom gate (BG) TFT with ALD In0.83Sn 0.11 Zn 0.06 O channel and channel length (L ch ) of 40 nm demonstrates remarkably optimized performance characteristics, including positive threshold voltage (V th ) of 0.38 V, excellent subthreshold swings (SS) value of 66.4 mV/dec, and high electron mobility (μEE) of 48 cm2/V-s. Moreover, the maximum on-state current density (ION) of 686 μA/μm at VDS=2V and impressively low drain-induced barrier lowering (DIBL) performance of 22 mV/V were exhibited. These results are among the most competitive values reported for TFTs based on quaternary ultrathin (Tch < 5 nm) amorphous oxide semiconductors. Furthermore, the TFT demonstrates highly stable device characteristics, as evidenced by threshold voltage shift (ΔV th ) of -40 mV and 60 mV (L ch = 700 nm) after 3600 seconds of negative gate bias stress (NBS) and positive gate bias stress (PBS) with |VG-V th | of 3 V.