In amorphous oxide semiconductors such as indium gallium zinc oxide (a-IGZO), subgap states often play an important role in determing thin-film transistor (TFT) operation metrics such as turn-on voltage, mobility, hysteresis, and bias-stress stability. Using a-IGZO TFTs, we are able to resolve (over 6 orders of magnitude) the photoconduction response of a-IGZO spanning the subgap region (from 0.3-3.5 eV) in order to elucidate the subgap density of states (DoS).¹ In this presentation, we will focus on two significant findings related to the nature of a-IGZO subgap states. First, hydrogen is found to act as a negative-U donor in a-IGZO, contributing an electron to the conduction band before bonding with valence band oxygen 2p states, thereby forming an [O_O ^2-H⁺]^- defect complex whose Gaussian-like DoS are positioned 0.4 eV above the valence band mobility edge.² Second, bottom-gate, back-channel etched a-IGZO TFTs are found to have a Gaussian-like zinc vacancy-related subgap state centered roughly 2.3 eV below the conduction band mobility edge, a feature that is not present in top-gate a-IGZO TFTs. K. Vogt et al. Phys. Rev. Research 2 (Sep. 2020). 10.1103/PhysRevResearch.2.033358 G. Mattson et al. arXiv:2111.08588 Figure 1