An experimental study is conducted in which the subgap trap density of states (DoS) is measured by ultrabroadband photoconduction (UBPC) to examine hydrogen incorporation into the channel layer of top-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). UBPC reveals that hydrogen incorporation leads to the creation of a spectrally broad ([Formula: see text] eV FWHM) distribution of electronic states in the bandgap centered at [Formula: see text] eV above the valence band mobility edge and to an increase in valence band tail state density. Concomitantly, drain current–gate voltage transfer curves demonstrate that hydrogen incorporation results in a negative shift in the turn-on voltage. Quantitatively, electronic state densities estimated by UBPC and the turn-on voltage shift are identical. These experimental findings imply that hydrogen acts as a donor in a-IGZO, but that its donor ionization energy is extraordinarily large, i.e., [Formula: see text] eV, inconsistent with that of a normal donor. It is proposed that this anomalous donor behavior is a consequence of the negative-U property of hydrogen in a-IGZO in which hydrogen ionization precedes its incorporation into the lattice network.