  Within the subgap of amorphous oxide semiconductors like amorphous indium
gallium zinc oxide (a-IGZO) are donor-like and acceptor-like states that
control the operational physics of optically transparent thin-film transistors
(TFTs). Hydrogen incorporation into the channel layer of a top-gate a-IGZO TFT
exists as an electron donor that causes an observed negative shift in the drain
current-gate voltage ($\mathrm{I_{D} - V_{G}}$) transfer curve turn-on voltage.
Normally, hydrogen is thought to create shallow electronic states just below
the conduction band mobility edge, with the donor ionization state controlled
by equilibrium thermodynamics involving the position of the Fermi level with
respect to the donor ionization energy. However, hydrogen does not behave as a
normal donor as revealed by the subgap density of states (DoS) measured by the
photoconduction response of top-gate a-IGZO TFTs to within 0.3 eV of the CBM
edge. Specifically, the DoS shows a subgap peak above the valence band mobility
edge growing at the same rate that $\mathrm{I_{D} - V_{G}}$ transfer curve
measurements suggest that hydrogen was incorporated into the channel layer.
Such hydrogen donor behavior in a-IGZO is anomalous and can be understood as
follows: Non-bonded hydrogen ionization precedes its incorporation into the
a-IGZO network as a bonded species. Ionized hydrogen bonds to a charged
oxygen-on-an-oxygen-site anion, resulting in the formation of a defect complex
denoted herein as, $\mathrm{{[{O_{O}^{2-}}{H^+}]}^{1-}}$. Formation of an
$\mathrm{{[{O_{O}^{2-}}{H^+}]}^{1-}}$ defect complex creates a spectrally-broad
($\sim$0.3 eV FWHM) distribution of electronic states observed in the bandgap
centered at 0.4 eV above the valence band mobility edge.
