A sub-bandgap optoelectronic differential ideality factor technique is proposed for extraction of the intrinsic density-of-states (DOS) over the bandgap in amorphous semiconductor thin-film transistors (TFTs). In the proposed technique, the gate bias-dependent differential change in the difference of ideality factors ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d\Delta \eta (V_{\mathrm {GS}})/dV_{\mathrm {GS}}$ </tex-math></inline-formula> ) between dark and sub-bandgap photonic excitation condition is employed. With the sub-bandgap photons ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$h\nu < E_{g}$ </tex-math></inline-formula> ), the photonic excitation of electrons is confined only from the localized DOS over the bandgap. We applied the proposed technique to a-InGaZnO TFTs with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W/L = 50/25 \mu $ </tex-math></inline-formula> m/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> m and extracted the energy distribution of the intrinsic DOS for the localized states over the bandgap.