We propose a differential ideality factor technique (DIFT) for extraction of subgap density of states (DOS) over the bandgap in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by using the differential ideality factor <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$d\eta/dV_{\rm GS}$</tex></formula> on behalf of the ideality factor itself. Contrary to the subthreshold current method which requires an accurate threshold voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(V_{T})$</tex></formula> , the DIFT is free from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$V_{T}$</tex></formula> itself and considerably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$W/L = \hbox{200}\ \mu \hbox{m}/\hbox{30}\ \mu\hbox{m}$</tex></formula> , the subgap DOS is extracted to be a superposition of exponential deep and tail states with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$N_{\rm DA} = \hbox{7.1} \times \hbox{10}^{15}\ \hbox{cm}^{-3} \cdot \hbox{eV}^{-1}$</tex></formula> , <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$kT_{\rm DA} = \hbox{0.6}\ \hbox{eV}$</tex></formula> , <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$N_{\rm TA} = \hbox{1.5} \times \hbox{10}^{16}\ \hbox{cm}^{-3} \cdot \hbox{eV}^{-1}$</tex></formula> , and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$kT_{\rm TA} = \hbox{0.024}\ \hbox{eV}$</tex></formula> .