Differential Ideality Factor Technique for Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors

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
$d\eta/dV_{\rm GS}$
on behalf of the ideality factor itself. Contrary to the subthreshold current method which requires an accurate threshold voltage
$(V_{T})$
, the DIFT is free from
$V_{T}$
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
$W/L = \hbox{200}\ \mu \hbox{m}/\hbox{30}\ \mu\hbox{m}$
, the subgap DOS is extracted to be a superposition of exponential deep and tail states with
$N_{\rm DA} = \hbox{7.1} \times \hbox{10}^{15}\ \hbox{cm}^{-3} \cdot \hbox{eV}^{-1}$
,
$kT_{\rm DA} = \hbox{0.6}\ \hbox{eV}$
,
$N_{\rm TA} = \hbox{1.5} \times \hbox{10}^{16}\ \hbox{cm}^{-3} \cdot \hbox{eV}^{-1}$
, and
$kT_{\rm TA} = \hbox{0.024}\ \hbox{eV}$
.