Relation Between Low-Frequency Noise and Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors

The relation between the low-frequency noise (LFN) and subgap density of states (DOS) of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated by changing the postannealing temperature from 150
$^{\circ}\hbox{C}$
to 300
$^{ \circ}\hbox{C}$
. It is found that the density of the tail states in the TFT annealed at 300
$^{\circ}\hbox{C}$
(showing the lowest LFN) is prominently lower than those in the TFTs annealed at 250
$^{\circ}\hbox{C}$
and 150
$^{\circ}\hbox{C}$
. The densities of the tail states in the TFTs annealed at 250
$^{\circ}\hbox{C}$
and 150
$^{\circ}\hbox{C}$
(indicating similar LFN) are almost the same. In addition, it is clearly observed that the increased DOS of the a-IGZO TFT subjected to ac gate voltage stress results in a higher LFN compared with one without electrical stress. Hooge's parameters
$\alpha_{H}$
's are extracted to be
$\sim\!\! \hbox{4.5} \times \hbox{10}^{-3}$
(for the TFT annealed at 300
$^{\circ}\hbox{C}$
) and
$\sim \!\!\hbox{1} \times \hbox{10}^{-2}$
(for the TFTs annealed at 250
$^{\circ}\hbox{C}$
and 150
$^{\circ}\hbox{C}$
as well as for the TFT annealed at 300
$^{\circ}\hbox{C}$
after the application of electrical ac stress). Therefore, the role of an a-IGZO subgap DOS on a LFN characteristic seems to be originated from the generation–recombination noise-induced carrier number fluctuation (via trap centers in the DOS tail states) while its correlation with the carrier mobility fluctuation is not clear except for the slope close to
$-$
1 in the logarithmic curve with the normalized power spectral density versus the gate overdrive voltage.