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°C to 300°C. It is found that the density of the tail states in the TFT annealed at 300°C (showing the lowest LFN) is prominently lower than those in the TFTs annealed at 250°C and 150°C. The densities of the tail states in the TFTs annealed at 250°C and 150°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 αH´s are extracted to be ~4.5 × 10^-3 (for the TFT annealed at 300°C) and ~1 × 10^-2 (for the TFTs annealed at 25°C and 150°C as well as for the TFT annealed at 300°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.