Thermopower Modulation Analyses of High-Mobility Transparent Amorphous Oxide Semiconductor Thin-Film Transistors

Transparent amorphous oxide semiconductor InSnZnOx (ITZO)-based thin-film transistors (TFTs) exhibit a high field-effect mobility (μFE). Although ITZO-TFTs have attracted increasing attention as a next-generation backplane of flat panel displays, the origin of the high μFE remains unclear due to the lack of systematic quantitative analyses using thermopower (S) as the measure. Here, we show that the high μFE originates from an extremely light carrier effective mass (m*) and a long carrier relaxation time (τ). The S measurements of several ITZO films with different carrier concentrations clarified that m* of ITZO films is ∼0.11 m0, which is ∼70% of that of a commercial oxide semiconductor, amorphous InGaZnO4 (∼0.16 m0). We then fabricated bottom-gate-top-contact ITZO-TFTs displaying excellent transistor characteristics (μFE ∼ 58 cm2 V–1 s–1) using amorphous AlOx as the gate insulator and demonstrated that the effective thickness increases with the gate voltage. This suggests that the bulk predominantly contributes to the drain current, which results in τ as long as ∼3.6 fs, which is quadruple that of amorphous InGaZnO4-TFTs (∼0.9 fs). The present results are useful to further improve the mobility of ITZO-TFTs.