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Electric field thermopower modulation analyses of the operation mechanism of transparent amorphous SnO$_2$ thin-film transistor

Transparent amorphous oxide semiconductors (TAOSs) based transparent thin-film transistors (TTFTs) with high field effect mobility are essential for developing advanced flat panel displays. Among TAOSs, amorphous (a-) SnO$_2$ has several advantages against current a-InGaZnO4 such as higher field effect mobility and being indium free. Although a-SnO$_2$ TTFT has been demonstrated several times, the operation mechanism has not been clarified thus far due to the strong gas sensing characteristics of SnO$_2$. Here we clarify the operation mechanism of a-SnO$_2$ TTFT by electric field thermopower modulation analyses. We prepared a bottom-gate top-contact type TTFT using 4.2-nm-thick a-SnO$_2$ as the channel without any surface passivation. The effective thickness of the conducting channel was ~1.7 + - 0.4 nm in air and in vacuum, but a large threshold gate voltage shift occurred in different atmospheres; this is attributed to carrier depletion near at the top surface (~2.5 nm) of the a-SnO$_2$ due to its interaction with the gas molecules and the resulting shift in the Fermi energy. The present results would provide a fundamental design concept to develop a-SnO$_2$ TTFT.