  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.
