Subthreshold swing behavior in amorphous indium-gallium-zinc-oxide transistors from room to cryogenic temperatures

Abstract

While cryogenic-temperature subthreshold swing (SS) in crystalline semiconductors has been widely studied, a careful study on the temperature-dependent SS in amorphous oxide semiconductors remains lacking. In this paper, a comprehensive analysis of the SS in thin-film transistors with an amorphous indium-gallium-zinc-oxide (IGZO) channel at temperatures from 300 K down to 4 K is presented. Main observations include the following: (1) at room temperature (300 K), the devices exhibit a SS of 61 mV/dec and a low interface trap density of <1011 cm−2. (2) A SS saturation around 40 mV/dec is observed between 200 and 100 K. It is well explained by the electron transport via band tail states with exponential decay (Wt) of 17 meV. (3) At deep cryogenic temperatures, the SS increases significantly, exceeding 200 mV/dec at 4 K. Such high SS values are actually limited by the measurement current range, confirmed by Id − Vg simulations based on the variable range hopping model. This work not only elucidates the SS behavior in amorphous IGZO devices but also provides a deep understanding of the physical mechanisms of electron transport in amorphous semiconductors.