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Tunnel field-effect transistors (TFETs) with a subthreshold swing (SS)<60 mV/dec are expected to be active devices in low-power flexible systems, potentially lowering operational voltage by virtue of steep switching behavior via band-to-band tunneling. In silicon (Si) channel materials, however, it still remains a challenge to obtain SS smaller than 60 mV/dec. In this study, we experimentally demonstrate the sub-60 mV/dec operation of a flexible semi-around gate TFET on a plastic substrate using Si nanowires (SiNWs) as the channel material. With the combined advantages of selectively thinned SiNW channels (width ~15 nm and height ~40 nm) and high-κ (Al2O3 ~7 nm) gate dielectric, in conjunction with an abrupt degenerate source junction, the device with a channel length of ~500 nm exhibits a minimal SS of ~42 mV/dec at room temperature. Moreover, mechanical bendability of the device indicates that it has stable and good fatigue properties, providing an important step towards the realization of steep-slope switches for low-power and energy-efficient flexible electronics. [ABSTRACT FROM AUTHOR]