We propose a new device structure of poly-silicon thin film channel metal-oxide-semiconductor field effect transistor where the transistor channel is enlarged vertically with an insulating pillar. Technology Computer Aided Design modeling was conducted to investigate the operational performance of the proposed device. For ultra-short lateral channel length of similar to 9.5 nm between source and drain, a remarkably low subthreshold swing of similar to 62.6 mV/dec was obtained at 0.5 V drain bias. The drain induced barrier lowering was also found to be quite small, similar to 13.7 mV V-1. Our device structure makes it possible to achieve the thermodynamic limit of subthreshold swing for the lateral device size of several nanometers