The electron distribution in an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with a floating metal-semiconductor (MS) back interface is analyzed using a technology computer-aided design (TCAD) model. The channel geometry (i.e., length and thickness) effect is carefully investigated. At a high work function (i.e., 5 eV) of the capping metal, electrons inside a-IGZO are mostly removed by the capping metal (electron depletion effect). The depletion of the IGZO film leads to an increase in threshold voltage in a-IGZO TFT. TCAD simulation reveals that increasing channel length and decreasing IGZO thickness significantly enhance such an electron depletion effect. Finally, the electron depletion effect is applied to a-IGZO TFT with a high-conductivity IGZO film to greatly suppress the leakage current by over 5 orders.