In this paper, we analyzed electrical characteristics of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with plasma-exposed source-drain (S/D) bulk region. The parasitic resistance and effective channel length characteristics exhibit similar behavior with that of crystalline silicon metal oxide-semiconductor field effect transistor (c-Si MOSFET) that has doped S/D bulk region. The transfer curves little changed with gate overlap variation, and the width-normalized parasitic resistance obtained from transmission line method was as low as 3 to 6 Omega. The effective channel length was shorter than the mask channel length and showed gate-to-source (V-GS) voltage dependency that is frequently observed for lightly doped drain (LDD) MOSFET. Experimental and simulation results showed that the plasma exposure caused an LDD-like doping effect in the S/D bulk region by inducing oxygen vacancy in the a-IGZO layer.