The amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) current - voltage (I-V) characteristics can be significantly distorted by either series resistance, R-s, associated with the source/drain (S/D) contact regions or/ and density of states. To isolate R-s contribution we used the five terminals coplanar homojunction TFT structure. Experimental results have shown this device structure has a low S/D contact resistance that do not contribute to observed I-V nonlinearity. We have shown using combination of the experimental data and two- dimensional simulations that the observed nonlinearity can be associated with the conduction band-tail states.