We investigated the lateral distribution of the equilibrium carrier concentration (n0) along the channel and the effects of channel length (L) on the source-drain series resistance (Rext) in the top-gate self-aligned (TG-SA) coplanar structure amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The lateral distribution of n0 across the channel was extracted using the paired gate-to-source voltage (VGS)-based transmission line method and the temperature-dependent transfer characteristics obtained from the TFTs with different Ls. n0 abruptly decreased with an increase in the distance from the channel edge near the source/drain junctions; however, much smaller gradient of n0 was observed in the region near the middle of the channel. The effect of L on the Rext in the TG-SA coplanar a-IGZO TFT was investigated by applying the drain current-conductance method to the TFTs with various Ls. The increase of Rext was clearly observed with an increase in L especially at low VGSs, which was possibly attributed to the enhanced carrier diffusion near the source/drain junctions due to the larger gradient of the carrier concentration in the longer channel devices. Because the lateral carrier diffusion and the relatively high Rext are the critical issues in the TG-SA coplanar structure-based oxide TFTs, the results in this work are expected to be useful in further improving the electrical performance and uniformity of the TG-SA coplanar structure oxide TFTs.