Study on the Lateral Carrier Diffusion and Source-Drain Series Resistance in Self-Aligned Top-Gate Coplanar InGaZnO Thin-Film Transistors

Abstract

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 (R_ext) 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 (V_GS)-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 R_ext 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 R_ext was clearly observed with an increase in L especially at low V_GSs, 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 R_ext 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.