Contact resistance of interface between the channel layers and various S/D electrodes was investigated by transmission line method. Different electrodes such as Ti/Au, a-IZO, and multilayer of a-IGZO/Ag/a-IGZO were compared in terms of contact resistance, using the transmission line model. The a-IGZO TFTs with a-IGZO/Ag/a-IGZO of S/D electrodes showed good performance and low contact resistance due to the homo-junction with channel layer. Amorphous oxide thin film transistors (TFTs) | have advantages of high mobility and low temperature in device fabrication. Since they also have high transparency in the visible range, they were regarded as transparent TFTs (TTFTs) [1 - 3] . Amorphous InGaZnO (a-IGZO) is regarded as channel layer of TTFT due their good performance, transparency, and stability [4] . There are many studies of channel layer as well as source and drain (S/ D) electrodes of TTFTs, because the S/D electrodes have an effect on the electrical performance and transparency of TTFTs. Transparent conductive oxides (TCOs) are suitable for the S/D electrodes of TTFTs due to their high optical bandgap (> 3.5 eV), good electrical conductivity, and high optical transparency of 80% in visible-infrared. Particularly, amorphous indium zinc oxide (a-IZO) was used for S/D electrode of ZnO TTFTs, due to high transparency and lower resistivity. Although there have been many researches on S/D electrodes of ZnO based TFTs, regarding their transparency, resistivity, and contact problem with channel layer, the relationship between the stability of ZnO based TFTs under the various stress and S/D electrodes is still unknown [5 - 8] . In this paper, the influence of S/D electrodes on contact resistance of a-IGZO TFTs has been investigated by transmission line method (TLM). The change in S/D electrode has an effect on interfacial traps and energy barrier between the channel layer and S/D electrodes. The different contact resistance in electrical performance of a-IGZO TFTs, depending on S/D electrode, has been studied using TLM. . The a-IGZO TFTs were fabricated by using a thermally grown SiO ₂ thin film on highly p-type Si (001) substrate as both substrate and gate electrode. The a-IGZO layer (50 nm) was deposited using rf magnetron sputtering with the channel width (250 μm) and channel length (L _ch ) ranging from 100 μm to 650 μm, as shown in Fig. 1 . All devices were annealed at 350℃ for 1 h in N2 ambient. Then, the different electrode was deposited and patterned using lift-off method. The Ti (10 nm) and Au (60 nm) as S/D electrodes was deposited by e-beam and thermal evaporation, respectively. The a-IZO electrodes and a-IGZO layer of a-IGZO/Ag/a-IGZO electrodes were grown by rf magnetron sputter method, and the Ag was deposited using the thermal evaporation. Figure 2 shows optical microscope image of real TLM patterned devices. Transfer characteristics are measured at a drain voltage VDS of 5.1 V with a single-sweep gate voltage V _GS mode from -20 to 40 V. The Vth is defined as the V _GS value, where a drain current IDS of 5 nA flows. Figure 3 shows the transfer curves of a-IGZO TFTs depending on L _ch with (a) Ti /Au electrodes, (b) a-IZO electrodes, and (c) aIGZO/ Ag/ a-IGZO electrodes. The inset shows output characteristics of a-IGZO TFTs measured at V _GS = 15 V with different L _chs . As L _ch increases, the off current (I _off ) and subthreshold swing (SS) were increased and the on current (I _on ) was decreased, since the resistivity of channel and total trap density were increased. It is observed that the electrical properties of a-IGZO TFTs were different depending on the contact between the a-IGZO and S/D electrode. In the case of the device with Ti/Au electrode, the on current was higher but the value of SS was also higher than those of a-IZO electrode. However, the a-IGZO TFT with Oxide/Metal/Oxide (OMO) electrode showed good performance in both respects. In order to investigate the specific contact resistance with various electrodes, the transfer line method (TLM) was used. Total resistance (R _tot ) was determined from the slope of the linear region of the output characteristics by using Equation (1) [9 , 10] . where L _ch is the separation of the electrodes, and W is the electrode width fixed at 250 μm, as shown in Fig. 4 . R _tot is a function of the contact resistances (R _SD ) of the two contacts and the sheet resistance (R _SH ) of the semiconducting layer outside the contact, and R _SD and R _SH were obtained from the y-axis intercept and slope the R _tot - L _ch plot of Fig. 5 , respectively. Figure 5 shows total resistance variation as a function of channel length depending on various electrodes. The R _SD with Ti /Au was higher than that with the a-IZO, and IGZO/Ag/IGZO electrodes and the ratio of R _SD /R _SH were also higher. This is mainly due to low energy barrier between the channel and S/D electrodes, so the oxide electrodes were easy for injecting the carriers into channel layer. Therefore, the IGZO/Ag/IGZO electrodes showed low contact resistance for S/D electrodes of a-IGZO TFTs. In conclusion, different S/D electrodes of the a-IGZO TFTs have been compared by TLM. The electrical properties and stability of a-IGZO TFT showed a different trend with S/D electrodes. The a-IGZO TFTs with a-IGZO/Ag/a-IGZO of S/D electrodes had a good performance and a low contact resistance due to the homo-junction with channel layer. It is clearly observed that the R _SD with Ti /Au was higher than that with the a-IZO and IGZO/Ag/IGZO electrodes, and the ratio of R _SD /R _SH was also higher. This is mainly due to low energy barrier between the channel and S/D electrodes, so the oxide electrodes were easy for injecting the carriers into channel layer. Therefore, the IGZO/Ag/IGZO electrodes showed low contact resistance for S/D electrodes of a-IGZO TFTs.