**2.1. Overview of TFT structure**

The structure of TFT is classified as top and bottom gate depending on the relative position of gate and active layer. Additionally, when the gate is on the same side of source/drain electrodes, it is called coplanar structure. Two types of oxide TFT structures are shown in **Figure 1**. Double-gate TFTs with both top and bottom gates are illustrated by **Figure 1(a)**. Gate and SD2 electrode function as a bottom and top gate, respectively. As the advantageous side, this structure was utilized to increase current flow and gets better output characteristics [7]. However, large parasitic capacitances between gates and source/drain metals are disadvantages, which are a big hurdle developing UHD OLED TV. Coplanar TFT was designed to reduce parasitic capacitance avoiding overlap between gate terminal and source/drain

**Figure 1.** The cross-section of oxide TFT structures. (a) Etch-stopper structure with double gates and (b) self-aligned coplanar structure with top gate.

terminals as shown in **Figure 1(b)**. Light shield (LS) layer acts as blocking light coming to active layer, which is the main source to cause device degradation under negative bias temperature illumination stress (NBTIS). a-IGZO TFT with coplanar structure needs active metallization process to make ohmic contact between a-IGZO semiconductor and source/drain metals. This process was optimized to generate increased oxygen vacancies inside active film [8]. After metallization process, we obtained effective channel length, which is found to be shorter than nominal channel length. Effective channel length should be managed with controlled process.
