Latest achievements in OLED drivers: BG-TFT technology and amorphous C12A7 electronic compounds

On September 11th, at the "China Beijing 2013 International Flat Panel Display Industry Summit Forum" co-sponsored by Nikkei BP and the China Optical Optoelectronics Industry Association LCD Branch, Chinese and foreign researchers shared the latest research results related to O LED drivers to the audience. .

Bridged die polysilicon thin film transistor

At present, thin film semiconductors driven by laser annealing low temperature polysilicon (LTPS) technology that drive AMO LEDs have problems such as uniformity and peripheral circuits, and are costly. Oxide thin film semiconductors have much lower cost, but the stability is insufficient. Long-term high current will cause changes in semiconductor performance. For example, the threshold voltage shift reaches 0.5~1V, which has a great influence on the resolution and the compensation effect is not satisfactory.

The research team led by Guo Haicheng, director of the Display Research Center of the Hong Kong University of Science and Technology, developed a technology called Bridged-Grain polysilicon thin film transistor, BG-TFT. It can improve the electrical properties of metal induced crystallization (MIC) and solid phase crystallization (SPC) TFTs, such as significantly lowering the threshold voltage (Vth) of SPC and MICTFT; reducing the pseudo-sub-threshold slope (SS) of SPC; increasing the current carrying current Sub-mobility (μ); increase the on-state current, reduce the off-state current; increase the source-drain current switching ratio by more than 10 times. This technology can be used in any polysilicon TFT, suitable for the production of large display screens, compatible with existing manufacturing processes, and the production cost is much lower than that of excimer laser crystallization (ELC).

Guo Haicheng's student Zhou Wei said in his speech that in the BG-TFT, the narrow high-concentration doped BG region is evenly distributed along the channel length, the doping type is the same as the source-drain region, and the active layer under the gate is covered. It is divided into many short channels, and the channel length is limited to about 2 μm by the exposure system. Adjacent BG regions are shorter and exhibit a strong short channel effect (SCE).

The BGTFT process does not require modification of existing lithographic plates. To form a heavily doped BG region, selective ion implantation of the polysilicon layer can be performed prior to formation of the active island, and the energy and depth of implantation can be calculated using simulation software for the fabrication process. Experiments have found that a depth of 400 nm is appropriate to ensure that there is no short circuit. The main steps are as shown in Fig. 1. First, a grating type photoresist layer is formed, then ion implantation is performed, and finally the photoresist is removed.

　

Figure 1 Main steps for selective ion implantation of a polysilicon layer

In order to form a photoresist layer with a relatively precise grating structure, laser interference photolithography or nanoimprint technology may be employed if a period of 1 μm or less is required. For a period of 2 μm or more, a general lithography apparatus can be used, and can be completed in synchronization with the gate fabrication process.

The principle of BG-TFT improving the electrical characteristics of MIC and SPCTFT is as follows. Taking PMOS as an example, the channel is n-doped, and the source and drain and BG regions are p+ doped. 1 BG-TFT leakage is very low when no power is applied. In the on state, the distance between the BG lines is short, the resistivity is lowered, and the carrier mobility is improved.