OLED Display Fundamentals and Applications

5

TFT Substrate for OLED Driving

Thin-film transistor technologies for active-matrix driving are discussed in this chapter.

5.1 TFT STRUCTURE

Figure 5.1 shows structural differences between polysilicon (polycrystalline silicon) and amorphous silicon (sometimes abbreviated as a-Si or α-Si).

Figure 5.1 Differences between (a) polysilicon (inside grain) and (b) (hydrogenated) amorphous silicon.

As shown in the figure, amorphous silicon has an irregular structure, so the potential energy is irregularly distributed. Therefore, the moving charge is scattered and mobility is low. The binding state fluctuates as a result of trapping and detrapping, so TFT characteristics change over time. On the other hand, polysilicon inside a grain has a cubic crystalline structure similar to that of single-crystal silicon, where the charges move within a periodical potential in the same way as in a single crystal, so the mobility is very high and stable.

Figure 5.2 shows a TFT substrate used in an actual active-matrix OLED display product. It is very similar to the LCD display TFT backplane except for the existence of a driver TFT in the OLED backplane.

Figure 5.2 An example of a polysilicon-driven OLED display panel.

There are two basic types of TFT structure, according to the electrode configuration: ...