A-Si:H TFT AM-PLEDs

Figure 9.7a shows an example of a-Si H TFT transfer characteristics in linear regime with different W/L ratios and an example of a-Si H TFT characteristics in saturation regime with W/L = 170 pm/6 kpm. A threshold voltage (Vth) of 10 = 11 V, a field-effect mobility (p) of 0.2 0.3 cm2/(V s), a subthreshold swing slope of 0.8 dec/V, and a current ON/OFF ratio of larger than 106 for VGS from —10 to 30 V were obtained from these curves for a-Si H TFTs. These devices were used in 4-a-Si H TFTs AM-PLEDs. The electrical properties of a-Si H TFTs used in 3-a-Si H TFTs AM-PLEDs are described in Ref. [18]. 

To demonstrate a-Si H TFT technology, a monochromatic red-light-emitting display (0.5 x 0.5 in.2) using 100 dpi 4-a-Si H TFTs AM-PLEDs with 50 x 50 pixels was fabricated. Here, 100 dpi represents the display resolution since the display has 50 dots (monochromatic pixels) for each row and column line. In this display, the pixel PLED structure is the same as the one shown in the insert of Figure 9.11a. In this a-Si H TFT AM-PLED, the cathode electrodes for each pixel were connected, and the polymer layers were removed from the contact pads with solvent. 

For display evaluation, 0-15 mA was applied through the /data lines to measure the display luminance at different data current levels. Figure 9.12b shows the demonstration of monochromatic red-light emission from a 100 dpi 4-a-Si H TFTs AM-PLED when the data current is 5 mA. A magnified image of discrete pixel light emission is also included in Figure 9.12b. 

Ldata was also measured at the /data lines of the pixel electrode circuit for various /data during select time (Figure 9.4). The variation of Fdata with /data is shown in Figure 9.13b. This measured voltage is related to the required compliance voltage of the current driver that supplies /data. For example, to apply /data up to 15 mA for a-Si H TFT AM-PLED, the current driver should have a capacity of at least 37 V for the compliance voltage (estimated from Figure 9.13b). 

In Figure 9.13c, the EL spectra of the red-light-emitting a-Si H TFT AM-PLED and PLED are shown. EL peak positions at 654 and 653 nm, and full-width-at-half-maximum (FWHM) of 101 and 105 nm were obtained, for the a-Si H TFT AM-PLED and PLED, respectively. From the obtained EL spectra, Commission Internationale de l Eclairage (CIE) color coordinates [31] were calculated for AM-PLED and PLEDs, as shown in the inset of Figure 9.13c, and were found to be (0.67, 0.33) and (0.67, 0.32), respectively. These very... 

The optoelectrical characteristics of 3-a-Si H TFTs AM-PLED have been measured using the same method as outlined above [30]. To light up the whole display, we continuously applied a DC signal (30 V) to all the scan lines and Fdata the signal was varied from 0 to 30 V for different gray scales. All the measurements have been performed in the air at RT [24]. 

FIGURE 9.15 (a) Top view and blowup of illuminated AM-PLED is shown. The bright area on the left is glare from the room light. The PLED was about 120 x 62.5 xm for pixel size of 127 x 127 xm. The fill factor was about 45%. (b) Measured current and luminance versus data voltage of 3-a-Si H TFTs AM-PLED are shown. (From Kim, J. H., Hong, Y., and Kanicki, J., IEEE Electron Device Lett., 24, 451, 2003. With permission.)... 

The estimated luminance values (L(iisp ay and L,.mi ior) at maximum luminous flux are about 50 and 120 cd/m2, respectively. These values can be increased similarly through optimization of a-Si H TFTs AM-PLED design and fabrication methods. 

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