**4. Increased resolution by pixel driving techniques**

To increase the display resolution, not only the technology (backplane and frontplane) but also the pixel driving techniques should be optimized. The OLED light output is dependent on the drain current of the driving TFTs of the AMOLED displays. Due to inherent variations in AMOLED displays, some compensation methods to the drain current of the TFTs are required to achieve uniform brightness. This can be implemented through either in-pixel compensation [10] or external compensation [11, 12]. Since in-pixel compensation schemes typically require more transistors inside the pixel, external compensation methods are preferred for high-resolution applications. **Figure 8** shows pixel circuits and a possible layout for in-pixel compensation, using an 8T1C [10] pixel, and external compensation using a 3T2C [11] and, respectively, a 2T1C [12] pixel. For all these layouts, the same design rules were used. It is clear from this figure that a display with external compensation, especially the 2T1C pixel circuit, can achieve a much higher pixel density.

The achievable pixel density depends on both the pixel circuit and the design rules imposed by the technology, such as the critical dimension (CD) of the lithography tool. **Figure 9** compares the achievable resolutions for different CDs for the 8T1C, the 3T2C, and the 2T1C pixel circuit. Although the CD of 1.5 μm, as currently achievable with typical i-line steppers, only yields a maximum pixel density of 565 ppi for the 8T1C pixel circuit, the same CD already yields a significant improvement for the pixel circuits using external compensation, namely, 847 ppi for the 3T2C pixel

**Figure 8.** *Pixel circuits and corresponding layouts for (left) 8T1C, (middle) 3T2C, and (right) 2T1C pixels.*

**Figure 9.** *Pixel resolution vs. critical dimension (CD) for various pixel schemes.*

circuit and 1210 ppi for the 2T1C pixel circuit, respectively. Furthermore, improvements in technology allowing smaller CD will even further increase the achievable pixel density, up to 9070 ppi for the 2T1C pixel circuit, when using a CD of 0.2 μm.

The compensation principle for the 3T2C pixel circuit relies on the fact that applying a voltage on the backgate of a transistor will shift the threshold voltage (VT) of that transistor. By applying the correct compensation voltage to the backgate of the drive transistor of each pixel, all VT variations can be eliminated, resulting in a more uniform display. This compensation method uses three different modes of operation for the display. The first mode of operation is the calibration mode. In this mode, the correct compensation voltage is determined for each pixel by applying a certain reference voltage to the frontgate and measuring the current through the pixel while varying the backgate. When the measured current matches a predetermined reference current, the voltage on the backgate is the correct compensation voltage, which will be stored both on the capacitor connected to the backgate and in external memory. Once the correct backgate voltage is set for every pixel, the display can be switched to normal operation. In this mode, the display is driven with the normal video data, which is written to the frontgate of each pixel. Since the CAL signal is low in this mode, the charge on the capacitor will remain, and hence the backgate voltage will be the compensated voltage. However, due to leakage, this charge will slowly change over time. Therefore, a third mode of operation is added, namely, the calibration refresh. In this mode, the SEL signal is kept low, but the CAL signal is running through the display, while the compensation data is applied to the data lines. This way the compensation voltage is restored on the backgate, to ensure the VT uniformity remains over time. This compensation method shows a significant improvement in current variation, as demonstrated in **Figure 10**.

The current through the drive TFT (IDS), and thus through the OLED, when operating in saturation regime can be calculated for a certain data voltage (VGS) by using Eq. (1):

from regime can be calculated for a certain data voltage \(V\_{GS}\) by 
$$I\_{DS} = \frac{\mu \ast C\_{ox}}{2} \ast \frac{W}{L} \left(V\_{GS} - V\_T\right)^2 = \mathbb{B} \ast \left(V\_{GS} - V\_T\right)^2\tag{1}$$

**131**

gray level by using Eq. (2):

**Figure 10.**

**Figure 11.**

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

*Current variation of a 3T2C display before and after compensation.*

to compensate for both β and VT variations. Similarly as the previous described compensation method, we will first characterize the current through each pixel for

For each pixel, the extracted β and VT values are stored. Based on these values, the VGS voltages can be calculated for each pixel by the driver IC for each desired

> \_ \_ *IDS*

This calculation is relative simple and straightforward, as it only requires a multiplication, a subtraction, and a square root calculation, which enables to display real-time video content by using this methodology. **Figure 11** shows the current variation improvements directly obtained from our AMOLED displays, by utilizing the VT-only compensation method and comparing it to the VT and β compensation method. As mentioned above, the simple VT compensation method provides good variation results for a small range, whereas the combined parameter method

<sup>β</sup> − *VT* (2)

multiple data voltages, whereafter the measurements are fitted to Eq. (1).

*Current variation of a 3T2C and 2T1C display before and after compensation.*

*VGS* = √

improves the variation across all desired gray levels.

Compensating only for VT can eliminate variations in current for one gray level; however, if the β-factor is different for each pixel, the current will still vary for different gray values, even after VT compensation. This is shown in **Figure 11** for the 3T2C pixel. As a consequence, we propose a new compensation method

*Liquid Crystals and Display Technology*

circuit and 1210 ppi for the 2T1C pixel circuit, respectively. Furthermore, improvements in technology allowing smaller CD will even further increase the achievable pixel density, up to 9070 ppi for the 2T1C pixel circuit, when using a CD of 0.2 μm. The compensation principle for the 3T2C pixel circuit relies on the fact that applying a voltage on the backgate of a transistor will shift the threshold voltage (VT) of that transistor. By applying the correct compensation voltage to the backgate of the drive transistor of each pixel, all VT variations can be eliminated, resulting in a more uniform display. This compensation method uses three different modes of operation for the display. The first mode of operation is the calibration mode. In this mode, the correct compensation voltage is determined for each pixel by applying a certain reference voltage to the frontgate and measuring the current through the pixel while varying the backgate. When the measured current matches a predetermined reference current, the voltage on the backgate is the correct compensation voltage, which will be stored both on the capacitor connected to the backgate and in external memory. Once the correct backgate voltage is set for every pixel, the display can be switched to normal operation. In this mode, the display is driven with the normal video data, which is written to the frontgate of each pixel. Since the CAL signal is low in this mode, the charge on the capacitor will remain, and hence the backgate voltage will be the compensated voltage. However, due to leakage, this charge will slowly change over time. Therefore, a third mode of operation is added, namely, the calibration refresh. In this mode, the SEL signal is kept low, but the CAL signal is running through the display, while the compensation data is applied to the data lines. This way the compensation voltage is restored on the backgate, to ensure the VT uniformity remains over time. This compensation method shows a significant

*Pixel resolution vs. critical dimension (CD) for various pixel schemes.*

improvement in current variation, as demonstrated in **Figure 10**.

*IDS* = \_ *μ* ∗ *Cox* 2 ∗ \_ *W*

The current through the drive TFT (IDS), and thus through the OLED, when operating in saturation regime can be calculated for a certain data voltage (VGS) by

*<sup>L</sup>* (*VGS* <sup>−</sup>*VT*)<sup>2</sup>

Compensating only for VT can eliminate variations in current for one gray level; however, if the β-factor is different for each pixel, the current will still vary for different gray values, even after VT compensation. This is shown in **Figure 11** for the 3T2C pixel. As a consequence, we propose a new compensation method

= β ∗ (*VGS* − *VT*)<sup>2</sup> (1)

**130**

using Eq. (1):

**Figure 9.**

**Figure 10.** *Current variation of a 3T2C display before and after compensation.*

**Figure 11.**

*Current variation of a 3T2C and 2T1C display before and after compensation.*

to compensate for both β and VT variations. Similarly as the previous described compensation method, we will first characterize the current through each pixel for multiple data voltages, whereafter the measurements are fitted to Eq. (1).

For each pixel, the extracted β and VT values are stored. Based on these values, the VGS voltages can be calculated for each pixel by the driver IC for each desired gray level by using Eq. (2): \_

$$\mathbf{V}\_{GS} = \sqrt{\frac{I\_{DS}}{\beta}} - \mathbf{V}\_T \tag{2}$$

This calculation is relative simple and straightforward, as it only requires a multiplication, a subtraction, and a square root calculation, which enables to display real-time video content by using this methodology. **Figure 11** shows the current variation improvements directly obtained from our AMOLED displays, by utilizing the VT-only compensation method and comparing it to the VT and β compensation method. As mentioned above, the simple VT compensation method provides good variation results for a small range, whereas the combined parameter method improves the variation across all desired gray levels.
