**5.3 Integrated readout circuit**

The integration of peripheral readout circuitry in panel side-by-side with the display peripherals is beneficial for resolution, connectivity, and potential lower system cost of the device. In this paragraph TFT-based integrated readout is demonstrated by using our IGZO n-type only TFTs.

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

*(MUX), and ADC. Power and driving electronics remain off-panel.*

**Figure 13.**

whereby the faster allows a width of 400 μm.

**5.4 Dual-gate metal-oxide technology**

metal layer, also used as anode layer.

The necessary blocks to implement an in-panel readout system are a charge sense amplifier (CSA), a multiplexer (MUX), and an analog to digital converter (ADC). The CSA is reading out the charge stored in each pixel of the array. The MUX is multiplexing the CSA outputs directly to the ADC, decreasing the number of the required ADC converters. The ADC is converting the analog voltage received from the CSA to a digital code. Various TFT-based analog blocks have been demonstrated in the literature [18–24]. Metal-oxide TFT technologies are preferable due to uniformity over large areas, very low leakage currents, and lower cost over area. In the following section, fast and small footprint ADCs and charge sense amplifiers (CSA) are discussed to meet the specifications of an in-panel fingerprint array. In **Figure 13**, the high-level block diagram of the in-panel readout system is shown. The five main blocks are detailed: a two-dimensional (2-D) image sensor array, the CSAs connect on each row of the array, a multiplexer connects four or more rows (MUX), and ADC connects to every MUX and off-panel power and driving electronics. Each column of the 2D image sensor array is readout from the corresponding row by a CSA. The pixels are readout subsequently enabled by the "SELECT" signals from the columns and converted to digital code through the MUX and an ADC. The MUX enables a larger footprint for the ADC, up to 4 to 8 times larger compared to a single line (50 μm). Hence, the ADC and MUX needs to be 4 to 8 times faster than the CSA. A 1–2 fps readout of 1M pixel imager (1000 × 1000 pixels) sets a readout speed of 1–2 kS/s per line for each CSA. This translates to 4–8 kS/s for the ADC if a 4:1 MUX is used or 8–16 kS/s for a 8:1 MUX. The slower ADC configuration sets the specification limitation to the width of the ADC to 200 μm,

*Block diagram of the proposed in-panel fingerprint sensor array with integrated CSA for each line, multiplexer* 

**Figure 14** (a) depicts the cross section of the dual-gate self-aligned metal-oxide

**Figure 14(b)** shows the distribution of the extracted on-current (ION) from the measured transfer characteristics of 480/20 (μm/μm) IGZO (red) and ITZO (orange) dual-gate TFTs. The IGZO TFTs exhibits a median ION of 54.4 μA, whereas

(MO) technology on a 15-μm-thick polyimide film [25, 26]. The metal-oxide (IGZO or ITZO) TFTs are fabricated with two metal gates (M0, M1) and sourcedrain metal contacts (M2). An additional metal layer (M3), not shown in the cross section, is beneficial for footprint but also for performance and noise. The CSA experimental results shown in the following sections are designed with an extra

**Figure 12.** *Fingerprint integration configurations in AMOLED displays.*

#### **Figure 13.**

*Liquid Crystals and Display Technology*

**5.1 Side by side/under/over display pixel**

**5.2 Passive/active sensor pixel**

**5.3 Integrated readout circuit**

strated by using our IGZO n-type only TFTs.

*Fingerprint integration configurations in AMOLED displays.*

of passive pixels.

**5. Adding a fourth pixel for finger/palmprint sensing**

ality, enabling detection of multiple fingers at once or even a palmprint.

Fingerprint sensor arrays (Figure 5.1) [13] are becoming a mainstream security mechanism for mobile devices and are today available as autonomous silicon-based component. The integration of the fingerprint sensor array together with AMOLED displays [14–16] would benefit the footprint of the mobile device and the function-

Fingerprint sensors combined with AMOLED displays can be realized in three different configurations for the sensor pixels: (1) in the same plane of the display pixels and (2) under and (3) over the display pixels (**Figure 12**). With sensors in the same plane, the display module gains optical sensing capability by incorporating photodetector pixels between OLED pixels. Sensors, under or above the display, require a separate fingerprint module. A fingerprint module under the display would need a semitransparent display and light scattering management. A fingerprint module over the display requires a transparent imager to avoid changes in display emission. In the previous section, we have demonstrated that a higher resolution backplane can be achieved at the same critical dimension by introducing external compensation methods. This combined with the photolitho-based patterning method of OPD and OLED will the crucial enablers for such a configuration as analyzed in Paragraph 3.

The pixel circuit architecture of the sensor array can be either passive or active [17]. The passive pixel is depicted in **Figure 13** and is comprised by the photoelement, a capacitor, and a select TFT. The main difference of the active pixel is that it requires an extra TFT acting as a local amplifier. For high-resolution applications, the active pixel is not recommended, since its footprint is larger than the footprint

The integration of peripheral readout circuitry in panel side-by-side with the display peripherals is beneficial for resolution, connectivity, and potential lower system cost of the device. In this paragraph TFT-based integrated readout is demon-

**132**

**Figure 12.**

*Block diagram of the proposed in-panel fingerprint sensor array with integrated CSA for each line, multiplexer (MUX), and ADC. Power and driving electronics remain off-panel.*

The necessary blocks to implement an in-panel readout system are a charge sense amplifier (CSA), a multiplexer (MUX), and an analog to digital converter (ADC). The CSA is reading out the charge stored in each pixel of the array. The MUX is multiplexing the CSA outputs directly to the ADC, decreasing the number of the required ADC converters. The ADC is converting the analog voltage received from the CSA to a digital code. Various TFT-based analog blocks have been demonstrated in the literature [18–24]. Metal-oxide TFT technologies are preferable due to uniformity over large areas, very low leakage currents, and lower cost over area. In the following section, fast and small footprint ADCs and charge sense amplifiers (CSA) are discussed to meet the specifications of an in-panel fingerprint array.

In **Figure 13**, the high-level block diagram of the in-panel readout system is shown. The five main blocks are detailed: a two-dimensional (2-D) image sensor array, the CSAs connect on each row of the array, a multiplexer connects four or more rows (MUX), and ADC connects to every MUX and off-panel power and driving electronics. Each column of the 2D image sensor array is readout from the corresponding row by a CSA. The pixels are readout subsequently enabled by the "SELECT" signals from the columns and converted to digital code through the MUX and an ADC. The MUX enables a larger footprint for the ADC, up to 4 to 8 times larger compared to a single line (50 μm). Hence, the ADC and MUX needs to be 4 to 8 times faster than the CSA. A 1–2 fps readout of 1M pixel imager (1000 × 1000 pixels) sets a readout speed of 1–2 kS/s per line for each CSA. This translates to 4–8 kS/s for the ADC if a 4:1 MUX is used or 8–16 kS/s for a 8:1 MUX. The slower ADC configuration sets the specification limitation to the width of the ADC to 200 μm, whereby the faster allows a width of 400 μm.
