Patent Publication Number: US-8115204-B2

Title: Photo elements and image displays

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Taiwan application Serial No. 96130694 filed Aug. 20, 2007, the subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a photo element, and more particularly to a photo element applied in an input display. 
     2. Description of the Related Art 
     In input displays, such as input liquid crystal displays, pixels generally comprise photo elements to serve as readout pixels with readout function, so that liquid crystal display panels can provide input and readout functions. The panels are referred to as in-cell input panels. Generally, photo elements are divided into two types: charge-mode photo elements and current-mode photo elements.  FIG. 1  shows a circuitry of a conventional charge-mode photo element  100 . The photo element  100  comprises a readout line Readout, a switch line SW, a bias line  30 , a switch thin film transistor (TFT)  10 , a photo TFT  20 , and a capacitor C 1 . The photo TFT  20  is used to receive environment light. When a scan signal on the switch line SW turns on the switch TFT  10 , the readout line Readout transmits charges to charge the capacitor C 1  through the switch TFT  10 . The brightness of the received environment light determines the conductivity of the photo TFT  20 . When the scan signal on the switch line SW turns off the switch TFT  10  and the brightness of the received environment light is higher, the photo TFT  20  discharges the capacitor C 1  more quickly. When the scan signal on the switch line SW turns off the switch TFT  10  and the brightness of the received environment light is lower, the photo TFT  20  discharges the capacitor C 1  more slowly. As described above, light sensitivity of the photo TFT  20  seriously affects performance of the photo element  100 . If the photo element  100  has high light sensitivity, the photo element  100  can operate in any light-receiving condition. Thus, it is important to enhance the light sensitivity of the photo TFT  20 . 
       FIG. 2  shows a circuitry of a conventional current-mode photo element  200 . The photo element  200  comprises a switch TFT STFT 2 , a photo TFT PTFT 2 , a readout line Readout 2  (a first line), a switch line SW 2  (a second line), and a bias line Bias 1  (a third line). A first electrode (for example, drain D) and a gate G of the photo TFT PTFT 2  are electrically coupled to the bias line Bias 1 , and a second electrode (for example, source S) thereof is electrically coupled to the switch TFT STFT 2 . A first electrode (for example, drain D), a second electrode (for example, source S), and a gate G of the switch TFT STFT 2  are electrically coupled to the source S of the photo TFT PTFT 2 , the readout line Readout 2 , and the switch line SW 2 , respectively. When the switch line SW 2  is at a high potential, the switch TFT STFT 2  is turned on, and the photo TFT PTFT 2  is turned on according to brightness of the received environment light to generate a corresponding light current. The corresponding light current can be provided to a signal detector (not shown in  FIG. 2 ) through the switch TFT STFT 2  and the readout line Readout 2  for detecting a degree of the received environment light. 
     Moreover,  FIG. 3  shows a circuitry of an improved photo element  300  of U.S. patent application Ser. No. 11/611,320. The photo element  300  comprises a switch TFT STFT 1 , a photo TFT PTFT 1 , a readout line Readout 1  (a first line), and a switch line SW 1  (a second line). A first electrode (for example, drain D), a second electrode (for example, source S), and a gate G of the switch TFT STFT 1  are electrically coupled to a second electrode (for example, source S) of the photo TFT PTFT 1 , the readout line Readout 1 , and the switch line SW 1 , respectively. A first electrode (for example, drain D) and a gate G of the photo TFT PTFT 1  are both electrically coupled to the switch line SW 1 . When the switch line SW 1  is at a high potential, the switch TFT STFT 1  and the photo TFT PTFT 1  are turned on, and the photo TFT PTFT 1  is turned on according to brightness of the received environment light to generate a corresponding light current. The corresponding light current can be provided to a signal detector (not shown in  FIG. 3 ) through the switch TFT STFT 1  and the readout line Readout 1  for detecting degree of the received environment light. As the photo element  100  of  FIG. 1 , the light currents of the photo elements  200  and  300  of  FIGS. 2 and 3  are affected by the brightness of the environment light received by the surfaces of the photo TFTs PTFT 1  and PTFT 2 . Generally, the light current is directly proportional to the degree of the environment light received by the photo TFT. Thus, enhancing light sensitivity of the photo TFTs PTFT 1  and PTFT 2  is very important. 
     BRIEF SUMMARY OF THE INVENTION 
     An exemplary embodiment of a photo element comprises a first line, a second line, a switch transistor, and a photo transistor. The switch transistor has a first electrode, a second electrode, and a first gate. One of the first electrode and the second electrode is electrically coupled to the first line, and the first gate is electrically coupled to the second line. The photo transistor is electrically coupled to the switch transistor and arranged to detect light. The photo transistor has a third electrode, a fourth electrode, and a second gate. At least one of the switch transistor and the photo transistor is an asymmetric transistor. 
     Another exemplary embodiment of an input device comprises a readout line, a switch line, a photo transistor, and a signal detector. The photo transistor is arranged to detect light and has a first electrode, a second electrode, and a gate. The first electrode is electrically coupled to the readout line, the gate is electrically coupled to the switch line, and the photo transistor is an asymmetric transistor. The signal detector is electrically coupled to the readout line and arranged to detect a light current generated by the photo transistor. 
     An exemplary embodiment of an image display comprises a first substrate and a pixel disposed on the first substrate. The pixel comprises a pixel electrode, a first line, a second line, a pixel transistor, a switch transistor, and a photo transistor. The pixel transistor is electrically coupled to the pixel electrode. The switch transistor has a first electrode, a second electrode, and a first gate. One of the first electrode and the second electrode is electrically coupled to the first line, and the first gate is electrically coupled to the second line. The photo transistor is electrically coupled to the switch transistor and has a third electrode, a fourth electrode, and a second gate. At least one of the switch transistor and the photo transistor is an asymmetric transistor. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a circuitry of a conventional charge-mode photo element; 
         FIG. 2  shows a circuitry of a conventional current-mode photo element; 
         FIG. 3  shows a circuitry of an improved photo element; 
         FIG. 4  shows exemplary embodiments of an asymmetric transistor; 
         FIG. 5  shows exemplary embodiments of a symmetric transistor; 
         FIG. 6  shows an exemplary embodiment of a photo element; 
         FIG. 7  is a relationship diagram showing light response intensity of a photo transistor in different channel lengths; 
         FIG. 8  shows another exemplary embodiment of a photo element; 
         FIG. 9  shows another exemplary embodiment of a photo element; 
         FIG. 10  shows an equivalent circuit of an exemplary embodiment of a photo element; 
         FIG. 11  shows another exemplary embodiment of a photo element; 
         FIG. 12  shows another exemplary embodiment of a photo element; 
         FIG. 13  shows an exemplary embodiment of a readout pixel circuit; 
         FIG. 14  shows another exemplary embodiment of a readout pixel circuit; 
         FIG. 15  shows another exemplary embodiment of a readout pixel circuit; 
         FIG. 16  shows another exemplary embodiment of a photo element; 
         FIG. 17  shows another exemplary embodiment of a photo element; 
         FIG. 18  shows an exemplary embodiment of a liquid crystal display panel in which a photo element is applied; and 
         FIG. 19  is cross-sectional view of an exemplary embodiment of a liquid crystal display. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIGS. 4 and 5  respectively show exemplary embodiments of an asymmetric transistor  400  and a symmetric transistor  500 . The asymmetric transistor  400  comprises a gate G, an amorphous-silicon layer  220 , a first electrode  230 , and a second electrode  240 . When the first electrode  230  is defined as a drain, the second electrode  240  is a source. On the contrary, when the first electrode  230  is defined as a source, the second electrode  240  is a drain. In following embodiments, the potential of the first electrode is compared with the potential of the second electrode, and the electrode with the higher potential serves as a drain while the electrode with the lower potential serves as a source. In  FIG. 4 , W 1  represents the length of the first electrode  230 , and W 2  represents the length of the second electrode  240 , wherein W 1 &gt;W 2 . W 3  represents the channel length of the asymmetric transistor  400 . W 3  is generally equal to the average value of the length W 1  of the first electrode  230  and the length W 2  of the second electrode  240 . In  FIG. 5 , the symmetric transistor  500  comprises a gate G, an amorphous-silicon layer  320 , a first electrode  330 , and a second electrode  340 . When the potential of the first electrode  330  is higher than that of the second electrode  340 , the first electrode  330  is defined as a drain, and the second electrode  340  is defined as a source. On the contrary, when the potential of the second electrode  340  is higher than that of the first electrode  330 , the second electrode  340  is defined as a drain, and the first electrode  330  is defined as a source. In  FIG. 5 , W 1  represents the length of the first electrode  330 , and W 2  represents the length of the second electrode  340 , wherein W 1 =W 2 . W 3  represents the channel length of the symmetric transistor  500  and is equal to W 1  and W 2 . 
     For the same area, since the asymmetric transistor  400  has a longer channel length than the symmetric transistor  500 , the asymmetric transistor  400  has larger conductivity ratio than the symmetric transistor  500 . The larger conductivity ratio can decrease signal transmission delay. Thus, a switch transistor and/or a photo transistor within a photo element of the embodiments of the invention have the asymmetric structure as shown in  FIG. 4 . In the preferred embodiments, the switch transistor and photo transistor are implemented by thin film transistors or amorphous-silicon thin film transistors, but not limited to that. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 drain (V d ) = gate (V g ) = 15 V, L = 5 um 
                 First type 
                 Second type 
               
               
                   
               
             
            
               
                 Structure 
                 Wd &lt; Ws 
                 Wd &gt; Ws 
               
               
                 current I d1  (dark, A) 
                 3.60E−06 
                 2.20E−06 
               
               
                 current I d2  (2150 cd/cm{circumflex over ( )}2(bright), A) 
                 6.60E−06 
                 5.28E−06 
               
               
                 current I d2  − current I d1  (current difference, A) 
                 3.01E−06 
                 3.08E−06 
               
               
                 current I d2 /current I d1  (current ratio) 
                 1.84E−00 
                 2.40E−00 
               
               
                   
               
            
           
         
       
     
     Moreover, the switch transistor in the photo element requires a large current to alleviate loading effect, and the photo transistor requires a large bright to dark current ratio to increase light sensitivity and signal/noise ratio. Table 1 shows measured results of an asymmetric transistor, wherein Wd represent the length of a drain of a thin film transistor, and Ws represent the length of a source thereof. The bias condition in this measurement is Vd=Vg=15V and Vs=0V, which the drain has higher potential than the source. According to Table 1, for the almost same current differences (the bright environment current I d2  minus the dark environment current I d1 ) of the first type and the second type, the current ratio for the second type is larger than that for the first type, thus, the photo transistor is appropriately implemented by the second type (Wd&gt;Ws). Referring to  FIG. 4 , the first electrode  230  can serve as a drain of the photo transistor, while the second electrode  240  can serve as a source thereof, wherein the potential of the first electrode  230  is higher than that of the second electrode  240 . Moreover, the switch transistor operates in a dark environment. According to Table 1, the current I d1  in the first type is larger. Thus, the switch transistor is appropriately implemented by the first type (Wd&lt;Ws). Referring to  FIG. 4 , the first electrode  230  can serve as a source of the switch transistor, while the second electrode  240  can serve as a drain thereof, wherein the potential of the second electrode  240  is higher than that of the first electrode  230 . 
       FIG. 6  shows an exemplary embodiment of a photo element according to the above transistor characteristics. The equivalent circuit of the photo element of  FIG. 6  is shown in  FIG. 3 . In the embodiment, the switch transistor STFT 1  and the photo transistor PTFT 1  are implemented by asymmetric transistors, such as U-shape (also referring to C-shape or semicircle-shaped) transistors. The switch transistor STFT 1  is the first type of transistor, that is the length of the source S is longer than the length of the drain D (Wd&lt;Ws). In the structure of the switch transistor STFT 1 , the source S has an approximate U, C or semicircle shape, and the drain D has an approximate rectangle or bar shape. The photo transistor PTFT 1  is the second type of transistor, that is the length of the source S is shorter than the length of the drain D (Wd&gt;Ws). The potential of the drain of each of the transistors in the photo element is set as higher than that of the source. In the structure of the photo transistor PTFT 1 , the drain D has an approximate U, C or semicircle shape, and the source S has an approximate rectangle or bar shape. In other embodiments, the drain D and the source S can have any asymmetric shape and are not limited to the above shapes. The gate G of the switch transistor STFT 1  and the gate G of the photo transistor PTFT 1  are electrically coupled together. Labels  601  and  602  represent amorphous-silicon layers of the switch transistor and the photo transistor, respectively. Under the same area of thin film transistors, compared with the conventional symmetric transistors, the structure of  FIG. 6  has preferred light response speed and performance.  FIG. 7  is a relationship diagram showing light response intensity of a photo transistor of a photo element in different channel lengths. The channel length of a switch transistor (the average value of the drain length and the source length) is set as 30 um. The X axis of  FIG. 7  represents the channel length of the photo transistor from 5 um to 50 um, while the Y axis of  FIG. 7  represents the light response intensity of the photo transistor. The point P represents the light response intensity as the channel lengths of the switch transistor and the photo transistor are the same. According to the curve  710 , when the channel length of the photo transistor is longer than the channel length of the switch transistor, the light response intensity is degraded due to the loading effect of the switch transistor. In order to provide the photo element with a preferred light sensitivity, the channel length of the photo transistor is designed to be less than or equal to the channel length of the switch transistor. When the channel length of the photo transistor was equal to the channel length of the switch transistor, the light response intensity reached a threshold value. Thus, in the embodiment of  FIG. 6 , the channel length of the photo transistor PTFT 1  of the photo element is designed to be less than or equal to the channel length of the switch transistor STFT 1 . 
       FIG. 8  shows another exemplary embodiment of a photo element. The equivalent circuit of the photo element of  FIG. 8  is shown in  FIG. 2 . In the embodiment, the switch transistor STFT 2  and the photo transistor PTFT 2  are implemented by asymmetric transistors, such as U-shape (also referring to C-shape or semicircle-shaped) transistors. The switch transistor STFT 2  is the first type of transistor, that is the length of the source S is longer than the length of the drain D (Wd&lt;Ws). The photo transistor PTFT 2  is the second type of transistor, that is the length of the source S is shorter than the length of the drain D (Wd&gt;Ws). The gate G of the switch transistor STFT 2  and the gate G of the photo transistor PTFT 2  are not electrically coupled together. Amorphous-silicon layers  801  and  802  are represented by blank areas in  FIG. 8 . Also, the potential of the drain of each of the transistors is set as higher than that of the source. In the embodiment of  FIG. 8 , the channel length of the photo transistor PTFT 2  is designed to be equal to the channel length of the switch transistor STFT 2 , however, without limitation. 
       FIG. 9  shows another exemplary embodiment of a photo element. The equivalent circuit of the photo element of  FIG. 9  is shown in  FIG. 10 . The equivalent circuit of  FIG. 10  is similar to that in  FIG. 2 , except that a source S of a photo transistor PTFT 3  is electrically coupled to a bias line (a third line) Biase 2 , and a drain D and a gate G thereof are electrically coupled to a source S of a switch transistor STFT 3 . The photo element  1000  of  FIG. 10  can be applied in a system in which the potential of the readout line Readout 3  (a first line) is higher than the potential of the bias line Biase 2 . The photo element of  FIG. 2  can be applied in a system in which the potential of the bias line Biase 1  is higher than potential of the readout line Readout 2 . 
     In the embodiment of  FIG. 9 , the switch TFT STFT 3  and the photo TFT PTFT 3  of the photo element are implemented by asymmetric transistors, such as U-shape (also referring to C-shape or semicircle-shaped) transistors, however, without limitation. Also, the potential of the drain of each of the transistors is set as higher than that of the source. As shown in  FIG. 9 , the switch transistor STFT 3  is the first type of transistor, that is the length of the source S is longer than the length of the drain D (Wd&lt;Ws). The photo transistor PTFT 3  is the second type of transistor, that is the length of the source S is shorter than the length of the drain D (Wd&gt;Ws). The gate G of the switch transistor STFT 3  and the gate G of the photo transistor PTFT 3  are not electrically coupled together. Amorphous-silicon layers  901  and  902  are represented by blank areas in  FIG. 9 . 
     In above embodiments, the readout lines Readout 1 , Readout 2 , and Readout 3  can be electrically coupled to respective signal detectors for detecting and reading the light current generated by the photo transistors. For example, in  FIG. 10 , the readout line Readout 3  is electrically coupled to a signal detector  1001 , so that the light current generated by the photo transistor PTFT 3  is transmitted to the signal detector  1001  through the switch transistor STFT 3  and the readout line Readout 3  for detecting and reading of degree of the received light. 
     Moreover, in the above embodiments, both the photo transistor and the switch transistor of each of the photo elements are asymmetric transistors. However, in some embodiments, only one of the photo transistor and the switch transistor of each of the photo elements is asymmetric transistors.  FIG. 11  shows another exemplary embodiment of a photo element. The equivalent circuit of the photo element of  FIG. 11  is shown in  FIG. 3 . A switch transistor STFT 4  is a symmetric transistor, while a photo transistor PTFT 4  is an asymmetric transistor, such as a U-shape transistor. Similarly, as the aforementioned, in another embodiment, the channel length of the photo transistor PTFT 4  is preferably to be designed shorter than or equal to the channel length of the switch transistor STFT 4 . In contrast to the embodiment of  FIG. 11 ,  FIG. 12  shows another exemplary embodiment of a photo element. The equivalent circuit of the photo element of  FIG. 12  is shown in  FIG. 3 . A switch transistor STFT 5  is an asymmetric transistor, such as a U-shape transistor, while a photo transistor PTFT 5  is a symmetric transistor. Also, in another embodiment, the channel length of the photo transistor PTFT 5  is preferably to be designed shorter than or equal to the channel length of the switch transistor STFT 5 . Similarly, in the photo elements of  FIGS. 2 and 10 , only one of the photo transistor and the switch transistor of each of the photo elements is needed to be designed as an asymmetric transistor, and the detailed description is omitted herein. 
     The photo element  200 ,  300 , or  1000  can be built in each or some of the general pixels (pixels without built-in photo elements) of an image displaying system (such as a liquid crystal display panel), so that the pixels with the photo element can be served as readout pixels with readout or input function. Thus, the liquid display panel can provide input and readout functions. The panel may be referred to as an in-cell input panel. The number of photo elements required for the in-cell input panel is determined according to the input function and the required resolution. 
       FIG. 13  shows an exemplary embodiment of a readout pixel circuit, wherein the photo element  300  is given as an example to be applied in a readout pixel. The readout pixel is disposed in a thin film transistor array substrate (or an array substrate). Generally, the thin film transistor array substrate comprises a plurality of gate lines and a plurality of data lines formed on it. The gate lines and the data lines define a plurality of general pixels and readout pixels. To simplify and clearly describe the readout pixel,  FIG. 13  shows only one readout pixel defined by a gate line  420  and a data line  410 . The description of a general pixel (a pixel without a built-in photo element) is omitted herein. The readout pixel comprises a liquid crystal capacitor C LC , a storage capacitor C ST , a pixel thin film transistor (TFT)  450 , and a photo element  300 . The pixel TFT  450  is arranged to be a switch element to control voltage required for a pixel electrode of a liquid crystal display. A drain and gate of the pixel TFT  450  are electrically coupled to the data line  410  and the gate line  420  respectively. The photo element  300  comprises a photo TFT  430  and a switch TFT  435  and is electrically coupled between a readout line  440  and a switch line  421 . In some embodiments, the switch line  421  can be another gate line or a line for other functions. In the embodiment of  FIG. 13 , at least one of the switch TFT  435  and the photo TFT  430  is an asymmetric transistor. As described above, since the light sensitivity of the photo element of the embodiment is higher than that of the conventional photo element, a light current of the readout pixel can be detected and read out easily. Moreover, the switch TFT  435  is disposed directly under a black matrix of an upper substrate of the liquid crystal display, so that the switch TFT  435  can not be affected by environment brightness changes.  FIG. 19  is cross-sectional view of an exemplary embodiment of a liquid crystal display. As shown in  FIG. 19 , the switch TFT 435  is disposed directly under a black matrix B/M of an upper substrate (the upper glass), thereby minimizing interference of environment light to the switch TFT  435 . 
     Similarly,  FIG. 14  shows another exemplary embodiment of a readout pixel circuit. The difference between  FIGS. 13 and 14  is the disposition of the photo elements  300  and  300   a . The photo element  300   a  is electrically coupled between a gate line  420   a  and a readout line  440   a . The photo element  300   a  comprises a photo TFT  430   a  and a switch TFT 435   a . At least one of the switch TFT  435   a  and the photo TFT  430   a  is an asymmetric transistor. Moreover, a drain and a gate of the pixel TFT  450   a  are electrically coupled to a data line  410   a  and a gate line  420   a  respectively. 
       FIG. 15  shows another exemplary embodiment of a readout pixel circuit, wherein the photo element  200  of  FIG. 2  is given as an example to be applied in a readout pixel. The photo element  300   b  comprises a photo TFT  430   b  and a switch TFT  435   b . The photo TFT  430   b  is electrically coupled to a gate line  421   b , a common voltage line Vcom (or a bias line), and a readout line  440   b . A drain and a gate of a pixel TFT  450   b  are electrically coupled to the data line  410   b  and the gate  420   b  respectively. And in this embodiment, the potential of the common voltage line can be set higher than a potential of the readout line  440   b . In some embodiments, the photo element  1000  as shown in  FIG. 10  can be also integrated in an image displaying system, as the photo element  200 , however, a detailed description is omitted. 
       FIG. 16  shows another exemplary embodiment of a photo element  1600 . The difference between the photo element  1600  and the above photo elements is that the photo element  1600  does not comprise a switch transistor STFT 6  and a photo transistor PTFT 6  is an asymmetric transistor. The photo transistor PTFT 6  is electrically coupled between a readout line Readout 4  (a first line) and a bias line Biase 3  (a third line). A gate of the photo transistor PTFT 6  is electrically coupled to a switch line SW 4  (a second line), and a drain and a source thereof are electrically coupled to a high potential and a low potential respectively. Moreover, the readout line Readout 4  is electrically coupled to a signal detector  1101  so that a light current generated by the photo transistor PTFT 6  can be provided to the signal detector  1101  through the readout line Readout 4  for detecting and reading the degree of the received light. 
       FIG. 17  shows another exemplary embodiment of a photo element  1700 . The photo element  1700  comprises a photo transistor PTFT 7 , and the photo transistor PTFT 7  is an asymmetric transistor. A gate G and a drain D of the photo transistor PTFT 7  are electrically coupled to a switch line SW 5  (a second line), and source S thereof are electrically coupled to a readout line Readout 5  (a first line). Moreover, the readout line Readout 5  is electrically coupled to a signal detector  1102  so that a light current generated by the photo transistor PTFT 7  can be provided to the signal detector  1102  through the readout line Readout 5  for detecting and reading the degree of the received light. 
     Specially, if the photo transistor PTFT 6  or PTFT 7  has a large switch current ratio I on /I off , the photo transistor PTFT 6  or PTFT 7  can serve as a switch transistor and a photo transistor as described above at the same time. This is because when photo transistor PTFT 6  or PTFT 7  has a large switch current ratio I on /I off , the light current generated by the photo transistor PTFT 6  or PTFT 7  is much larger than the leakage current generated by the turned-off photo transistor PTFT 6  or PTFT 7 . In other words, when the photo transistor PTFT 6  or PTFT 7  is applied in a display panel, the leakage current does not affect the light current generated by the photo transistor PTFT 6  or PTFT 7  significantly, which is one of the reasons why the photo element  1600  or  1700  does not need a switch transistor. 
       FIG. 18  shows an exemplary embodiment of a liquid crystal display panel in which the photo element of  FIG. 16  is applied. For simplicity, only connections between a photo element, switch lines n, n+1, n+2 and a readout line are shown in each pixel, and the descriptions of other elements, such pixel transistors, is omitted. According to  FIG. 18 , when the n-th photo element is read, a current Ireadout on the readout line is the sum of currents flowing though the photo transistor PTFT (Ireadout=Ion+Ioff (n+1) +Ioff (n+2) + . . . ). Thus, when each photo transistor has a large switch current ratio I on /I off , the photo transistor can serve as a switch transistor and a photo transistor at the same time. The value of current Ion is much larger than the values of leakage current Ioff (n+1) , I (n+2)  . . . , so that a signal detector (not shown) electrically coupled to the readout line can still detect the position of the touched pixel. In this embodiment, the photo transistor PTFT has a first electrode, a second electrode, and a gate, and the first electrode is electrically coupled to the readout line, the gate is electrically coupled to the switch line (such as n, n+1, n+2), and the photo transistor PTFT is preferably an asymmetric transistor, which a length of the first electrode is shorter than a length of the second electrode, and a potential of the second electrode is higher than a potential of the first electrode. Besides, in other embodiment, the second electrode of the photo transistor PTFT can be electrically coupled to a conductive line (a bias line or a common voltage line), and a potential of the conductive line is higher than a potential of the readout line. Similarly, in another embodiment, the second electrode of the photo transistor PTFT can also be electrically coupled to the switch line (such as n, n+1, n+2) and a potential of the switch line is higher than a potential of the readout line. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.