Abstract:
A photo detector has a sensing TFT (thin film transistor) and a photodiode. The sensing TFT has a gate and a base. The photodiode has an intrinsic semiconductor region electrically connected to the gate and the base of the sensing TFT. The sensing TFT and the photodiode both have a structure comprising low temperature poly-silicon. A display panel contains the photo detector is also disclosed.

Description:
RELATED APPLICATIONS 
     This application claims priority to Taiwan Application Serial Number 96126222, filed Jul. 18, 2007, which is herein incorporated by reference. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a photo detector. More particularly, the present invention relates to a photo detector having a low temperature poly-silicon (LTPS) material for a display panel. 
     2. Description of Related Art 
       FIG. 1  is a schematic diagram illustrating a traditional PIN photo diode  100 . A PIN photo diode  100  comprises an N-type semiconductor region  102 , a P-type semiconductor region  104  and an intrinsic semiconductor region  106  between the N-type semiconductor region  102  and the P-type semiconductor region  104 . When light irradiates the intrinsic semiconductor region  106 , the holes (h + ) and electrons (e − ) generated by the photoelectric effect would respectively move to the P-type semiconductor region  104  and the N-type semiconductor region  102 , thus generating a photoelectric current. It is well known that by measuring such photoelectric current, the PIN photo diode  100  could be used as a photo detector. 
     Generally, photo detectors manufactured by the LTPS thin film transistor (TFT) technique have the disadvantage of low photosensitivity. The low photosensitivity is because the thickness of the substrate used for photo-absorption is only about 50 nm. In addition, the traditional PIN photo diode suffers from the problem of excessive dark current due to its manufacturing process. Moreover, in consideration of the manufacturing process, a CMOS process that could carry out the implantation of the n+ ion and p+ ion at the same time would increase the complexity of the manufacturing process of the PIN photo diode. Therefore, the known solutions for increasing or improving the performance of photo detectors could not be completely adapted to or compatible with the current TFT manufacturing process of the display device. 
     SUMMARY 
     According to one embodiment of the present invention, a photo detector is provided. The photo detector comprises a sensing TFT and a photo diode. The sensing TFT has a gate and a base. The photo diode has an intrinsic semiconductor region electrically connected to the gate and the base of the sensing TFT. The sensing TFT and photo diode both have a structure comprising LTPS. 
     According to another embodiment of the present invention, a display panel is provided. The display panel comprises a pair of substrates correspondingly disposed, a liquid crystal layer and a plurality of pixel units. The liquid crystal layer is disposed between the pair of substrates, and the pixel units are disposed on the pair of substrates. Each pixel unit comprises a photo sensing region and a display region. The photo sensing region comprises a photo detector and a read-out TFT. The photo detector comprises a sensing TFT and a photo diode. The sensing TFT has a gate and a base, and the photo diode has an intrinsic semiconductor region electrically connected to the gate and the base of the sensing TFT. The read-out TFT reads the signal generated by the photo detector. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows: 
         FIG. 1  is a schematic diagram illustrating the traditional PIN photo diode; 
         FIG. 2A  is a top view diagram illustrating a photo detector according to one embodiment of the present invention; 
         FIG. 2B  is a schematic diagram illustrating the corresponding structure of the photo detector of  FIG. 2A ; 
         FIG. 3A  is a top view diagram illustrating a display panel according to one embodiment of the present invention; 
         FIG. 3B  is a cross-sectional view of the Line AA′ of  FIG. 3A ; 
         FIG. 3C  is a schematic diagram illustrating the corresponding structure of the photo detector of  FIG. 3A ; and 
         FIG. 4  is schematic diagram illustrating an active pixel sensor architecture according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2A  is a top view diagram illustrating a photo detector according to one embodiment of the present invention.  FIG. 2B  is a schematic diagram illustrating the corresponding structure of the photo detector of  FIG. 2A . Refer to both  FIG. 2A  and  FIG. 2B . The photo detector  200  comprises a sensing TFT  210  and a photo diode  220 . The sensing TFT  210  has a base  212 , agate  214 , a drain  216 , and a source  218 . The photo diode  220  has an intrinsic semiconductor region  222  and two N-type semiconductor regions  226  and  228 . The intrinsic semiconductor region  222  electrically connects to the base  212  and the gate  214 , and the drain  216  and the source  218  electrically connect to the N-type semiconductor regions  226  and  228  respectively. The sensing TFT  210  and the photo diode  220  both have an LTPS structure. 
     More specifically, the substrate of the photo detector  200  of  FIG. 2A  is an LTPS material, and the photo detector  200  could be divided into three regions: an intrinsic semiconductor region  232 , and N-type semiconductor regions  236  and  238  that are disposed on both sides of the intrinsic semiconductor region  232 . The intrinsic semiconductor region  232  corresponds to the base  212  and intrinsic semiconductor region  222  of  FIG. 2B . An H-shaped metal layer  234  is disposed on the intrinsic semiconductor region  232  and corresponds to the gate  214  of  FIG. 2B . The intrinsic semiconductor region  232  that is not shielded by the H-shaped metal layer  234  corresponds to the intrinsic semiconductor region  222  of the photo diode  220  which is used to receive light. The N-type semiconductor region  236  corresponds to the drain  216  and the N-type semiconductor region  226  in  FIG. 2B . The N-type semiconductor region  238  corresponds to the source  218  and the N-type semiconductor region  228  in  FIG. 2B . 
     When the intrinsic semiconductor region  232  is irradiated by light the electron/hole pairs resulted from the photoelectric effect move toward the two opposite ends of the photo diode  220  and turn on the sensing TFT  210  thereby generating a positive feedback in order to amplify the photoelectric current of the photo detector  200 . In this way, the photosensitivity of the photo detector  200  is significantly improved. In this embodiment, the sensing TFT  210  could be an N-type metal-oxide-semiconductor (NMOS) TFT, and the photo diode  220  could be an N-type-intrinsic-N-type (NIN) semiconductor diode. However, according to other embodiments, a P-type metal-oxide-semiconductor (PMOS) TFT can be used as a sensing TFT, and P-type-intrinsic-P-type (PIP) semiconductor diode can be used as a photo diode. Hence, the architecture of the photo detector of this embodiment could be implanted by the PMOS-LTPS technique, which is more cost-effective in the display panel manufacturing process. 
     Moreover, the aforementioned sensing TFT and photo diode could be used in a display panel. For example, the intrinsic semiconductor region  232  (i.e., the base  212  and the intrinsic semiconductor region  222 ) could be disposed in the LTPS layer of the display panel, and the H-shaped metal layer  234  (i.e., the gate  214 ) could be disposed in the first metal layer of the display panel. In addition, the H-shaped metal layer  234  (i.e., the gate  214 ) could electrically connect to the intrinsic semiconductor region  232  (i.e., the intrinsic semiconductor region  222  of the photo diode  220 ) by means of at least one contact plug  245  of the second metal layer (such as the metal layer  244 ) of the display panel. In addition, the second metal layer (such as the metal layers  246 ,  248 ) of the display panel could also be used as a contact electrode of the N-type semiconductor regions  236  and  238 . 
       FIG. 3A  is a top view diagram illustrating a display panel according to one embodiment of the present invention.  FIG. 3B  is a cross-sectional view taken along the line AA′ of  FIG. 3A .  FIG. 3C  is a schematic diagram illustrating the corresponding structure of the photo detector of  FIG. 3A . Refer to  FIG. 3A ,  FIG. 3B  and  FIG. 3C . The display panel  300  comprises a pair of correspondingly disposed substrates  330  and  340 , a liquid crystal layer  350  and a plurality of pixel units  360 . For the purpose of simplicity and clarity, only one pixel unit  360  is shown in  FIG. 3A  and  FIG. 3B . The liquid crystal layer  350  is disposed between the pair of substrates  330  and  340 , and the pixel unit is disposed on the pair of substrates  330  and  340 . Each pixel unit  360  comprises a photo sensing region  362  and a display region  364 . The photo sensing region  362  comprises a photo detector  372  and a read-out TFT  382 . The read-out TFT  382  reads the signal generated by the photo detector  372 . As shown in  FIG. 3C , the corresponding structure of the photo detector  372  comprises a sensing TFT  310  and a photo diode  320 . The sensing TFT  310  has a gate  314  and a base  312 , and the photo diode  320  has an intrinsic semiconductor region  322  electrically connected to the gate  314  and the base  312  of the sensing TFT  310 . 
     The material of at least one of the substrates  330  and  340  is transparent material (such as glass, quartz, or the like), non-transparent material (such as Silica sheet, ceramics, or the like), flexible material (such as polyester, polyalkene, polyamide, polyol, poly cycloalkane, poly aromatics, or the like or combinations thereof), or combinations thereof. According to one example of this embodiment, the substrates  330  and  340  are glass substrates. 
     A common electrode  331  is disposed on the substrate  330 . A material of the common electrode  331  could be a transparent conductive material, such as indium tin oxide, aluminum zinc oxide, cadmium tin oxide, indium zinc oxide, aluminum tin oxide, or the like or combinations thereof. A light shielding layer  332  (such as black matrix) and a color filter layer  334  are disposed on the common electrode  331  and correspond respectively to the photo sensing region  362  and the display region  364 . A semiconductor layer  341 , an insulating layer  342 , a first metal layer  343 , an insulating layer  344 , an insulating layer  345 , a second metal layer  346 , a protective layer  347 , and an insulating layer  348  are sequentially formed on the substrate  340  and patterned respectively so as to form a photo detector  372 , a read-out TFT  382 , a display TFT  374 , a storage capacitor  384 , N-type semiconductor regions  391   a - 391   f , and contact plugs  393   a - 393   g.    
     The material of the semiconductor layer  341  could be, for example, polycrystalline silicon, micro-crystalline silicon, single-crystalline silicon, amorphous silicon, or combinations thereof. In this embodiment, the material of the semiconductor layer  341  is an LTPS. The material of at least one of the insulating layer  342 , the insulating layer  344 , the insulating layer  345 , the protective layer  347 , and the insulating layer  348  is organic material (such as a photoresist material, polyarylene ether (PAE), polyamide, polyester, polyol, polyalkene, benzocyciclobutene (BCB), hydrogen silsesquioxane (HSQ), methyl silesquioxane (MSQ), silicone hydrocarbon (SiOC—H), or the like or combinations thereof), inorganic material (such as silicon oxide, silicon nitride, silicon nitrogen oxide, silicon carbide, hafnium oxide, or the like or combinations thereof), or combinations thereof. 
     More specifically, the semiconductor layer  341  of  FIG. 3A  and  FIG. 3B  correspond to the base  312  and the intrinsic semiconductor region  322  of  FIG. 3C . The first metal layer  343  of  FIG. 3A  and  FIG. 3B  corresponds to the gate  314  of  FIG. 3C  and electrically connects to the semiconductor layer  341  by means of such as at least one contact plug  393   a - 393   g  of the second metal layer  346 . The semiconductor layer  341  of the photo detector  372  of  FIG. 3A  and  FIG. 3B , which is not shielded by the first metal layer  343 , corresponds to the intrinsic semiconductor region  322  of the photo diode  320  of  FIG. 3C  for receiving light. The N-type semiconductor region  391 c of  FIG. 3A  and  FIG. 3B  corresponds to the drain  316  and the N-type semiconductor region  326  of  FIG. 3C , while the N-type semiconductor region  391   d  of  FIGS. 3A and 3B  corresponds to the source  318  and the N-type semiconductor region  328  of  FIG. 3C . 
     In this embodiment, the sensing TFT  310  is an NMOS TFT, and the photo diode  320  is an NIN diode. However, in other embodiments, it is possible to use a PMOS TFT as the sensing TFT, and a PIP diode as the photo diode. Moreover, the aforementioned pixel unit  360  comprises at least one sub-pixel unit, wherein it is defined that each sub-pixel unit corresponds to a single display region. In other words, when there is more than one color within a single pixel unit, it is possible to selectively dispose one photo sensing region at each display region with a different color. Alternatively, a plurality of display regions within a pixel unit could share a photo sensing region. 
     The actuating relationship of the pixel unit  360  in response to changing light is discussed. As shown in  FIG. 3B , the light shielding layer  332  has an opening  333  corresponding to the photo detector  372  (such as the intrinsic semiconductor region  322  within the photo diode  320  which is used for receiving the light). The opening  333  detects the external changing light of the pixel unit  360 . The photoelectric current generated by the photo diode  320  as a result of light irradiation could pass through and turn on the sensing TFT  310  and thereby amplify the photoelectric current of the photo detector  372 . Afterward, the photoelectric current is read by the read-out TFT  382  so as to be transferred to the external circuit (such as processor). 
     For example, when the light is not shielded from the exterior of the pixel unit  360  (i.e., when the opening  333  is not blocked by an obstacle such as a finger or a foreign matter), the light intensity received by the intrinsic semiconductor region  322  is increased and thereby amplifies the photoelectric current signal that the sensing TFT  310  received. In this case, the read-out TFT  382  would transfer a signal representing non-obstacle to the external processor. When an obstacle approaches the pixel unit  360  and thus blocks the light, the light intensity received by the intrinsic semiconductor region  322  is decrease and thereby reduces the photoelectric current signal that the sensing TFT  310  received. In this case, the read-out TFT  382  would transfer a signal representing an obstacle to the external processor. 
     If the display panel  300  is a touch display panel, the processor would determine the position of the pixel unit within the display panel  300  which the finger or the foreign matter is pressed against based on such signal. Alternatively, if the display panel utilizes such a pixel unit  360  or the photo sensing region  362  to detect the light quantity of the external environment, the processor would determine whether the light quantity of the external environment where the display panel  300  is situated in is sufficient based on such signal. Furthermore, the processor could adjust the light intensity of the back light module based on such signal. 
     The aforementioned photo sensing region  362  could adapt an active pixel sensor (APS) architecture, a passive pixel sensor (PPS) architecture, or other suitable sensor architecture to accomplish the connection relation between the photo detector  372  and the read-out TFT  382 .  FIG. 4  is schematic diagram illustrating an active pixel sensor architecture  400  according to one embodiment of the present invention. Signal lines  420  of different rows control a plurality of pixel units  410 . The read-out TFTs  412  transfer signals to the processor through the reset transistor  430  by means of a common line  440 . 
     Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.