Abstract:
When a photosensor was conventionally provided in a display device, separate modules manufactured in separate steps were installed in the same case. However, decreases in the number of parts and in cost could not be achieved, and a compact size and thinning of the display device was not proceeded. A photosensor is realized by a TFT provided on an insulating substrate. Photocurrent caused by incidence of external light onto a TFT when the TFT is turned-off is detected so that the TFT is used as a photosensor. By performing laser-annealing for a semiconductor layer of the photosensor, an average grain size of crystal particles of the semiconductor layer of the photosensor is made larger than those of a crystal particle of a display portion and a light emission element, thereby improving crystal properties. Thus, a generation efficiency of the photocurrent of the photosensor can be increased.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a display device and a method of fabricating the same, and more particularly to a display device installing a photosensor in the same substrate and a method of fabricating the same.  
         [0003]     2. Description of the Related Arts  
         [0004]     As for current display devices, displays have been spread by demands for a compact size, light weight and thin thickness thereof from the market. Among such display devices, there haven been many devices installing a photosensor therein, which include, for example, an optical touch panel for detecting input coordinates by blocking light, and devices senses external light to control brightness of a display.  
         [0005]      FIG. 9  shows an example of the optical touch panel. Optical touch panel  301  arranges light emitter  303  for emitting infrared ray and the like and light detector  304  for receiving the infrared ray and the like in the periphery of display plane  302 . Such an optical touch panel  301  detects a point as input coordinates, from which the infrared ray does not reach to the light detector  304 , by blocking the infrared rays emitted from the light emitter  303  with user&#39;s fingers which are trying to perform coordinate input. This technology is described for instance in Japanese Patent Application Publication No. Hei 5-35402 (page 2 to 2, FIG. 2).  
         [0006]     In conventional displays, a display portion and a photosensor were manufactured as separate module parts after having undergone different manufacturing processes using separate production facilities. These separate module parts were assembled into the same case, and thus a finished product was manufactured. Therefore, there was inevitably a limitation to a decrease in the number of parts of the device and a reduction of a manufacturing cost for each module part.  
         [0007]     Particularly, the dissemination of mobile terminal devices such as PDAs is currently remarkable, and therefore a further compacted size, lighter weight and thinner thickness are required for the displays. Accordingly, a decrease in the number of parts and provision of cheap displays are desired.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention provides a display device that includes a display portion comprising a plurality of pixels, each of the pixels comprising a first thin film transistor comprising a first semiconductor layer, and a photosensor comprising a second thin film transistor comprising a second semiconductor layer, wherein a grain size of crystals forming the second semiconductor layer is larger than a grain size of crystals forming the first semiconductor layer.  
         [0009]     The invention also provides a display device that includes a display portion comprising a plurality of pixels, each of the pixels comprising a first thin film transistor comprising a first semiconductor layer, and a photosensor comprising a second thin film transistor comprising a second semiconductor layer, wherein a crystal length in a predetermined direction of crystals forming the second semiconductor layer is longer than a crystal length in the predetermined direction of crystals forming the first semiconductor layer.  
         [0010]     The invention further provides a method of fabricating a display device. The method includes forming an amorphous semiconductor layer on an insulating substrate, crystallizing the amorphous semiconductor layer so as to form a first semiconductor layer of a first grain size and a second semiconductor layer of a second grain size that is larger than the first grain size, forming a first thin film transistor comprising the first semiconductor layer, forming a photosensor comprising a second thin film transistor comprising the second semiconductor layer, and forming a pixel comprising the first thin film transistor in a display portion of the display device.  
         [0011]     The invention further provides a method of fabricating a display device. The method includes forming an amorphous semiconductor layer on an insulating substrate, crystallizing the amorphous semiconductor layer so as to form a first semiconductor layer comprising crystals having a first crystal length in a predetermined direction and a second semiconductor layer comprising crystals having a second crystal length in the predetermined direction which is longer than the fist crystal length, forming a first thin film transistor comprising the first semiconductor layer, forming a photosensor comprising a second thin film transistor comprising the second semiconductor layer so that an electric conduction of the second thin film transistor is in the predetermined direction, and forming a pixel comprising the first thin film transistor in a display portion of the display device.  
         [0012]     According to the display device of the present invention, an average grain size of crystal particles of a semiconductor layer in a TFT functioning as a photosensor is set to be larger than a semiconductor layer in a TFT constituting a display portion and a light emission element, whereby a probability of occurrence of electron-hole pairs increases during light radiation, and crystal properties are improved. Thus, very small current can be detected more easily.  
         [0013]     Particularly, since a high precision photosensor can be materialized with a TFT formed in an insulating substrate, the photosensor can be arranged in the same substrate where a display device is formed, and a compacted size and thin thickness of the device can be realized. Since enlargement of the average grain size of crystal particles can be realized by performing laser-annealing only for the photosensor portion two times, such enlargement can be carries out without complicating fabrication processes, and can be further contributed to decreases of the numbers of parts and fabrication processes compared to a structure in which the photosensor is installed as a separate module.  
         [0014]     Furthermore, the grain size of a crystal particle of the TFT constituting the display portion and the light emission element should not be made large beyond necessity. By performing the laser-annealing two times only for the photosensor portion which is a region fully smaller than the display portion, it is possible to provide the method of fabricating a display device, which is capable of detecting very small photocurrent while preventing an increase in cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1A  and  FIG. 1B  are a plan view of a touch panel and a section view taken along the line B-B of  FIG. 1A .  
         [0016]      FIG. 2A  and  FIG. 2B  are a plan view of one pixel in a display portion and a section view taken along the line B-B of  FIG. 2A .  
         [0017]      FIG. 3  is a section view of a photosensor.  
         [0018]      FIG. 4  is a section view for explaining a method of fabricating a display device according to an embodiment of the invention.  
         [0019]      FIG. 5  is a section view for explaining a method of fabricating a display device according to an embodiment of the invention.  
         [0020]      FIG. 6  is a section view for explaining a method of fabricating a display device according to an embodiment of the invention.  
         [0021]      FIG. 7  is a section view for explaining a method of fabricating a display device according to an embodiment of the invention.  
         [0022]      FIG. 8  is a section view for explaining a method of fabricating a display device according to an embodiment of the invention.  
         [0023]      FIG. 9  is a section view for explaining a conventional art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     A first embodiment of the present invention will be described with reference to FIGS.  1  though  8  in detail while taking a touch panel using an organic EL element as an example.  FIG. 1A  is a plan view of a touch panel, and  FIG. 1B  is a section view taken along the line A-A of  FIG. 1A . Note that a reflection material of  FIG. 1B  is omitted in  FIG. 1A .  
         [0025]     Display device  250  of this embodiment of the present invention comprises photosensor  100 , display portion  200  and light emission element  240 , which are arranged in the same insulating substrate  10 .  
         [0026]     The display portion  200  has a switching TFT and a driving TFT, and arranges a plurality of pixels formed of an organic EL element in a matrix therein, which is connected to the driving TFT. The plurality of light emission elements  240  are arranged along two sides at the periphery of the display portion  200 . The light emission elements  240  are arranged, for example, in a rectangular region of  FIG. 1A  at certain intervals, and the photosensors  100  receive light emitted from the corresponding light emission elements  240 . Each of the light emission element  240  is formed of an organic EL element which is identical to that constituting the display portion  200 . Alternatively, when it is intended to active-drive each of the light emission element  240 , a TFT constituting the display portion  200  may be further provided in the organic EL element. The photosensors  100  are a TFT, and arranged along opposite two sides of the display portion  200 , which are different from those where the light emission elements  240  are arranged. The plurality of photosensors  100  are arranged at certain intervals so as to correspond respectively to the light emission elements  240  in the rectangular region of  FIG. 1A .  
         [0027]     As shown in  FIG. 1B , the display portion  200 , the light emission elements  240  and the photosensors  100  are sealed by transparent cover material  310 , such as glass, while interposing sealing material  311  provided in the periphery of the substrate therebetween.  
         [0028]     Each of the light emission elements  240  emits light in the upward direction perpendicularly to the plane of the display portion  250  illustrated in  FIG. 1B . Therefore, reflection material  260  such as an mirror is provided on the substrate  10  so that the light from the light emission element  240  travels above the display portion  200  and reaches the photosensor  100 . Note that a grain size of a crystal particle of the semiconductor layer constituting each TFT is schematically illustrated below the substrate  10  of the drawing. In this embodiment, each TFT is constituted by the first semiconductor layer ps 1  and the second semiconductor layer ps 2 , which have different grain size of a crystal particle, and descriptions as to the grain size of a crystal particle will be made later.  
         [0029]     Describing an example of a method of detecting input coordinates, among the light emission elements  240 , each of the light emission elements  240  arranged in one side of the display portion  200  first performs light emission sequentially in the order thereof, and subsequently each of the light emission elements  240  arranged in the other side thereof performs light emission sequentially in the order thereof. If there exists no body above the display portion  200 , the light emitted from each of the light emission elements  240  is always received by the photosensor  100 . Meanwhile, when fingers and an entering pen touch a predetermined position of the display portion  200 , the light emission of a specific light emission element  240  is blocked, and the light emitted from the specific light emission element  240  comes not to be received by a specific photosensor  100 . The region blocking the light is two-dimensionally sensed based on a timing of the light emission of the specific light emission element  240  and an output of the specific photosensor  100 , and thus input coordinates are detected.  
         [0030]      FIGS. 2A and 2B  show one pixel of the display portion of  FIGS. 1A and 1B .  FIG. 2A  is a plan view, and  FIG. 2B  is a section view taken along the line B-B of  FIG. 2B .  
         [0031]     As shown in  FIG. 2A , each of pixels P is formed in a region surrounded by gate signal line  151  and drain signal line  152 . Switching TFT  210  is provided in the vicinity of an intersection of both signal lines, and source  113   s  of the TFT  210  functions also as capacitor electrode  155 . The capacitor electrode  155  constitutes holding capacitor  170  together with holding capacitor electrode line  154  to be described later. The source  113   s  is connected to gate  141  of driving TFT  220  of organic EL element  171 . Source  143   s  of the driving TFT  220  is connected to anode  161  of the organic EL element  171 , and drain  143 d is connected to driving power supply line  153  for driving the organic EL element.  
         [0032]     Furthermore, the holding capacitor electrode line  154  is arranged in parallel with the gate signal line  151  in the vicinity of the switching TFT  210 . This holding capacitor electrode line  154  stores charges between itself and the capacitor electrode  155  through gate insulating film  12 , and constitutes the holding capacitor  170 . The holding capacitor  170  is provided so as to hold a voltage applied to the gate  141  of the driving TFT  220 . The capacitor electrode  155  is connected to the source  113   s  of the switching TFT  210 .  
         [0033]     As shown in  FIG. 2B , the switching TFT  210  provides insulating film  14 , which functions as a buffer layer, on the insulating substrate  10  made of quartz glass, non-alkali glass and the like. Semiconductor layer  113  formed of the first p-Si film ps 1  is formed on the insulating substrate  10 . In the semiconductor layer  113 , channel  113   c  which is intrinsic or substantially intrinsic is provided on a region overlapping a gate electrode  111 . The source  113   s  and the drain  113   d  are provided on both sides of the channel  113   c . Furthermore, the semiconductor layer  113  may be so called a LDD (lightly doped drain) structure. In this case, both sides of the channel  113   c  is a low concentration impurity region, and the outside of both sides thereof is a high concentration impurity region.  
         [0034]     The gate insulating film  12  is provided on the semiconductor layer  113 , and the gate signal line  151  (not shown here) functioning also as the gate electrode  111  made of a high melting point metal, and the holding capacitance electrode line  154  are also provided on the gate insulating film  12 .  
         [0035]     Interlayer insulating film  15  is laminated on the entire surfaces of the gate insulating film  12 , the gate electrode  111 , the gate signal line  151  and the holding capacitor electrode line  154 , and a metal is filled in a contact hole of the gate insulating film  12  and the interlayer insulating film  15 , which is provided so as to correspond to the drain  113   d , and thus drain electrode  116  functioning as the drain signal line  152  is provided. Note that the source  113   s  is extended so as to form the holding capacitor  170 .  
         [0036]     The second TFT  220  is provided on the insulating substrate  10  and the buffer layer  14 , where the switching TFT  210  is formed. Specifically, a semiconductor layer  143  formed of the first p-Si film ps 1  is provided. In the semiconductor layer  143 , a channel  143   c  which is intrinsic or substantially intrinsic is formed in the semiconductor layer  143 , and both sides of the channel  143   c  undergoes ion-doping, and thus the source  143   s  and the drain  143   d  are formed on both sides of the channel  143   c  in the semiconductor layer  143 .  
         [0037]     The gate insulating film  12  and the gate electrode  141  made of a high melting point are formed on the semiconductor layer  143 .  
         [0038]     Subsequently, the interlayer insulating film  15  is formed as in the case of the switching TFT  210 , and a metal is filled in a contact hole provided so as to correspond to the drain  143   d , and thus the driving power supply line  153  connected to a driving power supply is arranged. Furthermore, source electrode  158  is provided in the contact hole provided so as to correspond to the source  143   s . Planarized insulating film  17  is formed on the entire surface of the resultant structure, and a contact hole is formed at a position in the planarized insulating film  17  corresponding to the source electrode  158 . A first electrode having a reflection function by mixing Ag compound into ITO (indium tin oxide) and IZO (indium zinc oxide), which contacts the source electrode  158  through the contact hole, that is, the first electrode (anode)  161 , is provided. The organic EL element  171  is constituted by the first electrode  161 , organic EL layer  165  and second electrode  166 .  
         [0039]     The organic EL layer  165  is formed by laminating hole transportation layer  162 , light emission layer  163  and electron transportation layer  164  on the anode  161  in this order. Furthermore, the second electrode, that is, cathode  166 , which is made of magnesium indium alloy and made thin in its thickness so as to transmit light therethrough, is laminatedly formed thereon. The cathode  166  is provided on the entire surface of the substrate  10  forming an organic EL display portion shown in  FIG. 2A . Furthermore, the light emission layer  163  emits R, G and B color lights by using different materials for each pixel.  
         [0040]     In the organic EL element  171 , holes injected from the anode  161  and electrons injected from the cathode  166  are recombined within the light emission layer  163 , and organic molecules forming the light emission layer  163  are excited, thus generating excitons. Light is emitted from the light emission layer  163  in the course these excitons are radiated to be inactivated, and this light is emitted from the cathode  166  having light permeability to the outside, so that light emission is performed by the organic EL element  171 .  
         [0041]     Since the light emission element  240  may use the same constituent components as those of the above described pixel, illustrations and detailed descriptions are omitted. Specifically, the buffer layer  14 , the semiconductor layer formed of the first p-Si film ps 1 , the gate insulating film and the gate electrode are laminated on the substrate, and the channel, the source and the drain are provided in the semiconductor layer. The drain electrode connected to the drain and the source electrode connected to the source are provided, and thus the organic EL element is formed. The organic EL element has a structure that the organic EL layer is formed on the anode and the cathode is laminated thereon. The anode is connected to the source electrode.  
         [0042]     However, the light emission layer of the pixel emits any of three color lights of R, G and B, as described above, and the pixels are arranged sequentially in order. Meanwhile, the light emission element may emits light simply, and may emit one color lights. For example, since it is necessary to allow the light to pass above the display portion  200  to reach the photosensor  100 , the light emission element should emit strong light as possible, and a light emission element emitting the blue color (B) light with high energy in terms its waveform should be employed.  
         [0043]      FIG. 3  is a section view showing the photosensor  100 . The photosensor  100  is a TFT constituted by a gate electrode  11 , the insulating film  12  and a semiconductor layer  13 .  
         [0044]     The photosensor  100  provides the buffer layer  14  on the insulating substrate  10  where the display portion  200  is formed, and the semiconductor layer  13  formed of the second p-Si film ps 2  and the gate insulating film  12  are laminated thereon. On the gate insulating film  12 , the gate electrode  11  made of a high melting point metal such as chromium (Cr) and molybdenum (Mo) is provided.  
         [0045]     The semiconductor layer  13  of the photosensor  100  is formed of the second p-Si film ps 2  having the grain size of a crystal particle larger than that of the first p-Si film ps 1  (semiconductor layers  113 ,  143 ) of each TFT constituting the display portion  200  and the light emission element  240 . To be concrete, when the one obtained by averaging grain size of crystal particles of the plurality of p-Si crystals contained in a unit area is called an average grain size of crystal particles, the average grain size of crystal particles of the semiconductor layer  13  which is the second p-Si film ps 2  shall be larger than that of the semiconductor layers  113  and  143  of the display portion and the light emission element, which are the first p-Si film ps 1  (see  FIG. 1B ).  
         [0046]     In the semiconductor layer  13 , a channel  13   c  which is located below the gate electrode  11  and intrinsic or substantially intrinsic is provided, and a source  13   s  and a drain  13   d , which are an n + -type diffusion region, are provided on both sides of the channel  13   c.    
         [0047]     The interlayer insulating film  15 , in which a SiO 2  film, a SiN film and a SiO 2  film are laminated in this order, is provided on the entire surfaces of the semiconductor layer  13 , the gate insulating film  12  and the gate electrode  11 . Furthermore, a drain electrode  16  connected to the drain  13   d  is provided. A source electrode  18  connected to the source  13   s  is provided, and the planarized insulating film  17  (not shown here) is provided on the drain and source electrodes  16  and  18 .  
         [0048]     Photo current amplified by the photosensor  100  is outputted from the source electrode  18  side or the drain electrode  16  side.  
         [0049]     The plurality of photosensors  100  are provided so as to correspond to the respective light emission elements  240 . When the plurality of photosensors  100  are arranged in this manner, the photosensors  100  are connected in parallel with each other. By providing the plurality of TFTs, redundancy and averageness of light receiving as the photosensors  100  are secured.  
         [0050]     In the TFT having the above described structure, when light is incident onto the semiconductor layer  13  from the outside during turning-off of the TFT, a junction region occurs in the vicinity of the boundary between the channel  13   c  and the source  13   s  or between the channel  13   c  and the drain  13   d . In the junction region, electron-hole pairs are separated by electric field, and photoelectromotive force is generated. Thus, photocurrent is obtained. The TFT is utilized as the photosensor by detecting an increase of such photocurrent.  
         [0051]     However, when carriers (electrons or holes) move between grain boundaries, large energy is required. Furthermore, an amount of resistance components increases due to traps of the carriers at the grain boundaries. Particularly, in the touch panel shown in  FIGS. 1A and 1B , light from the light emission element  240  is reflected and the light passing through the display portion  200  is received by the photosensor  100 . Specifically, attenuation of the light caused until the light reaches the photosensor  100  is unavoidable, and the photosensor  100  senses very weak light.  
         [0052]     Accordingly, as in the case of this embodiment, by making the average grain size of crystal particles of the semiconductor layer  13  of the photosensor  100  large, the number of times of movements of the carriers between the grain boundaries becomes small, and the grain boundaries in an electric conduction direction also decrease. Since a crystal property of the semiconductor layer  13  is improved by approximating a crystallinity of the semiconductor layer  13  to that of a single crystal, a probability of occurrence of electron-hole pairs increases during light irradiation, and it is possible to sense even very small current.  
         [0053]     On the other hand, drivability of the TFT constituting the display portion  200  and the light emission element  240  may be sufficiently equal to that of a semiconductor layer polycrystallized by a known crystallization process in terms of its operation speed. That is, to improve the current drivability unnecessarily is not desirable because it is thought that variations of TFT characteristics such as drivability of the TFT becomes large. Furthermore, if the grain size of a crystal particle is made large, the TFT may be formed on the crystal boundaries. Such a TFT is apt to cause breaking and short-circuiting, leading to a problem of pixel defect.  
         [0054]     Accordingly, in this embodiment, the average grain size of crystal particles of the semiconductor layer  13  of the TFT constituting the photosensor  100  is made larger than those of the semiconductor layers  113  and  143  of the switching and driving TFTs  210  and  220  provided in the display portion  200 , as shown in FIG. LA When a TFT for active-driving the light emission element  240  is present, the average grain size of crystal particles of the semiconductor layer  13  thereof is also made larger than that of a semiconductor layer forming this TFT. Even when TFTs positioned in grain boundaries exist by making the grain size of a crystal particle large, the plurality of photosensors are coupled to each other in parallel, and hence the characteristics of light receiving of the photosensors are averaged, sense capability of them are only slightly influenced.  
         [0055]     Next, a method of fabricating a displaying device according to an embodiment of the present invention will be described by use of  FIGS. 4 through 8 . Although the TFT constituting light emission element  240  and the TFT constituting the photosensor  100  may be formed in different processes from the following processes, these TFTs are formed in the same processes as the following processes in this embodiment. Furthermore, since the light emission element  240  has the same structure as that of the driving TFT, illustrations and descriptions are omitted.  
         [0000]     First Process (See  FIG. 4 ): Process of Forming an Amorphous Semiconductor Layer on an Insulating Substrate  
         [0056]     A buffer layer  14  made of SiN, SiO 2  and the like is formed on a insulating substrate  10  made of quartz glass, non-alkali glass and the like, and an amorphous silicon film is deposited on the buffer layer  14 , followed by patterning the amorphous silicon film. Thus, amorphous semiconductor layers  80 ,  180  and  280  respectively used for constituting switching TFT  210 , driving TFT  220  and TFT of the photosensor  100  are formed.  
         [0000]     Second Process (See  FIG. 5 ): Process of Polycrystallizing Amorphous Semiconductor Layer  
         [0057]     The amorphous semiconductor layers are polycrystallized by laser-annealing, and thus semiconductor layers  113  and  143  constituting the switching TFT  210  and the driving TFT  220 , and a semiconductor layer  13  constituting the photosensor  100  are formed. In this state, the semiconductor layers have grain size of a crystal particle equal to each other.  
         [0000]     Third Process (See  FIG. 6 ): Process of Forming First and Second Semiconductor Layers Having Different Grain Size of a Crystal Particle by Crystallizing Parts of Polycrystallized Semiconductor Layer  
         [0058]     Resist film PR is formed, and only the semiconductor layer  13  of the photosensor  100  is exposed. Laser annealing is performed again, and the grain size of a crystal particle is enlarged. Thus, a first p-Si film ps 1  and a second p-Si film ps 2  having the grain size of a crystal particle different from each other are formed. An average grain size of crystal particles of the second p-Si film ps 2  (semiconductor layer  13 ) is larger than that of the first p-Si film ps 1  (the semiconductor layers  113  and  143 ).  
         [0059]     In this embodiment, although a method is used, in which resist mask PR is formed and a laser is radiated onto the entire region of the resultant structure to enlarge the grain size of a crystal particle selectively, other methods achieving the same effects as this method may be employed. For example, regions where the grain size of a crystal particle are different from each other may be formed by limiting a region onto which the laser is radiated, by changing the number of times of laser radiations, and by changing an amount of energy given to each of semiconductor layers by the laser radiation. Furthermore, a scanning speed for the laser radiation is decreased for a region where a photosensor is formed, or alternatively a laser output is changed, whereby the amount of the energy given to the semiconductor layer by the laser radiation may be changed.  
         [0000]     Fourth Process (See  FIG. 7 ): Process of Forming First Thin Film Transistor Having First Gate Electrode and First Semiconductor Layer  
         [0060]     SiN, SiO 2  or the like, which constitutes gate insulating film  12 , is formed on the buffer layer  14  and the semiconductor layer  113 , and a high melting point metal such as Cr, and Mo is evaporated thereon, thus forming a gate electrode  111 . Note that a holding capacitor electrode line  154  for forming a holding capacitor may be formed at a predetermined position on the semiconductor layer  113  which is the first p-Si film ps 1 . Subsequently, a region of the semiconductor layer  113  overlapping the gate electrode  111 , the semiconductor layer  113  constituting the switching TFT  210 , is used as channel  113   c , and one conductivity type impurities of a high concentration are diffused onto both sides of the channel  113   c , thus forming the source  113   s  and drain  113   d . Furthermore, the semiconductor layer  113  may be formed to so-called a LDD (lightly doped drain) structure. In this case, one conductivity type impurities of a low concentration are diffused onto both sides of the channel  113   c  at a low concentration, and impurities of a high concentration are diffused onto both sides of the channel  113   c.    
         [0061]     In the same manner described above, the gate insulating film  12  and a gate electrode  141  are laminatedly formed on the buffer layer  14  and the semiconductor layer  143  which is the first p-Si film ps 1 , and thus the driving TFT  220  of the display portion  200  is formed. Subsequently, a channel  143   c  is formed in a region of the semiconductor layer  143  overlapping the gate electrode  141 , the semiconductor layer  143  constituting the driving TFT  220 , and a source  143   s  and a drain  143   d  are formed on both sides of the channel  143   c.    
         [0000]     Fifth Process (See  FIG. 7 ): Process of Forming Photosensor Constituted by Second Thin Film Transistor Having Second Gate Electrode and Second Semiconductor Layer  
         [0062]     In the same manner as the fourth process, SiN, SiO 2  or the like, which is used as the gate insulating film  12 , is formed on the semiconductor layer  13  (second p-Si film), and a gate electrode  11  is formed thereon, thus forming the TFT functioning as the photosensor  100 . A region of the semiconductor layer  13 , which overlaps the gate electrode  11 , is used as a channel  13   c , and one conductivity type impurities of a high concentration are diffused onto both sides of the channel  13   c , thus forming a source  13   s  and a drain  13   d . At this time, the source side which serves as current outputting side may take a LDD structure.  
         [0000]     Sixth Process (See  FIG. 8 ): Process of Forming Pixel Including First Thin Film Transistor in Region Where First Thin Film Transistor is Formed, and Forming Display Portion Constituted by Pixels  
         [0063]     Thereafter, interlayer insulating film  15  is formed on the entire surfaces of the semiconductor layers  13 ,  113 , and  143 , the gate insulating film  12  and the gate electrodes  11 ,  111 , and  141  by sequentially laminating a SiO 2  film, a SiN film and a SiO 2  film in this order. Furthermore, a contact hole is formed so as to correspond to the drain  113   d  of the switching TFT, and a metal such as aluminum (Al) is filled in the contact hole, thus providing drain electrode  116  functioning also as a drain signal line  152 . At the same time, a metal such as aluminum (Al) is filled in a contact hole formed so as to correspond to the drain  143   d  of the driving TFT, thus forming a driving power supply line  153  connected to a driving power supply.  
         [0064]     Furthermore, a contact hole corresponding to the drain  13   d  of the photosensor  100  is formed, and thus a drain electrode  16  is formed. In the same manner described above, a metal such as aluminum (Al) is filled also in a contact hole corresponding to the source  143   s  of the driving TFT  220 , and hence source electrode  158  is formed. A metal such as aluminum (Al) is filled also in a contact hole corresponding to the source  13   s  of the photosensor  100 , thus forming a source electrode  18 . Still furthermore, planarized insulating film  17  made of organic resin, a surface of which is planarized, is formed on the entire surface of the resultant structure.  
         [0065]     In the driving TFT  220 , a contact hole corresponding to the source electrode  158  is formed in the planarized insulating film  17 , and an anode  161  obtained by giving a reflection function to ITO, IZO or the like is formed. Furthermore, in order to EL layer  165  from being divided by a step difference in an edge of the anode  161 , second planarized film  56  is formed on the entire surface of the resultant structure, thus exposing a formation region of the EL layer  165 . The EL layer  165  forms a hole transportation layer  162  composed of first and second transportation layers on the anode  161 . Furthermore, a metallic mask having an opening is placed in a light emission region, and a light emission layer emitting a certain one color among R, G and B colors, which composes a display pixel, is deposited. Thereafter, the metallic mask is moved, and light emission layers emitting colors other than the certain one color are deposited. Thus, light emission layers  163  of R, G and B colors are formed.  
         [0066]     Furthermore, electron transportation layer  164  is laminated, and cathode  166  made of magnesium-indium alloy, which has light permeability, is laminatedly formed, thus forming the display portion  200  and the light emission element  240 . At this time, an organic EL layer  171  of the light emission element  240  (not shown) is also formed. Since the light emission layer  163  of the light emission element  240  may emit any color and is not required to emit different colors, the light emission layers  163  of all light emission elements  240  are formed when a light emission layer emitting any one of the colors R, G and B is formed. When the light emission layer  163  is monochrome and colorized by use of a color filter and a color changing layer, it is possible to fabricate all EL elements of the display portion  200  and the light emission element  240  by common material and structure.  
         [0067]     In order to allow the light of the light emission element  240  to reach the photosensor  100 , the reflection material  260  such as a mirror is formed as shown in  FIG. 1 , thus obtaining the display device shown in  FIG. 1 .  
         [0068]     Next, a second embodiment will be described. In this embodiment of the present invention, a semiconductor layer having not only difference in the grain size of a crystal particle but also that anisotropy in a crystal length (average crystal length) may be used.  
         [0069]     A schematic view of the device of this embodiment is the same as that in  FIGS. 1 through 3 , and descriptions are omitted. A method of obtaining a semiconductor layer having not only difference in the grain size of a crystal particle but also that the anisotropy in the crystal length will be described.  
         [0000]     (1) CLC (CW-Laser Lateral Crystallization) Method  
         [0070]     A CLC method is the one in which a DPSS (diode-pumped solid state) laser is radiated onto amorphous silicon and a crystal is grown in a scanning direction of the laser. According to this method, a crystal length of the crystal in the scanning direction is made longer by controlling a speed at which the laser is scanned.  
         [0000]     (2) SELAX (Selectively Enlarging Laser X&#39;Tallization) Method  
         [0071]     A SELAX method is the one in which polysilicon having a small grain size of a crystal particle is formed by radiating an excimer laser onto amorphous silicon, and a solid state pulse laser is radiated onto the polysilicon, thus forming the polysilicon which is long in a scanning direction thereof.  
         [0000]     (3) SLS (Sequential Lateral Solidification) Method  
         [0072]     A SLS method is the one in which a line-shaped excimer laser is radiated onto amorphous silicon to grow a crystal which is long in a direction parallel to a scanning direction, and this crystal overlaps the crystal grown by the previous laser radiation a little, whereby the crystals are continuously formed. In the CLC and SELAX methods, while the lower-power solid state laser is used, the SLS method is a useful method because the excimer laser having power higher than that of the solid state laser is radiated in the SLS method.  
         [0073]     The above described methods can obtain the semiconductor layers having the difference equal to the grain size of the crystal particle and the anisotropy in the crystal length because the difference of the energy given to the semiconductor layers is produced even when the laser is radiated onto the entire surface in the foregoing second step and the third process (polycrystallization process). The TFTs constituting a photosensor  100  are arranged so that the direction where the crystal length is long (the direction where the number of the crystal boundaries are small) and the electric conduction direction of the TFTs constituting the photosensor  100  (the source-drain direction) are parallel, and the electric conduction directions of driving TFTs  220  and switching TFTs  210  are arranged in a direction different from the electric conduction direction of the TFTs constituting the photosensor  100 , for example, a vertical direction to the electric conduction direction of the TFTs for the photosensor  100 . Where TFTs for driving light emission element  240  are present, also an electric conduction direction of these TFTs is set to be the same as that of the switching TFTs  210 . Thus, it is possible to make the crystal length of the TFTs for the photosensor  100  different from those of other TFTs without masking (see  FIG. 6 ) locally as described in the first embodiment.  
         [0074]     The embodiment above corresponds to a structure in which all the crystals of the polycrystallized semiconductor layer are elongated in one direction, the electric conduction direction of the photosensor TFT is aligned in this elongation direction, and the electric conduction direction of the driving and switching TFTs is aligned in a direction different of the elongation direction, preferably, normal to the elongation direction. However, it is also possible that the semiconductor layer corresponding to the driving and switching TFTs of the pixel is crystallized to have a normal grain shape, i.e., not intentionally elongated in one direction, and that the semiconductor layer corresponding to the photosensor TFT is crystallized to have the elongated shape so that the sensor configuration can take advantage of this elongated crystal shape. Furthermore, when the photosensors are formed along two edges of the insulating substrate normal to each other, the corresponding semiconductor layers may be crystallized to have different elongation directions.  
         [0075]     In this embodiment, the descriptions were made by taking the touch panel as an example. However, the display device is not limited to the touch panel, as long as the display device adopts the structure in which the photosensor  100  is formed on the same substrate in which the display portion  200  is formed. For example, the present invention can be applied to a display device in which a photosensor is provided on the same plane on which a display portion is formed, and a brightness of the display portion is adjusted by detecting external light.  
         [0076]     As for the pixel TFT, the TFT for the photosensor, the TFT connected to the light emission element, if there is a gate electrode on their light receiving plane, it is difficult for them to receive light because of the existence of the gate electrode. Accordingly, the gate electrode should be provided opposite to the light receiving plane.  
         [0077]     In the foregoing embodiment, though the descriptions of the pixel TFT, the TFT for the photosensor, and the TFT connected to the light emission element were made as to so-called a top gate type TFT, the same is true for a bottom gate type TFT in which the lamination order of the gate electrode, the gate insulating film and the semiconductor layer is inverse to that of the top gate type TFT.  
         [0078]     The pixel TFT, the TFT for the photosensor, and the TFT connected to the light emission element need not to be the top gate type or the bottom gate type all together, and the combination of the top and bottom gate types may be adopted.  
         [0079]     Furthermore, in the above described embodiment, the descriptions were made for the top emission type in which the light from the organic EL layer  165  is emitted in a reverse direction to the insulating substrate  10 . The embodiment of the present invention is not limited to this, and a bottom emission type in which the light from the organic EL layer  165  is emitted via the insulating substrate  10  may be employed.