Abstract:
In an optical sensor device employing an amorphous silicon photodiode, an external amplifier IC and the like are required due to low current capacity of the sensor element in order to improve the load driving capacity. It to increase in cost and mounting space of the optical sensor device. In addition, noise may easily superimpose since the photodiode and the amplifier IC are connected to each other over a printed circuit board. According to the invention, an amorphous silicon photodiode and an amplifier configured by a thin film transistor are formed integrally over a substrate so that the load driving capacity is improved while reducing cost and mounting space. Superimposing noise can also be reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/829,709, filed Jul. 27, 2007, now allowed, which is a continuation of U.S. application Ser. No. 10/939,998, filed Sep. 14, 2004, now U.S. Pat. No. 7,253,391 B2 (Aug. 7, 2007), which claims the benefit of foreign priority applications filed in Japan on Sep. 19, 2003, as Serial No. 2003-327629 and on Sep. 30, 2003, as Serial No. 2003-342632, all of which are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical sensor device, and particularly to an optical sensor device configured by a thin film semiconductor element. Further, the invention relates to an electronic apparatus using the optical sensor device. 
         [0004]    2. Description of the Related Art 
         [0005]    In recent years, cellular phones have been widely used with the advance of telecommunication technology. In future, transmission of moving images and transmission of a larger volume of information are expected. In addition, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PDAs originated in electronic notebooks have also been produced in large quantities and widely used. With the development of display devices, the majority of portable information devices is equipped with a flat panel display. 
         [0006]    In such a display device, brightness of the periphery thereof is detected so as to control the display luminance. The suitable display luminance obtained by detecting the peripheral brightness enables power reduction. An optical sensor device for controlling luminance is applied to cellular phones, personal computers, and the like. (e.g., Patent Document 1). 
         [0007]    Meanwhile, an optical sensor device is used for the convergence control in a display device having a projector. In the convergence control, an image is controlled so as not to cause the deviation of respective images of each color of RGB. Each image position of RGB is detected to dispose the image at an appropriate position by using an optical sensor. (e.g., Patent Document 2). 
         [0008]    These optical sensor devices adopt an amorphous silicon photodiode. Compared to a single crystal silicon photodiode, an amorphous silicon photodiode is less sensitive to the light on the long wavelength side, namely in the infrared region. Respective sensitive characteristics of the amorphous silicon photodiode and the single crystal silicon photodiode are shown in  FIG. 13 . The amorphous silicon photodiode is less sensitive to the light in the region except the visible light region, which is similar to the human visual sensitivity. On the other hand, the single crystal silicon photodiode is much sensitive even to the light in the infrared region. Therefore, the single crystal silicon photodiode reacts differently from the visual sensitivity in the case where there is some infrared light. This is why the optical sensor device using the amorphous silicon photodiode is suitable for this case. 
         [0009]    The above-described optical sensor device configured by the amorphous silicon photodiode has the following problem. As for an amorphous silicon photodiode, output current thereof is smaller than that of a single crystal silicon photodiode while the light sensitivity is close to the human visual sensitivity as mentioned above. Thus, it is difficult for the amorphous silicon photodiode to drive another circuit directly. The optical sensor device is, consequently, configured by the combination of an amorphous silicon photodiode  502 , an external amplifier circuit  501 , and a feedback resistor  503  so that output current of the amorphous silicon photodiode  502  is converted into voltage by the feedback resistor  503 , which is then outputted from an output terminal  504  as a voltage output.
   [Patent Document 1] Japanese Patent Laid-Open No. 2003-60744   [Patent Document 2] Japanese Patent Laid-Open No. 2003-47017   
 
       SUMMARY OF THE INVENTION 
       [0012]    The above-described conventional optical sensor device is configured by the combination of the amorphous silicon photodiode  502 , the external amplifier circuit  501 , and the feedback resistor  503 , causing problems such as high cost and large mounting space. In addition, noise may easily superimpose since an output of the amorphous silicon photodiode  502  is connected to the external amplifier circuit  501  over a printed circuit board. 
         [0013]    In view of solving the above problem, according to the invention, an optical sensor element and an amplifier circuit using a thin film transistor are formed integrally. An optical sensor element using amorphous silicon (e.g., an amorphous silicon photodiode) is generally formed over an insulating substrate. Similarly, a thin film transistor is generally formed over an insulating substrate as well as the optical sensor element, therefore, they are common in many points. In an optical sensor device according to the invention, an optical sensor element and an amplifier circuit using a thin film transistor (abbreviated to a TFT hereinafter) are formed integrally over a substrate so that reductions in cost and mounting space can be realized. In addition, noise is less likely to superimpose in the optical sensor device according to the invention since the optical sensor element and the amplifier circuit are directly connected to each other over a sensor substrate. 
         [0014]    The invention is applicable to an optical sensor element using polysilicon (e.g., a polysilicon photodiode) as well as an optical sensor element using amorphous silicon. 
         [0015]    Described below is a structure of the invention. 
         [0016]    According to the invention, in an optical sensor device comprising an optical sensor element and an amplifier circuit, the optical sensor element and the amplifier circuit are formed integrally over a substrate. 
         [0017]    According to the invention, in the above configuration, an optical sensor element comprises amorphous silicon. 
         [0018]    According to the invention, in the above configuration, an optical sensor element is configured by an amorphous silicon photodiode. 
         [0019]    According to the invention, in the above configuration, an optical sensor element comprises polysilicon. 
         [0020]    According to the invention, in the above configuration, an optical sensor element is configured by a polysilicon photodiode. 
         [0021]    According to the invention, in an optical sensor device comprising an optical sensor element and an amplifier circuit, the amplifier circuit is configured by a thin film transistor, and the optical sensor element and the amplifier circuit are formed integrally over a substrate. 
         [0022]    According to the invention, in an optical sensor device comprising an optical sensor element and an amplifier circuit, the amplifier circuit is an operational amplifier configured by a thin film transistor, and the optical sensor element and the amplifier circuit are formed integrally over a substrate. 
         [0023]    According to the invention, in an optical sensor device comprising an optical sensor element, an amplifier circuit, and a feedback resistor, the amplifier circuit is an operational amplifier configured by a thin film transistor, the optical sensor element and the amplifier circuit are formed integrally over a substrate, and the feedback resistor is provided outside of the substrate. 
         [0024]    According to the invention, in an optical sensor device comprising an optical sensor element, an amplifier circuit, and a current-voltage conversion resistor (abbreviation: I-V conversion resistor), the amplifier circuit is an operational amplifier configured by a thin film transistor, the optical sensor element and the amplifier circuit are formed integrally over a substrate, and the I-V conversion resistor is provided outside of the substrate. 
         [0025]    According to the invention, in an optical sensor device comprising an optical sensor element, an amplifier circuit, and a level shift circuit, the amplifier circuit and the level shift circuit are configured by a thin film transistor, and the optical sensor element, the amplifier circuit, and the level shift circuit are fouled integrally over a substrate. 
         [0026]    According to the invention, in the above configuration, the level shift circuit is configured by a P-channel TFT and a constant current source. 
         [0027]    According to the invention, in the above configuration, the level shift circuit is configured by an N-channel TFT and a constant current source. 
         [0028]    According to the invention, in the above configuration, the substrate comprises four electrode terminals for connection thereon. 
         [0029]    According to the invention, in the above configuration, the substrate comprises four electrode terminals for connection thereon, and two of them are power source terminals. 
         [0030]    According to the invention, in an optical sensor device comprising an optical sensor element and an amplifier circuit, the amplifier circuit is a current mirror circuit configured by a thin film transistor, and the optical sensor element and the amplifier circuit are formed integrally over a substrate. 
         [0031]    According to the invention, in the above configuration, the current mirror circuit is configured with the multigate structure in which a TFT has a plurality of gates. 
         [0032]    According to the invention, in the above configuration, the current mirror circuit is configured with the cascode connection. 
         [0033]    According to the invention, in the above configuration, the current mirror circuit is configured with the Wilson connection. 
         [0034]    According to the invention, in the above configuration, the current mirror circuit is configured with the improved Wilson connection. 
         [0035]    According to the invention, in the above configuration, the amplification ratio can be controlled by arbitrarily changing the number, the gate length L, and the gate width W of TFTs of a current mirror circuit. 
         [0036]    According to the invention, in the above configuration, the current mirror circuit is configured by an N-channel TFT. 
         [0037]    According to the invention, in the above configuration, the current mirror circuit is configured by a P-channel TFT. 
         [0038]    According to the invention, in the above configuration, the substrate comprises two electrical electrode terminals for connection thereon. 
         [0039]    According to the invention, in the above configuration, the optical sensor element and the amplifier circuit are formed integrally over a plastic substrate. 
         [0040]    According to the invention, in the above configuration; the optical sensor element and the amplifier circuit are formed integrally over a glass substrate. 
         [0041]    The invention provides an electronic apparatus having the above-described optical sensor device. 
         [0042]    As described hereinbefore, in an optical sensor device according to the invention, a photodiode and an amplifier circuit using a TFT are formed integrally over a sensor substrate so that reductions in cost and mounting space can be realized and superimposing noise can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0044]      FIG. 2  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0045]      FIG. 3  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0046]      FIG. 4  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0047]      FIG. 5  is a diagram showing an embodiment mode of a conventional optical sensor device. 
           [0048]      FIGS. 6A to 6C  are views each showing an outward of an optical sensor device of the invention. 
           [0049]      FIG. 7  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0050]      FIG. 8  is a diagram showing an embodiment mode of an optical sensor device of the invention. 
           [0051]      FIG. 9  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0052]      FIG. 10  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0053]      FIG. 11  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0054]      FIG. 12  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0055]      FIG. 13  is a graph showing sensitive characteristics of photodiodes. 
           [0056]      FIG. 14  is a diagram showing an equivalent circuit of an amplifier circuit adopted in the invention. 
           [0057]      FIGS. 15A and 15B  are views of a cellular phone to which an optical sensor device of the invention is applied. 
           [0058]      FIG. 16  is a view of a personal computer to which an optical sensor device of the invention is applied. 
           [0059]      FIG. 17  is a diagram showing an example of a structure of an optical sensor device of the invention. 
           [0060]      FIGS. 18A and 18B  are views each showing an example of a cross-sectional structure of an optical sensor device of the invention. 
           [0061]      FIGS. 19A and 19B  are diagrams showing an embodiment mode of an optical sensor device of the invention. 
           [0062]      FIGS. 20A and 20B  are diagrams showing an embodiment mode of an optical sensor device of the invention. 
           [0063]      FIGS. 21A and 21B  are diagrams showing an embodiment mode of an optical sensor device of the invention. 
           [0064]      FIGS. 22A and 22B  are diagrams showing an embodiment mode of an optical sensor device of the invention. 
           [0065]      FIGS. 23A and 23B  are diagrams showing an embodiment mode of an optical sensor device of the invention. 
           [0066]      FIGS. 24A and 24B  are diagrams showing an embodiment of an optical sensor device of the invention. 
           [0067]      FIG. 25  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0068]      FIG. 26  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0069]      FIG. 27  is a diagram showing an embodiment of an optical sensor device of the invention. 
           [0070]      FIG. 28  is a diagram showing an embodiment of an optical sensor device of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0071]    Although the invention is fully described by way of Embodiment Modes and Embodiments with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the invention hereinafter defined, they should be constructed as being included therein. 
         [0072]    In Embodiment Modes of the invention, the case where reverse voltage (reverse bias) is applied to a photodiode to drive is described. That is, voltage is applied in the direction from a second electrode toward a first electrode. The first electrode means an electrode that contacts with a p-type layer of an amorphous silicon layer of the photodiode, while the second electrode means an electrode that contacts with an n-type layer thereof. 
       Embodiment Mode 1 
       [0073]      FIG. 1  shows the first embodiment mode of the invention. In this embodiment mode, an amplifier circuit  101  configured by a TFT and a photodiode  102  are formed integrally over a sensor substrate. An operation thereof is described below. A non-inverting input terminal of the amplifier circuit  101  is connected to an external power source VBB. The external power source VBB has a potential between a high-potential-side power source VDD and a low-potential-side power source VSS of the amplifier circuit  101 . A first electrode of the photodiode  102  is connected to the external power source VBB and a second electrode thereof is connected to an inverting input terminal of the amplifier circuit  101  and a first terminal of a feedback resistor  103 . A second terminal of the feedback resistor  103  is connected to an output terminal  104  of the amplifier circuit  101 . Note that the feedback resistor  103  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage of the amplifier circuit  101 , however, the feedback resistor  103  may be formed over the sensor substrate in the case where the fluctuation is within an acceptable range. 
         [0074]    When light is inputted to the photodiode  102 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  102 . Consequently, current flows through the output terminal  104  of the amplifier circuit  101  toward the feedback resistor  103 , generating voltage between both the terminals of the feedback resistor  103 . 
         [0075]    Note that high driving capacity may be provided with the amplifier circuit  101  in this embodiment mode, therefore, an optical sensor circuit may drive a load connected to the output terminal  104 . 
         [0076]    In  FIG. 7 , a photodiode is connected in inverse to the photodiode  102  in  FIG. 1 . An optical sensor circuit shown in  FIG. 7  comprises an amplifier circuit  701 , a photodiode  702 , and a feedback resistor  703 . A first electrode of the photodiode  702  is connected to an inverting input terminal of the amplifier circuit  701  and the feedback resistor  703  and a second electrode thereof is connected to an external power source VBB and a non-inverting input terminal of the amplifier circuit  701 . Current flows from the external power source VBB toward an output teinrinaI  704  of the amplifier circuit  701  through the photodiode  702  and the feedback resistor  703 . As described above, the direction of a photodiode is not exclusively limited. 
         [0077]    Note that the amplifier circuits  101  and  701  are operational amplifiers in this embodiment mode, however, the amplifier circuits  101  and  701  are not limited to the operational amplifiers. 
         [0078]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 2 
       [0079]      FIG. 2  shows the second embodiment mode of the invention. In this embodiment mode, an amplifier circuit  201  configured by a TFT and a photodiode  202  are formed integrally over a sensor substrate. An operation thereof is described below. A non-inverting input terminal of the amplifier circuit  201  is connected to a first terminal of an I-V conversion resistor  203  and a second electrode of the photodiode  202 . An output terminal  204  of the amplifier circuit  201  is connected to an inverting terminal thereof, that is, the amplifier circuit  201  is a voltage follower. A first electrode of the photodiode  202  is connected to an external power source VBB 2 . A second terminal of the I-V conversion resistor  203  is connected to an external power source VBB 1 . Note that the I-V conversion resistor  203  is fainted outside of the sensor substrate in order to reduce the fluctuation of an output voltage, however, the I-V conversion resistor  203  may be fotuied over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0080]    When light is inputted to the photodiode  202 , optical current flows from the external power source VBB 1  in the direction from the second electrode toward the first electrode of the photodiode  202  through the I-V conversion resister  203 . Consequently, voltage is generated between both the terminals of the I-V conversion resistor  203 . 
         [0081]    That is, a potential equal to that of the VBB 1  is outputted to the output terminal  204  in the case where no light is inputted and no current flows into the photodiode  202  whereas in the case where light is inputted and current flows into the photodiode  202 , potential of the output terminal  204  drops in proportion to the amount of current. 
         [0082]    Note that high driving capacity may be provided with the amplifier circuit  201  in this embodiment mode, therefore, an optical sensor circuit may drive a load connected to the output terminal  204 . 
         [0083]    In  FIG. 8 , a photodiode is connected in inverse to the photodiode  202  in  FIG. 2 . An optical sensor circuit shown in  FIG. 8  comprises an amplifier circuit  801 , a photodiode  802 , and an I-V conversion resistor  803 . A first electrode of the photodiode  802  is connected to a non-inverting input terminal of the amplifier circuit  801  and the I-V conversion resistor  803  and a second electrode thereof is connected to an external power source VBB 2 . Current flows from the external power source VBB 2  toward an external power source VBB 1  through the photodiode  802  and the I-V conversion resistor  803 . As described above, the direction of a photodiode is not exclusively limited. 
         [0084]    Note that the amplifier circuits  201  and  801  are operational amplifiers in this embodiment mode, however, the amplifier circuits  201  and  801  are not limited to the operational amplifiers. 
         [0085]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 3 
       [0086]      FIG. 3  shows the third embodiment mode of the invention. In this embodiment mode, an amplifier circuit  301  configured by a TFT, a photodiode  302 , and level shift circuits  305  and  306  are formed integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit  305  is connected to a low-potential-side power source VSS and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  301 . An input of the level shift circuit  306  is connected to a second electrode of the photodiode  302  and a first terminal of a feedback resistor  303  and an output thereof is connected to an inverting input terminal of the amplifier circuit  301 . A second terminal of the feedback resistor  303  is connected to an output terminal  304  of the amplifier circuit  301 . A first electrode of the photodiode  302  is connected to the low-potential-side power source VSS. Note that the feedback resistor  303  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage of the amplifier circuit  301 , however, the feedback resistor  303  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0087]    When light is inputted to the photodiode  302 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  302 . Consequently, current flows through the output terminal  304  of the amplifier circuit  301  toward the feedback resistor  303 , generating voltage between both the terminals of the feedback resister  303 . 
         [0088]    Note that high driving capacity may be provided with the amplifier circuit  301  in this embodiment mode, therefore, an optical sensor circuit may drive a load connected to the output terminal  304 . 
         [0089]    Such a level shift circuit brings the following advantages. 
         [0090]      FIGS. 6A to 6C  are outward views of an integrated sensor chip  600 . As shown in  FIG. 6A , a photodiode portion  601  and an amplifier circuit portion  602  are formed on a top surface of the integrated sensor chip  600 . As shown in a side view of  FIG. 6B  and a bottom view of  FIG. 6C , a TFT forming region  604  and an electrode terminal for connection  605  are formed in this order on a bottom surface of a substrate  603  of the integrated sensor chip  600 . In view of strength, it is preferable that an electrode terminal for connection is formed at four points over such a sensor chip, then the sensor chip is mounted on a printed circuit board and the like. However, the aforementioned first embodiment mode does not conform to the above since the number of terminals is five. 
         [0091]    Furthermore, the power source VBB is required in addition to the high-potential-side power source VDD and the low-potential-side power source VSS in the first embodiment mode. The number of power sources can be reduced by means of a level shifter as shown in  FIG. 3 . 
         [0092]    Note that the amplifier circuit  301  is an operational amplifier in this embodiment mode, however, the amplifier circuit  301  is not limited to the operational amplifier. 
         [0093]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 4 
       [0094]      FIG. 4  shows the fourth embodiment mode of the invention. In this embodiment mode, an amplifier circuit  401  configured by a TFT, a photodiode  402 , and level shift circuits  405  and  406  are formed integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit  405  is connected to a first terminal of an I-V conversion resistor  403  and a second electrode of the photodiode  402  and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  401 . An input of the level shift circuit  406  is connected to an output terminal  404  and an output thereof is connected to an inverting input terminal of the amplifier circuit  401 , so that the amplifier circuit  401  is a voltage follower. A first electrode of the photodiode  402  is connected to a low-potential-side power source VSS of the amplifier circuit  401 . A second terminal of the I-V conversion resistor  403  is connected to an external power source VBB. Note that the I-V conversion resistor  403  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage, however, the I-V conversion resistor  403  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0095]    When light is inputted to the photodiode  402 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  402 . Consequently, current flows in the I-V conversion resister  403  and voltage is generated between both the terminals of the I-V conversion resistor  403 . 
         [0096]    That is, a potential equal to that of the VBB is outputted to the output terminal  404  in the case where no light is inputted and no current flows into the photodiode  402  whereas in the case where light is inputted and current flows into the photodiode  402 , potential of the output terminal  404  drops in proportion to the amount of current. 
         [0097]    Note that high driving capacity may be provided with the amplifier circuit  401  in this embodiment mode, therefore, an optical sensor circuit may drive a load connected to the output terminal  404 . 
         [0098]    Such a level shift circuit brings the following advantages. 
         [0099]      FIGS. 6A to 6C  are outward views of an integrated sensor chip  600 . As shown in  FIG. 6A , a photodiode portion  601  and an amplifier circuit portion  602  are formed on a top surface of the integrated sensor chip  600 . As shown in a side view of  FIG. 6B  and a bottom view of  FIG. 6C , an electrode for connection  605  is formed on a bottom surface of the integrated sensor chip  600 . In view of strength, it is preferable that an electrode terminal for connection is formed at four points over such a sensor chip, then the sensor chip is mounted on a printed circuit board and the like. However, the aforementioned second embodiment mode does not conform to the above since the number of terminals is five. 
         [0100]    Furthermore, the power source VBB 2  is required in addition to the high-potential-side power source VDD, the low-potential-side power source VSS, and the external power source VBB 1  in the second embodiment mode. The number of power sources can be reduced by means of a level shifter as shown in  FIG. 4 . 
         [0101]    Note that the amplifier circuit  401  is an operational amplifier in this embodiment mode, however, the amplifier circuit  401  is not limited to the operational amplifier. 
         [0102]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 5 
       [0103]      FIGS. 19A and 19B  show the fifth embodiment mode of the invention. In this embodiment mode, an amplifier circuit  1901  configured by TFTs and a photodiode  1902  are formed integrally over a sensor substrate. An operation thereof is described below. Source regions of TFTs in the amplifier circuit  1901  are connected to an external power source GND. Drain regions of the Y-side TFT and the X-side TFT in the amplifier circuit  1901  are connected to an output terminal  1904  and a first electrode of the photodiode  1902 , respectively. A second electrode of the photodiode  1902  is connected to the output terminal  1904 . 
         [0104]    When light is inputted to the photodiode  1902 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  1902 . Consequently, current flows into the X-side TFT in the amplifier circuit  1901 , generating voltage for flowing the current at each gate thereof. 
         [0105]    As for the X-side TFT and the Y-side TFT, when the number of TFTs connected in parallel, the gate length L, and the channel width W are equal to each other, the same amount of current flows into the X-side TFT and the Y-side TFT since respective gate voltages are equal to each other in a saturation region. For example, the number of the TFTs is determined so as to satisfy X-side:Y-side=1: n (assuming that other respective characteristics of the X-side TFT and the Y-side TFT are identical to each other), so that the amplification becomes n times as large. The desired amplification can be obtained in this manner. 
         [0106]      FIG. 19A  shows a circuit mounted with N-channel TFTs while  FIG. 19B  shows a circuit mounted with P-channel TFTs. An optical sensor circuit shown in  FIG. 19B  comprises the amplifier circuit  1901  and the photodiode  1902 . A drain region of the Y-side TFT in the amplifier circuit  1901  is connected to the external power source GND and a source region thereof is connected to the output terminal  1904 . A drain region of the X-side TFT in the amplifier circuit  1901  is connected to the second electrode of the photodiode  1902 . The first electrode of the photodiode  1902  is connected to the GND. In this embodiment mode, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 6 
       [0107]      FIGS. 20A and 20B  show the sixth embodiment mode of the invention. In this embodiment mode, an amplifier circuit  2001  configured by TFTs and a photodiode  2002  are formed integrally over a sensor substrate. An operation thereof is described below. As for TFTs in each of the X-side and the Y-side in the amplifier circuit  2001 , they are disposed in series by connecting a source region and a drain region to each other and a multigate structure is formed by commonly connecting their gate electrodes. In addition, source regions of the TFTs at the low-voltage-side stage are connected to an external power source GND. Drain regions of the Y-side TFT and the X-side TFT at the high-voltage-side stage are connected to an output terminal  2004  and a first electrode of the photodiode  2002 , respectively. A second electrode of the photodiode  2002  is connected to the output terminal  2004 . 
         [0108]    When light is inputted to the photodiode  2002 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  2002 . Consequently, current flows into the X-side TFTs in the amplifier circuit  2001 , generating voltage for flowing the current at each gate thereof. 
         [0109]    As for the X-side TFTs and the Y-side TFTs, when the number of TFTs connected in parallel, the gate length L, and the channel width W are equal to each other, the same amount of current flows into the X-side TFTs and the Y-side TFTs since respective gate voltages are equal to each other in a saturation region. For example, the number of the TFTs connected in parallel is determined so as to satisfy X-side:Y-side=1: n (assuming that other respective characteristics of the X-side TFT and the Y-side TFT are identical to each other), so that the amplification becomes n times as large. The desired amplification can be obtained in this manner. 
         [0110]    Note that two stages of TFTs are connected in series to each other in the amplifier circuit  2001  in this embodiment mode, however, the number of stages is not limited to two. In addition, the characteristics of TFTs are not necessarily identical per stage, however, the relationship between the X-side and the Y-side has to be identical per stage. 
         [0111]      FIG. 20A  shows a circuit mounted with N-channel TFTs while  FIG. 20B  shows a circuit mounted with P-channel TFTs. In this embodiment mode, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 7 
       [0112]      FIGS. 21A and 21B  show the seventh embodiment mode of the invention. In this embodiment mode, an amplifier circuit  2101  configured by TFTs and a photodiode  2102  are formed integrally over a sensor substrate. An operation thereof is described below. As for TFTs in each of the X-side and the Y-side in the amplifier circuit  2101 , they are disposed in series by connecting a source region and a drain region to each other. Gates of the X-side TFTs are diode-connected, which are then connected in series to each other. Gates of the Y-side TFTs are connected to the gates of the opposite X-side TFTs, respectively. In addition, source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2101  are connected to an external power source GND. Drain regions of the Y-side TFT and the X-side TFT at the high-voltage-side stage are connected to an output terminal  2104  and a first electrode of the photodiode  2102 , respectively. A second electrode of the photodiode  2102  is connected to the output terminal  2104 . 
         [0113]    When light is inputted to the photodiode  2102 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  2102 . Consequently, current flows into the X-side TFTs in the amplifier circuit  2101 , generating voltage for flowing the current at each gate thereof. 
         [0114]    As for the X-side TFTs and the Y-side TFTs, when the number of TFTs connected in parallel, the gate length L, and the channel width W are equal to each other, the same amount of current flows into the X-side TFTs and the Y-side TFTs since respective gate voltages are equal to each other per stage in a saturation region. For example, the number of the TFTs connected in parallel is determined so as to satisfy X-side Y-side=1: n (assuming that other respective characteristics of the X-side and the Y-side TFT are identical to each other), so that the amplification becomes n times as large. The desired amplification can be obtained in this manner. 
         [0115]    Note that two stages of TFTs are connected in series to each other in the amplifier circuit  2101  in this embodiment mode, however, the number of stages is not limited to two. In addition, the characteristics of TFTs are not necessarily identical per stage, however, the relationship between the X-side and the Y-side has to be identical per stage. 
         [0116]      FIG. 21A  shows a circuit mounted with N-channel TFTs while  FIG. 21B  shows a circuit mounted with P-channel TFTs. In this embodiment mode, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 8 
       [0117]      FIGS. 22A and 22B  show the eighth embodiment mode of the invention. In this embodiment mode, an amplifier circuit  2201  configured by TFTs and a photodiode  2202  are formed integrally over a sensor substrate. An operation thereof is described below. The amplifier circuit  2201  is a Wilson current mirror circuit and source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2201  are connected to an external power source GND. Drain regions of the Y-side TFT and the X-side TFT are connected to an output terminal  2204  and a first electrode of the photodiode  2202 , respectively. A second electrode of the photodiode  2202  is connected to the output terminal  2204 . 
         [0118]    When light is inputted to the photodiode  2202 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  2202 . Consequently, current flows into the X-side TFT in the amplifier circuit  2201 , generating voltage for flowing the current at each gate thereof. 
         [0119]    As for the X-side TFT and the Y-side TFTs in the Wilson current mirror circuit, when the number of TFTs connected in parallel, the gate length L, and the channel width W are equal to each other, the same amount of current flows into the X-side TFT and the Y-side TFTs since respective gate voltages are equal to each other per stage in a saturation region. For example, the number of the TFTs connected in parallel is determined so as to satisfy X-side:Y-side=1: n (assuming that other respective characteristics of the X-side TFT and the Y-side TFT are identical to each other), so that the amplification becomes n times as large. The desired amplification can be obtained in this manner. 
         [0120]    Note that two stages of TFTs are connected in series to each other in the amplifier circuit  2201  in this embodiment mode, however, the number of stages is not limited to two. In addition, the characteristics of TFTs are not necessarily identical per stage, however, the relationship between the X-side and the Y-side has to be identical per stage. 
         [0121]      FIG. 22A  shows a circuit mounted with N-channel TFTs while  FIG. 22B  shows a circuit mounted with P-channel TFTs. In this embodiment mode, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment Mode 9 
       [0122]      FIGS. 23A and 23B  show the ninth embodiment mode of the invention. In this embodiment mode, an amplifier circuit  2301  configured by TFTs and a photodiode  2302  are formed integrally over a sensor substrate. An operation thereof is described below. The amplifier circuit  2301  is an improved Wilson current mirror circuit. In the improved Wilson current mirror circuit, the same number of TFTs are provided at the X-side and the Y-side, thereby each source-drain voltage of corresponding TFTs is equal to each other, so that the same amount of current flows into the X-side and the Y-side even when each TFT is a finite output resistor. The improved Wilson current mirror circuit is different from the Wilson current mirror circuit shown in  FIGS. 22A and 22B  in this point. Source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2301  are connected to an external power source GND. Drain regions of the Y-side TFT and the X-side TFT are connected to an output terminal  2304  and a first electrode of the photodiode  2302 , respectively. A second electrode of the photodiode  2302  is connected to the output terminal  2304 . 
         [0123]    When light is inputted to the photodiode  2302 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  2302 . Consequently, current flows into the X-side TFTs in the amplifier circuit  2301 , generating voltage for flowing the current at each gate thereof. 
         [0124]    As for the X-side TFTs and the Y-side TFTs, when the number of TFTs connected in parallel, the gate length L, and the channel width W are equal to each other, the same amount of current flows into the X-side TFTs and the Y-side TFTs since respective gate voltages are equal to each other per stage in a saturation region. For example, the number of the TFTs connected in parallel is determined so as to satisfy X-side:Y-side=1: n (assuming that other respective characteristics of the X-side and the Y-side-TFT are identical to each other), so that the amplification becomes n times as large. The desired amplification can be obtained in this manner. 
         [0125]    Note that two stages of TFTs are connected in series to each other in the amplifier circuit  2301  in this embodiment mode, however, the number of stages is not limited to two. In addition, the characteristics of TFTs are not necessarily identical per stage, however, the relationship between the X-side and the Y-side has to be identical per stage. 
         [0126]      FIG. 23A  shows a circuit mounted with N-channel TFTs while  FIG. 23B  shows a circuit mounted with P-channel TFTs. In this embodiment mode, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 1 
       [0127]      FIG. 9  shows the first embodiment of the invention. This embodiment describes Embodiment Mode 3 employing a level shift circuit in detail. In this embodiment, an amplifier circuit  901  configured by a TFT, a photodiode  902 , and two level shift circuits including P-channel TFTs  905  and  906  and constant current sources  907  and  908  are fainted integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit including the P-channel TFT  905  and the constant current source  907  is connected to a low-potential-side power source VSS and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  901 . An input of the level shift circuit including the P-channel TFT  906  and the constant current source  908  is connected to a second electrode of the photodiode  902  and the first terminal of a feedback resistor  903  and an output thereof is connected to an inverting input terminal of the amplifier circuit  901 . A first electrode of the photodiode  902  is connected to the low-potential-side power source VSS. A second terminal of the feedback resistor  903  is connected to an output terminal  904  of the amplifier circuit  901 . Note that the feedback resistor  903  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage of the amplifier circuit  901 , however, the feedback resistor  903  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0128]    When light is inputted to the photodiode  902 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  902 . Consequently, current flows through the output terminal  904  of the amplifier circuit  901  toward the feedback resistor  903 , generating voltage between both the terminals of the feedback resistor  903 . 
         [0129]    High driving capacity may be provided with the amplifier circuit  901  in this embodiment, therefore, an optical sensor circuit may drive a load connected to the output terminal  904 . 
         [0130]    In this embodiment, an electrode terminal for connection of the optical sensor device can be formed at four points, namely a high-potential-side power source VDD, the low-potential-side power source VSS, the output terminal  904  of the amplifier circuit  901 , and a connecting terminal between the feedback resistor  903  and the photodiode  902 , so that the aforementioned mounting strength can be improved. In addition, the number of power sources can be reduced to two by means of a level shifter. 
         [0131]    Note that the amplifier circuit  901  is an operational amplifier in this embodiment, however, the amplifier circuit  901  is not limited to the operational amplifier. 
         [0132]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 2 
       [0133]      FIG. 10  shows the second embodiment of the invention. This embodiment describes Embodiment Mode 3 employing a level shift circuit in detail. In this embodiment, an amplifier circuit  1001  configured by a TFT, a photodiode  1002 , and two level shift circuits including N-channel TFTs  1005  and  1006  and constant current sources  1007  and  1008  are formed integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit including the N-channel TFT  1005  and the constant current source  1007  is connected to a high-potential-side power source VDD and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  1001 . An input of the level shift circuit including the N-channel TFT  1006  and the constant current source  1008  is connected to a first electrode of the photodiode  1002  and a first terminal of a feedback resistor  1003  and an output thereof is connected to an inverting input terminal of the amplifier circuit  1001 . A second electrode of the photodiode  1002  is connected to the high-potential-side power source VDD. The second terminal of the feedback resistor  1003  is connected to an output terminal  1004  of the amplifier circuit  1001 . Note that the feedback resistor  1003  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage of the amplifier circuit  1001 , however, the feedback resistor  1003  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0134]    When light is inputted to the photodiode  1002 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  1002 . Consequently, current flows through the feedback resistor  1003  toward the output terminal  1004  of the amplifier circuit  1001 , generating voltage between both the terminals of the feedback resistor  1003 . 
         [0135]    High driving capacity may be provided with the amplifier circuit  1001  in this embodiment, therefore, an optical sensor circuit may drive a load connected to the output terminal  1004 . 
         [0136]    In this embodiment, an electrode terminal for connection of the optical sensor device can be formed at four points, namely the high-potential-side power source VDD, a low-potential-side power source VSS, the output terminal  1004  of the amplifier circuit  1001 , and a connecting terminal between the feedback resistor  1003  and the photodiode  1002 , so that the aforementioned mounting strength can be improved. In addition, the number of power sources can be reduced to two by means of a level shifter. 
         [0137]    Note that the amplifier circuit  1001  is an operational amplifier in this embodiment, however, the amplifier circuit  1001  is not limited to the operational amplifier. 
         [0138]    In addition, other optical sensor element may be employed as a substitute for the photodiode  1002 . 
       Embodiment 3 
       [0139]      FIG. 11  shows the third embodiment of the invention. This embodiment describes Embodiment Mode 4 in detail. In this embodiment, an amplifier circuit  1101  configured by a TFT, a photodiode  1102 , and two level shift circuits including TFTs  1105  and  1106  and constant current sources  1107  and  1108  are formed integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit including the P-channel TFT  1105  and the constant current source  1107  is connected to a first electrode of the photodiode  1102  and a first terminal of an I-V conversion resistor  1103  and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  1101 . An input of the level shift circuit including the P-channel TFT  1106  and the constant current source  1108  is connected to an output terminal  1104  of the amplifier circuit  1101  and an output thereof is connected to an inverting input terminal of the amplifier circuit  1101 , so that the amplifier circuit  1101  is a voltage follower. A second electrode of the photodiode  1102  is connected to a high-potential-side power source VDD. A second terminal of the I-V conversion resistor  1103  is connected to an external power source VBB. Note that the I-V conversion resistor  1103  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage, however, the I-V conversion resistor  1103  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0140]    When light is inputted to the photodiode  1102 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  1102 . Consequently, current flows into the I-V conversion resistor  1103 , generating voltage between both the terminals of the I-V conversion resistor  1103 . 
         [0141]    That is, a potential equal to that of the VBB is outputted to the output terminal  1104  in the case where no light is inputted and no current flows into the photodiode  1102  whereas in the case where light is inputted and current flows into the photodiode  1102 , the potential of the output terminal  1104  rises in proportion to the amount of current. 
         [0142]    High driving capacity may be provided with the amplifier circuit  1101  in this embodiment, therefore, an optical sensor circuit may drive a load connected to the output terminal  1104 . 
         [0143]    In this embodiment, an electrode terminal for connection of the optical sensor device can be formed at four points, namely the high-potential-side power source VDD, a low-potential-side power source VSS, the output terminal  1104  of the amplifier circuit  1101 , and a connecting terminal between the I-V conversion resistor  1103  and the photodiode  1102 , so that the aforementioned mounting strength can be improved. In addition, the number of power sources can be reduced to two by means of a level shifter. 
         [0144]    Note that the amplifier circuit  1101  is an operational amplifier in this embodiment, however, the amplifier circuit  1101  is not limited to the operational amplifier. 
         [0145]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 4 
       [0146]      FIG. 12  shows the fourth embodiment of the invention. This embodiment describes Embodiment Mode 4 in detail. In this embodiment, an amplifier circuit  1201  configured by a TFT, a photodiode  1202 , and two level shift circuits including TFTs  1205  and  1206  and constant current sources  1207  and  1208  are formed integrally over a sensor substrate. An operation thereof is described below. An input of the level shift circuit including the N-channel TFT  1205  and the constant current source  1207  is connected to a second electrode of the photodiode  1202  and a first terminal of an I-V conversion resistor  1203  and an output thereof is connected to a non-inverting input terminal of the amplifier circuit  1201 . An input of the level shift circuit including the N-channel TFT  1206  and the constant current source  1208  is connected to an output terminal  1204  of the amplifier circuit  1201  and an output thereof is connected to an inverting input terminal of the amplifier circuit  1201 , so that the amplifier circuit  1201  is a voltage follower. A first electrode of the photodiode  1202  is connected to a low-potential-side power source VSS. A second terminal of the I-V conversion resistor  1203  is connected to an external power source VBB. Note that the I-V conversion resistor  1203  is formed outside of the sensor substrate in order to reduce the fluctuation of an output voltage, however, the I-V conversion resistor  1203  may be formed over the sensor substrate in the case where the fluctuation is within the acceptable range. 
         [0147]    When light is inputted to the photodiode  1202 , optical current flows in the direction from the second electrode toward the first electrode of the photodiode  1202 . Consequently, current flows into the I-V conversion resistor  1203 , generating voltage between both the terminals of the I-V conversion resistor  1203 . 
         [0148]    That is, a potential equal to that of the VBB is outputted to the output terminal  1204  in the case where no light is inputted and no current flows into the photodiode  1202  whereas in the case where light is inputted and current flows into the photodiode  1202 , potential of the output terminal  1204  drops in proportion to the amount of current. 
         [0149]    High driving capacity may be provided with the amplifier circuit  1201  in this embodiment, therefore, an optical sensor circuit may drive a load connected to the output terminal  1204 . 
         [0150]    In this embodiment, an electrode terminal for connection of the optical sensor device can be formed at four points, namely the high-potential-side power source VDD, the low-potential-side power source VSS, the output terminal  1204  of the amplifier circuit  1201 , and a connecting terminal between the I-V conversion resistor  1203  and the photodiode  1202 , so that the aforementioned mounting strength can be improved. In addition, the number of power sources can be reduced to two by means of a level shifter. 
         [0151]    Note that the amplifier circuit  1201  is an operational amplifier in this embodiment, however, the amplifier circuit  1201  is not limited to the operational amplifier. 
         [0152]    In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 5 
       [0153]      FIG. 14  is an equivalent circuit diagram of an amplifier circuit, in particular, an operational amplifier circuit configured by a thin film semiconductor element, in particular, a TFT. The operational amplifier comprises a differential circuit including TFTs  1401  and  1402 , a current mirror circuit including TFTs  1403  and  1404 , a constant current source including TFTs  1405  and  1409 , a common source circuit including a TFT  1406 , an idling circuit including TFTs  1407  and  1408 , a source follower circuit including TFTs  1410  and  1411 , and a phase compensation capacitor  1412 . 
         [0154]    An operation of the operational amplifier circuit in  FIG. 14  is explained below. When a positive signal is inputted to a non-inverting input terminal, drain current of the TFT  1401  becomes larger than that of the TFT  1402  since the constant current source including the TFT  1405  is connected to sources of the TFTs  1401  and  1402  in the differential circuit. Drain current of the TFT  1403  becomes equal to that of the TFT  1402  since the TFTs  1404  and  1403  configure the current mirror circuit. Consequently, a drain current difference between the TFT  1403  and the TFT  1401  causes gate potential of the TFT  1406  to drop. The TFT  1406  is a P-channel T 11  therefore, it is turned ON and the amount of drain current thereof increases as the gate potential drops. Consequently, gate potential of the TFT  1410  rises, and according to this, source potential of the TFT  1410 , namely potential of an output terminal rises. 
         [0155]    When a negative signal is inputted to the non-inverting input terminal, drain current of the TFT  1401  becomes smaller than that of the TFT  1402  while drain current of the TFT  1403  is equal to that of the TFT  1402 . Consequently, a drain current difference between the TFT  1403  and the TFT  1401  causes gate potential of the TFT  1406  to rise. The TFT  1406  is a P-channel TFT, therefore, it is turned OFF and the amount of drain current thereof decreases as the gate potential rises. Consequently, gate potential of the TFT  1410  drops, and according to this, source potential of the TFT  1410 , namely potential of the output terminal drops. A signal having the same phase as that of a signal of a non-inverting input terminal is outputted from the output terminal in this manner. 
         [0156]    When a positive signal is inputted to an inverting input terminal, drain current of the TFT  1401  becomes smaller than that of the TFT  1402  while drain current of the TFT  1403  is equal to that of the TFT  1402 . Consequently, a drain current difference between the TFT  1403  and the TFT  1401  causes gate potential of the TFT  1406  to rise. The TFT  1406  is a P-channel TFT, therefore, it is turned OFF and the amount of drain current thereof decreases as the gate potential rises. Consequently, gate potential of the TFT  1410  drops, and according to this, source potential of the TFT  1410 , namely potential of the output terminal drops. 
         [0157]    When a negative signal is inputted to the inverting input terminal, drain current of the TFT  1401  becomes larger than that of the TFT  1402  while drain current of the TFT  1403  is equal to that of the TFT  1402 . Consequently, a drain current difference between the TFT  1403  and the TFT  1401  causes gate potential of the TFT  1406  to drop. The TFT  1406  is a P-channel TF 1 ; therefore, it is turned ON and the amount of drain current thereof increases as the gate potential drops. Consequently, gate potential of the TFT  1410  rises, and according to this, source potential of the TFT  1410 , namely potential of the output terminal rises. A signal having the reverse phase to that of an inverting input terminal is outputted from the output terminal in this manner. 
         [0158]    In this embodiment, the differential circuit and the current mirror circuit are configured by N-channel TFT and P-channel TFTs, respectively. However, the invention is not limited to this and polarities of these circuits may be reversed. In addition, a circuit configuration of the amplifier circuit is not limited to the above either so long as it has a function of an amplifier circuit. 
         [0159]    This embodiment may be freely combined with the aforementioned embodiment modes and embodiments. 
       Embodiment 6 
       [0160]    An example of a structure of an optical sensor device of the invention is described below.  FIG. 17  shows an optical sensor device in which a photodiode and an amplifier circuit are formed integrally. This figure is seen from a side for mounting the optical sensor device to a printed circuit board. 
         [0161]    An amplifier circuit  1702  and a photodiode  1703  are fainted over a substrate  1701 , and an electrode terminal for connection  1704  is formed thereon. The electrode terminal for connection  1704  is connected to the amplifier circuit  1702  and the photodiode  1703  through a contact hole  1705 . 
         [0162]    An enlarged view of a portion  1706  in which the amplifier circuit  1702  and the photodiode  1703  are connected is shown using a line. A TFT  1707  comprises source and drain regions  1708  and  1709 , a channel forming region (not shown), source and drain electrodes  1710  and  1711 , and a gate electrode  1712 . 
         [0163]    Over the gate electrode  1712 , an interlayer insulating film (not shown) is formed, and a wiring  1713  is formed thereon to be connected to the gate electrode  1712  through a contact hole  1714 . A first electrode  1715  of the photodiode  1703  is formed over the wiring  1713 . 
         [0164]      FIG. 18A  is a cross-sectional view of the optical sensor device in which the photodiode  1703  and the amplifier circuit  1702  are formed integrally. The electrical connection between the TFT  1707  of the amplifier circuit  1702  and the photodiode  1703  is described next in more detail with reference to  FIG. 18A .  FIG. 18A  is a cross-sectional view along a line A-A′, in which an identical portion to  FIG. 17  is denoted by the same reference numeral. A base insulating film  1801  is formed on the substrate  1701  and the TFT  1707  and the photodiode  1703  are formed over the base insulating film  1801 . 
         [0165]    A semiconductor layer  1802  corresponds to the channel forming region. The source and drain regions  1708  and  1709  are formed on the frontside and the backside of the channel forming region in the figure. A gate insulating film  1803 , the gate electrode  1712 , a first interlayer insulating film  1804 , and a second interlayer insulating film  1805  are formed on the semiconductor layer  1802  in this order. 
         [0166]    The gate electrode  1712  of the TFT  1707  is connected to the first electrode  1715  of the photodiode  1703  through the contact hole  1714  by the wiring  1713 . 
         [0167]    A P-type semiconductor layer  1806  is formed so as to contact with the first electrode  1715  of the photodiode  1703 . A photoelectric conversion layer  1807 , an N-type semiconductor layer  1808 , and a second electrode  1809  of the photodiode  1703  are formed on the P-type semiconductor layer  1806  in this order. A third interlayer insulating film  1810  is formed on the second electrode  1809  of the photodiode  1703  and the second electrode  1809  is connected to the electrode terminal for connection  1704  through the contact hole  1705 . 
         [0168]    A material having light translucency and conductivity such as ITO (Indium Tin Oxide) is preferably used for the first electrode  1715  of the photodiode  1703  in order to prevent the incident light to the photodiode  1703  from being shielded. In addition, a material having light reflectivity such as Ti is preferably used for the second electrode  1809  of the photodiode  1703 , so that light passing through the photoelectric conversion layer  1807  and the N-type semiconductor layer  1808  without being absorbed into the photoelectric conversion layer  1807  among the incident light from the P-type semiconductor layer  1806  is reflected to be absorbed into the photoelectric conversion layer  1807 . The P-type semiconductor layer  1806  can be formed by a P-type amorphous silicon film (a-Si:H) or a P-type microcrystal semiconductor (μ c-Si: H), the photoelectric conversion layer  1807  can be formed by an amorphous silicon film (a-Si:H), and the N-type semiconductor layer  1808  can be formed by an N-type amorphous silicon film (a-Si:H) or an N-type microcrystal semiconductor (μ c-Si:H) 
         [0169]      FIG. 18B  is a cross-sectional view along a line B-B′ in  FIG. 17 , which is seen from the A side of a line A-A′ and the third interlayer insulating film  1810  which is assumed to transmit light. 
         [0170]    The TFT  1707  comprises the semiconductor layer  1802 , the source and drain regions  1708  and  1709 , the source and drain electrodes  1710  and  1711 , and the gate electrode  1712 . The gate electrode  1712  extends to the backside of the figure to be connected to the first electrode  1715  of the photodiode  1703  through the contact hole  1714  by the wiring  1713 . The P-type semiconductor layer  1806  is formed so as to contact with the first electrode  1715  of the photodiode  1703 . The photoelectric conversion layer  1807 , the N-type semiconductor layer  1808 , and the second electrode  1809  of the photodiode  1703  are formed on the P-type semiconductor layer  1806  in this order. Furthermore, the third interlayer insulating film  1810  and the electrode terminal for connection  1704  are formed on the second electrode  1809  of the photodiode  1703 . The electrode terminal for connection  1704  is connected to the second electrode  1809  of the photodiode  1703  through a contact hole (not shown). 
         [0171]    Note that the invention is not limited to the structure of the optical sensor device described in Embodiment 6. For example, the amplifier circuit  1702  may be a current mirror circuit instead of the operational amplifier. In addition, the optical sensor element is not limited to the above structure and may be a polysilicon photodiode. Other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 7 
       [0172]      FIG. 24A  shows the seventh embodiment of the invention. This embodiment describes Embodiment Mode 5 in detail. 
         [0173]    In this embodiment, an amplifier circuit  2401  configured by a TFT and a photodiode  2402  are formed integrally over a sensor substrate. An operation thereof is described below. Source regions of TFTs in the amplifier circuit  2401  are connected to an external power source GND. The Y-side TFTs have N columns of parallel connections and drain regions thereof are connected to an output terminal  2404 . Drain region of the X-side TFT in the amplifier circuit  2401  is connected to a first electrode of the photodiode  2402 . A second electrode of the photodiode  2402  is connected to the output terminal  2404 . 
         [0174]    When light is inputted to the photodiode  2402 , optical current I flows in the direction from the second electrode toward the first electrode of the photodiode  2402 . Consequently, current I flows into the X-side TFT in the amplifier circuit  2401 , generating voltage for flowing the current I at each gate thereof. 
         [0175]    As for the X-side I and the Y-side TFTs, since the gate length L and the channel width W are equal to each other and each gate of the Y-side TFTs is connected to the gate of the X-side TFT, the current I flows into each column of the Y-side TFTs. Accordingly, when the optical current I flows into the photodiode  2402 , the amount of current flowing into the output terminal  2404  becomes (1+N)*I. 
         [0176]    Note that N-channel TFTs are employed in this embodiment, however, P-channel TFTs may be employed as well. For the amplification, the number of parallel connections is N times as large in the circuit shown in  FIG. 24A , however, ‘the channel width W/the channel length L’ may be N times as large as shown in  FIG. 24B . In addition, other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 8 
       [0177]      FIG. 25  shows the eighth embodiment of the invention. This embodiment describes Embodiment Mode 6 in detail. 
         [0178]    In this embodiment, an amplifier circuit  2501  configured by a TFT and a photodiode  2502  are f 111  aced integrally over a sensor substrate. An operation thereof is described below. As for TFTs in each of the X-side and the Y-side in the amplifier circuit  2501 , they are disposed in series by connecting a source region and a drain region to each other and a multigate structure is formed by commonly connecting their gate electrodes. In addition, source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2501  are connected to an external power source GND. The Y-side TFTs have N columns of parallel connections and drain regions thereof at the high-voltage-side stage are connected to an output terminal  2504 . A drain region of the X-side TFT at the high-voltage-side stage is connected to a first electrode of the photodiode  2502 . A second electrode of the photodiode  2502  is connected to the output terminal  2504 . 
         [0179]    When light is inputted to the photodiode  2502 , optical current I flows in the direction from the second electrode toward the first electrode of the photodiode  2502 . Consequently, current I flows into the X-side TFTs in the amplifier circuit  2501 , generating voltage for flowing the current I at each gate thereof. As for the X-side TFTs and the Y-side TFTs, since the gate length L and the channel width W are equal to each other and each gate of the Y-side TFTs is connected to respective gate of the X-side TFTs, the current I flows into each column of the Y-side TFTs. Accordingly, when the optical current I flows into the photodiode  2502 , the amount of current flowing into the output terminal  2504  becomes (1+N)*I. 
         [0180]    Note that N-channel TFTs are employed in this embodiment, however, P-channel TFTs may be employed as well. Other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 9 
       [0181]      FIG. 26  shows the ninth embodiment of the invention. This embodiment describes Embodiment Mode 7 in detail. 
         [0182]    In this embodiment, an amplifier circuit  2601  configured by a TFT and a photodiode  2602  are formed integrally over a sensor substrate. An operation thereof is described below. As for TFTs in each of the X-side and the Y-side in the amplifier circuit  2601 , they are disposed in series by connecting a source region and a drain region to each other. Source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2601  are connected to an external power source GND. The Y-side TFTs have N columns of parallel connections and drain regions thereof are connected to an output terminal  2604 , in particular, the drain regions thereof at the high-voltage-side stage are connected to an output terminal  2604 . A drain region of the X-side TFT at the high-voltage-side stage is connected to a first electrode of the photodiode  2602 . A second electrode of the photodiode  2602  is connected to the output terminal  2604 . 
         [0183]    When light is inputted to the photodiode  2602 , optical current I flows in the direction from the second electrode toward the first electrode of the photodiode  2602 . Consequently, current I flows into the X-side T 1  Is in the amplifier circuit  2601 , generating voltage for flowing the current I at each gate thereof. 
         [0184]    As for the X-side TFTs and the Y-side TFTs, since the gate length L and the channel width W are equal to each other and each gate of the Y-side TFTs is connected to respective gate of the X-side TFTs, the current I flows into each column of the Y-side TFTs. Accordingly, when the optical current I flows into the photodiode  2602 , the amount of current flowing into the output terminal  2604  becomes (1+N)*I. 
         [0185]    Note that N-channel TFTs are employed in this embodiment, however, P-channel TFTs may be employed as well. Other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 10 
       [0186]      FIG. 27  shows the tenth embodiment of the invention. This embodiment describes Embodiment Mode 8 in detail. 
         [0187]    In this embodiment, an amplifier circuit  2701  configured by a TFT and a photodiode  2702  are formed integrally over a sensor substrate. An operation thereof is described below. The amplifier circuit  2701  is a Wilson current mirror circuit and source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2701  are connected to an external power source GND. The Y-side TFTs have N columns of parallel connections and drain regions thereof are connected to an output terminal  2704 . A drain region of the X-side TFT is connected to a first electrode of the photodiode  2702 . A second electrode of the photodiode  2702  is connected to the output terminal  2704 . 
         [0188]    When light is inputted to the photodiode  2702 , optical current I flows in the direction from the second electrode toward the first electrode of the photodiode  2702 . Consequently, current I flows into the X-side TFT in the amplifier circuit  2701 , generating voltage for flowing the current I at each gate thereof. 
         [0189]    As for the X-side TFT and the Y-side TFTs, since the gate length L and the channel width W are equal to each other and each gate of the 1-side TFTs is connected to the gate of the X-side TFT, the current I flows into each column of the Y-side TFTs. Accordingly, when the optical current I flows into the photodiode  2702 , the amount of current flowing into the output terminal  2704  becomes (1+N)*I. 
         [0190]    Note that N-channel TFTs are employed in this embodiment, however, P-channel TFTs may be employed as well. Other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 11 
       [0191]      FIG. 28  shows the eleventh embodiment of the invention. This embodiment describes Embodiment Mode 9 in detail. 
         [0192]    In this embodiment, an amplifier circuit  2801  configured by a TFT and a photodiode  2802  are formed integrally over a sensor substrate. An operation thereof is described below. The amplifier circuit  2801  is an improved Wilson current mirror circuit and source regions of TFTs at the low-voltage-side stage in the amplifier circuit  2801  are connected to an external power source GND. The Y-side TFTs have N columns of parallel connections and drain regions thereof are connected to an output terminal  2804 . A drain region of the X-side TFT in the amplifier circuit  2801  is connected to a first electrode of the photodiode  2802 . A second electrode of the photodiode  2802  is connected to the output terminal  2804 . 
         [0193]    When light is inputted to the photodiode  2802 , optical current I flows in the direction from the second electrode toward the first electrode of the photodiode  2802 . Consequently, current I flows into the X-side TFTs in the amplifier circuit  2801 , generating voltage for flowing the current I at each gate thereof. 
         [0194]    As for the X-side TFTs and the Y-side TFTs, since the gate length L and the channel width W are equal to each other and each gate of the Y-side TFTs is connected to respective gate of the X-side TFTs, the current I flows into each column of the Y-side TFTs. Accordingly, when the optical current I flows into the photodiode  2802 , the amount of current flowing into the output terminal  2804  becomes (1+N)*I. 
         [0195]    Note that N-channel TFTs are employed in this embodiment, however, P-channel TFTs may be employed as well. Other optical sensor element may be employed as a substitute for the photodiode. 
       Embodiment 12 
       [0196]    An optical sensor element such as a photodiode and a TFT can be formed integrally over an insulating substrate by utilizing the known technology, specifically, the technology disclosed in Japanese Patent Laid-Open No. Hei 11-125841, Japanese Patent Laid-Open No. 2002-305296, or Japanese Patent Laid-Open No. 2002-305297. 
       Embodiment 13 
       [0197]    An optical sensor device of the invention having the above structures is applicable to a display portion of various electronic apparatuses as a luminance controller. Described below are electronic apparatuses using the optical sensor device of the invention. 
         [0198]    Such electronic apparatuses include a video camera, a digital camera, a head mounted display (a goggle type display), a game machine, a car navigation system, a personal computer, a personal digital assistant (a mobile computer, a cellular phone, an electronic book, etc.), and a television. 
         [0199]      FIG. 16  illustrates a personal computer which includes a main body  3201 , a housing  3202 , a display portion  3203 , a keyboard  3204 , an external connecting port  3205 , a pointing mouse  3206 , an optical sensor portion  3207 , and the like. The position of the optical sensor portion  3207  is not exclusively limited and another optical sensor portion may be additionally provided on the backside in the case where the display portion  3203  comprises a dual emission device that enables the image display on the backside, for example. 
         [0200]    Personal computers, in particular, have been widely used for various purposes and in various situations recently. Depending on the outside brightness of the place where the personal computer is used, various display luminances are required for the display. In addition, when a personal computer is carried about with a user, the power source thereof depends on a battery in many cases, which leads one of the considerations of suppressing power consumption in order to use for long period. Therefore, the optical sensor portion  3207  preferably adopts the optical sensor device of the invention so that the outside brightness is detected and the display portion  3203  displays an image at a luminance according to the outside brightness, whereby a personal computer with low power consumption can be fabricated. Furthermore, in the case of a display device using an EL light emitting element, degradation with time of the EL light emitting element can be suppressed due to low power consumption according to the invention. 
         [0201]      FIGS. 15A and 15B  each illustrates a cellular phone which includes housings  1501  and  1502 , a display portion  1503 , an audio input portion  1510 , an antenna  1507 , operating keys  1505  and  1509 , a speaker  1506 , a hinge  1508 , a battery  1511 , an optical sensor portion  1504 , and the like. The position of the optical sensor portion  1504  is not exclusively limited and another optical sensor portion may be additionally provided on the backside in the case where the display portion  1503  comprises a dual emission device that enables the image display on the backside, for example. 
         [0202]    Cellular phones have been widely used and developed with various functions recently as well as personal computers. Such functions include a game, a camera, and Internet, and are used by means of a display device in many cases in various situations. Depending on the outside brightness of the place where the cellular phone is used, various display luminances are required for the display. In addition, the power source of a cellular phone depends on a battery in many cases, which leads one of the considerations of suppressing power consumption in order to use for long period. Therefore, the optical sensor portion  1504  preferably adopts the optical sensor device of the invention so that the outside brightness is detected, and the display portion  1503  and the operating keys  1505  and  1509  display images at a luminance according to the outside brightness, whereby a cellular phone with low power consumption can be fabricated. Furthermore, in the case of a display device using an EL light emitting element, aging degradation with time of the EL light emitting element can be suppressed due to low power consumption according to the invention. 
         [0203]    The electronic apparatuses of this embodiment can be fabricated by using any combination of Embodiment Modes 1 to 9 and Embodiments 1 to 13. 
         [0204]    The application range of the invention is quite wide and the invention is applicable to electronic apparatuses in various fields as well as the personal computer and the cellular phone described above. 
         [0205]    This application is based on Japanese Patent Application serial no. 2003-327629 filed in Japan Patent Office on 19th, Sep., 2003 and Japanese Patent Application serial no. 2003-342632 filed in Japan Patent Office on 30th, Sep., 2003, the contents of which are hereby incorporated by reference.