Abstract:
Disclosed herein is a display device, including a display element, a first scanning line, a second scanning line, a data signal line, a switching element having a first terminal and a second terminal of a first conduction type, the first terminal being connected to the data signal line, for being held in a conducting state or a non-conducting state according to a voltage applied to the first scanning line, and a storage capacitance having a first electrode and a second electrode that shares the second scanning line, wherein the second terminal of the switching element is connected to the display element and connected to the first electrode of the storage capacitance including a semiconductor film of a second conduction type different from the second terminal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a display device and a method of manufacturing the same, and particularly to a liquid crystal display device and a method of manufacturing the same.  
         [0002]     With characteristics of being thin and low power consumption, liquid crystal display devices are widely used in notebook computers, display devices for car navigation, personal digital assistants (PDAs), portable telephones and the like. The liquid crystal display device is roughly classified into a transmissive type that controls light from a backlight for display, a reflective type that reflects extraneous light such as sunlight or the like for display, or a recent display device referred to as a transflective type that combines characteristics of both the transmissive type and the reflective type.  
         [0003]     Applications of these display devices require that the display devices consume low power, and therefore a high aperture ratio is required to maximize efficiency of use of the backlight. In the transflective type, a transmission part and a reflection part are formed in the same pixel, and thus multiple functions are incorporated within the pixel; it is therefore necessary to use space as effectively as possible.  
         [0004]     In displaying an image on a liquid crystal display device, a scanning pulse is applied from a scanning line to a switching element, for example a TFT (Thin Film Transistor) provided for each pixel, and the switching element is turned on/off, whereby display pixels are selected. A signal corresponding to a video signal is applied to a data line, and then applied via a source and a drain of the TFT to electrodes that have a liquid crystal sandwiched therebetween, whereby light entering the liquid crystal is modulated to display the image.  
         [0005]     Within a period until a next writing operation after a voltage corresponding to the video signal is written to each pixel, charge resulting from the voltage applied to the electrodes of the liquid crystal leaks through the liquid crystal and the switching-element and is thus changed. In order to insure display picture quality, the applied voltage needs to be retained. Accordingly, an auxiliary capacitance (CS) sufficient as compared with the amount of leakage is generally formed in the liquid crystal display device.  
         [0006]      FIG. 1  is an example of an equivalent circuit diagram of a conventional liquid crystal display device.  FIG. 2  is a plan view of a configuration of the liquid crystal display device shown in  FIG. 1 .  
         [0007]      FIG. 1  shows an equivalent circuit of 2×3 pixels. One pixel in the equivalent circuit includes a liquid crystal element and electrodes having the liquid crystal element sandwiched therebetween, a transistor Tr as a switching element, and an auxiliary capacitance CS. Ccl 1  to Ccl 6  denotes a capacitance of a liquid crystal capacitor formed by a liquid crystal element and a display electrode and a common electrode having the liquid crystal element sandwiched therebetween. CS 1  to CS 6  denotes a capacitance value of the auxiliary capacitance of each pixel.  
         [0008]     A plurality of scanning lines WLn−1, WLn, and WLn+1 are arranged in parallel with each other, and each connected to gate electrodes of transistors Tr 1  and Tr 4 , transistors Tr 2  and Tr 5 , or transistors Tr 3  and Tr 6  formed by TFTs, for example. The scanning lines WLn−1, WLn, and WLn+1 effect ON/OFF control of each of the transistors and thereby select pixels.  
         [0009]     Data signal lines BLn−1, BLn, and BLn+1 arranged in parallel with each other apply a voltage corresponding to a video signal to each pixel. The data signal lines BLn−1, BLn, and BLn+1 are connected to for example a source region of the transistors Tr 1 , Tr 2 , and Tr 3  or the transistors Tr 4 , Tr 5 , and Tr 6 . The data signal lines BLn−1, BLn, and BLn+1 apply voltage to electrodes on both sides of liquid crystal elements in pixels selected by the scanning line WLn−1, WLn, or WLn+1 while charging auxiliary capacitances CS in the pixels, whereby light entering the liquid crystal elements is modulated to display an image.  
         [0010]      FIG. 2  is a view of the configuration of scanning lines, data signal lines, and one pixel formed on a transparent substrate. As shown in  FIG. 2 , the auxiliary capacitance CS 1  is formed on an auxiliary capacitance line CSLn−1 as one electrode of the auxiliary capacitance CS 1 . One impurity region, for example the source region of the transistor Tr 1  is connected to the data signal line BLn−1 via a conductive material deposited in a contact hole H 1 . Another impurity region, for example a drain region of the transistor Tr 1  is connected to another electrode formed by a semiconductor, for example, of the auxiliary capacitance CS 1  and an ITO electrode of an upper layer not shown in the figure via conductive material deposited in contact holes H 2  and H 3 .  
         [0011]     An N-channel type thin film transistor TFT is generally used as the transistors Tr 1 , . . . , Tr 6 . Specifically, N-type source and drain impurity regions are formed by injecting phosphorus (P) or the like into semiconductor thin film on both sides of a gate electrode. When a positive voltage equal to or higher than a threshold value is applied to the gate electrode (scanning line), an N-channel formed by an N-type inversion layer is formed between the source and the drain, whereby the source and the drain are electrically connected to each other. That is, the transistor is in an ON state. When a voltage lower than the threshold value is applied to the gate electrode (scanning line), on the other hand, the channel for electrically connecting the source and the drain to each other is not formed, and therefore the transistor is in an OFF state.  
         [0012]     The auxiliary capacitance CS 1  is generally formed by a MOS structure of a semiconductor layer, an insulating film, and a metal, which structure can form a highest capacitance. In  FIG. 2 , the auxiliary capacitance CS 1  is for example formed by the auxiliary capacitance line CSLn−1 (metal), a gate insulating film forming the transistor Tr 1 , and the above-mentioned N-type semiconductor film having phosphorus or the like injected therein. Such a MOS capacitance will hereinafter be referred to as an N-type MOS structure.  
         [0013]     When the electrodes of the auxiliary capacitance are to be set at a fixed potential, the auxiliary capacitance portion is generally made to be of the N-type MOS structure.  
         [0014]     In a case of common-inversion driving in which auxiliary capacitance electrodes are oscillated in phase with a counter electrode, the semiconductor film forming the auxiliary capacitance CS does not form a sufficient capacitance in an intrinsic state. Therefore, the semiconductor layer is generally metalized, that is, made to contain a high concentration of phosphorus (made to be of an N+ type) or boron (made to be of a P+ type).  
         [0015]     With the above conventional method, high-concentration injection of phosphorus (allowing the semiconductor layer to be of the N+ type) or boron (allowing the semiconductor layer to be of the P+ type) is required to be performed only once, and therefore manufacturing cost can be reduced.  
         [0016]     However, the above structure requires an independent auxiliary capacitance line, thus presenting a problem of a decrease in an aperture ratio.  
         [0017]     Accordingly, a CS-on-gate structure is proposed in which a scanning line (gate line) in a preceding stage or a succeeding stage also serves as the auxiliary capacitance line.  
         [0018]      FIGS. 3A and 3B  show another example of a conventional liquid crystal display device.  FIG. 3A  is an equivalent circuit diagram of the liquid crystal display device, and  FIG. 3B  is a plan view of a configuration of the liquid crystal display device. In  FIGS. 3A and 3B , the same components as in  FIG. 1  are designated by using the same reference numerals, and their repeated description will be omitted where appropriate.  
         [0019]      FIG. 3A  shows an equivalent circuit of 2×2 pixels. In  FIG. 3A , auxiliary capacitances CS 1 , CS 4 , CS 2 , and CS 5  are directly connected to scanning lines WLn−1, WLn, and WLn+1 in place of the auxiliary capacitance lines CSLn−1, CSLn, and CSLn+1 shown in  FIG. 1 .  
         [0020]      FIG. 3B  shows a configuration of scanning lines, data signal lines, and one pixel formed on a transparent substrate. The auxiliary capacitance CS 1  is formed so as to overlap the scanning line WLn in place of the auxiliary capacitance line CSLn−1 shown in  FIG. 2 .  
         [0021]     Also in this case, an N-channel type thin film transistor TFT is generally used as transistors Tr 1 , . . . , Tr 6 . Also, the auxiliary capacitance CS 1  is an N-type MOS capacitance. Specifically, when a positive voltage equal to or higher than a threshold value is applied to gate electrodes (scanning lines) of the transistors Tr 1 , . . . , Tr 6 , the transistors Tr 1 , . . . , Tr 6  are brought into an ON state. When a voltage lower than the threshold value is applied to the gate electrodes, the transistors Tr 1 , . . . , Tr 6  are brought into an OFF state.  
         [0022]     As shown in  FIG. 3B , the auxiliary capacitance CS 1  is formed by the scanning line WLn (metal), a gate insulating film forming the transistor Tr 1 , and an N-type semiconductor film having phosphorus or the like injected therein.  
         [0023]     Such a CS-on-gate structure eliminates the need for forming the independent auxiliary capacitance line, and therefore has an advantage of increasing the aperture ratio.  
         [0024]     In order to maintain the NMOS transistor Tr 1  in an off state, the potential of the scanning lines WLn−1, WLn, . . . may generally be set to about 0 V to −6 V. In addition, the transistor Tr 1  in the liquid crystal display device is maintained in the OFF state during most of a period of display of one screen. That is, the potential of the scanning lines is maintained at 0 V or lower during most of the display period.  
         [0025]     However, in the case of the CS-on-gate structure as shown in  FIGS. 3A and 3B , in which structure the auxiliary capacitance CS 1  is formed with the scanning line (gate line) WLn in the succeeding stage, for example, and the potential as described above is applied, the N-type MOS structure formed by the scanning line, the gate insulating film, and the N-type semiconductor film cannot provide a sufficient capacitance.  
         [0026]      FIG. 4  is a graph showing capacitance-voltage characteristics of the N-type MOS structure.  
         [0027]     When −2 V, for example, is applied to the scanning lines WLn−1 and WLn shown in  FIG. 3B  and thereby Tr 1  is maintained in an OFF state, since CS 1  is charged while Tr 1  is in an ON state, the semiconductor electrode of CS 1  is at a higher potential than the scanning lines WLn−1 and WLn, and the gate voltage applied to CS 1  is a negative voltage. This causes majority carrier electrons to be repelled from a surface of the semiconductor film and a depletion layer (and/or an inversion layer) to be formed on the surface of the semiconductor film, which corresponds to an increase in thickness of the insulating layer of CS 1 . Thus, a resulting capacitance is small.  
         [0028]     This tendency is shown in  FIG. 4 . When the scanning potential is used in a range of about 1.5 V and lower, only a small capacitance is provided by the N-type MOS capacitance at all times.  
         [0029]     In order to increase the capacitance of the auxiliary capacitance CS 1 , phosphorus (for making an N+ type) or boron (for making a P+ type) needs to be injected at a high concentration into the semiconductor film electrode of the auxiliary capacitance CS 1 . This causes problems of an increase in the number of processes, a decrease in yield resulting from occurrence of defects, and the like.  
         [0030]      FIGS. 5A  to  7 B illustrate an example of a process of manufacturing the conventional liquid crystal display device.  
         [0031]     In  FIG. 5A , gate electrodes (scanning lines)  102   a  and  102   b  serving as scanning lines are formed on a glass substrate  101 . A metal such as Ta, Cr, Mo, Ti, Al or the like is used for the material of the gate electrodes, and the pattern is formed by wet etching or dry etching following a photoresist process.  
         [0032]     In  FIG. 5B , an gate insulating film  103  and a semiconductor layer  104   a  are formed over the gate electrodes  102   a  and  102   b . Examples of the gate insulating film  103  include silicon nitride film and silicon oxide film, as well as anodized film obtained by anodizing a gate electrode and the like. As the semiconductor film, amorphous silicon film, polysilicon film obtained by crystallizing amorphous silicon film, polysilicon film formed directly, or the like is used.  
         [0033]     In  FIG. 5C , a protective insulating film  105  is formed on the semiconductor film  104   a . As the protective insulating film  105 , silicon nitride film, silicon oxide film or the like is used.  
         [0034]     In  FIG. 5D , resists  107   a  and  107   b  are formed in a self-aligning manner with the gate electrodes  102   a  and  102   b  serving as a light shield mask. The protective insulating film  105  is thereafter removed by wet etching or dry etching. Then, using remaining protective insulating films  105   a  and  105   b  as a mask, the semiconductor film is doped with phosphorus (P) or the like at a low concentration. A doped portion of the semiconductor film is denoted as  104   b . The semiconductor film  104   b  is an n −  type semiconductor.  
         [0035]     In  FIG. 6A , a resist  108  having such a shape as to cover a portion forming an LDD region in the pixel transistor is formed. In order to remove the protective insulating film  105   b  remaining in a portion where the auxiliary capacitance is formed, wet etching or dry etching is thereafter performed.  
         [0036]     Then, phosphorus or the like is injected at a high concentration to thereby metalize the semiconductor layer  104   b . A metalized portion of the semiconductor film  104   b  is denoted as  104   c.    
         [0037]     Though not shown in the figures, a photoresist process and an injection process are carried out according to a portion for injection of a second type of dope (boron or the like). Thereafter heat treatment is performed as required to activate doped elements.  
         [0038]     The wet etching or the dry etching is usually performed by a process with etching selectivity to protect the semiconductor layer  104   b  from the etching. However, when there is a pin hole or the like in the semiconductor layer  104   b , the gate insulating film  103  serving as a base is etched. The etched portion, being greatly deteriorated in terms of withstand voltage, forms a path of current leakage, and causes a defect such as a point defect or the like. Besides, the process for removing the protective insulating film  105   b  is added, which represents a cost increasing factor.  
         [0039]     In  FIG. 6B , for device isolation, the semiconductor layer  104   c  on the outside of the gate electrodes  102   a  and  102   b  is removed using means such as photolithography, dry etching or the like.  
         [0040]     In  FIG. 6C , an interlayer insulating film  109  is formed by silicon nitride film, silicon oxide film or the like. Thereafter contact holes  110   a  and  110   b  are formed using means such as photolithography, wet etching or the like.  
         [0041]     In  FIG. 7A , a metal such as Al, Ta, W or the like is formed as data signal lines  111   a  and  111   b  and a connecting metal  112  for connection with a pixel electrode. Thereafter the metal is removed using means such as photolithography, dry etching or the like, whereby a pattern is formed.  
         [0042]     In  FIG. 7B , a second interlayer insulating film  113  is formed by silicon nitride film, silicon oxide film or the like. In order to provide this layer with a flattening effect, a photosensitive organic film, a photosensitive SOG (spin on glass) film or the like may be used. Also in this process, a contact hole for connection with the pixel electrode  114  is formed. Thereafter the pixel electrode  114  is formed using a transparent conductive film of ITO, IXO or the like.  
         [0043]     Then, though not shown in the figures, a corresponding color filter substrate prepared separately is superposed on the TFT substrate, an assembly process is carried out with a liquid crystal layer sandwiched between the color filter substrate and the TFT substrate, and further a polarizer and the like are attached, whereby the liquid crystal display device is completed.  
         [0044]     Thus, with the conventional manufacturing method, a special process is required to complete the structure of the auxiliary capacitance, which constitutes a cost increasing factor, and also leakage current that causes defects is increased. There are conventionally problems of such an increase in the number of processes and a decrease in yield resulting from occurrence of defects.  
         [0045]     While a conventional example of the bottom gate type transistor in which the scanning line (gate electrode) is formed under the semiconductor layer has been described above, there are also problems in a method of manufacturing the top gate type transistor in which the scanning line (gate electrode) is formed above the semiconductor layer.  
         [0046]      FIGS. 8A, 8B , and  8 C and  FIGS. 9A and 9B  illustrate a conventional example of a method of manufacturing a liquid crystal display device having the structure of the top gate type transistor.  
         [0047]     As shown in  FIG. 8A , a base layer  122  and a semiconductor layer  123   a  are formed on a glass substrate  121 . As the base layer  122 , silicon nitride film or silicon oxide film, for example, is used. As the semiconductor layer  123   a , amorphous silicon film, polysilicon film obtained by crystallizing amorphous silicon film, polysilicon film formed directly, or the like is used.  
         [0048]     As shown in  FIG. 8B , in order to obtain a region for device isolation, a part of the semiconductor layer  123   a  is removed using means such as photolithography, dry etching or the like.  
         [0049]     A gate insulating film  124  is then formed on the semiconductor film  123   a . Examples of the gate insulating film include silicon nitride film, silicon oxide film and the like.  
         [0050]     Then, gate electrodes  125   a  and  125   b  are formed in a region for forming a transistor TFT and a region for forming an auxiliary capacitance.  
         [0051]     Next, the semiconductor film is doped with phosphorus or the like at a low concentration in a self-aligning manner with the gate electrodes  125   a  and  125   b  serving as an injection mask. A doped portion of the semiconductor film is an n −  type semiconductor, and is denoted as  123   b.    
         [0052]     As shown in  FIG. 8C , a resist  126  having such a shape as to cover a portion forming an LDD region in the pixel transistor TFT is formed. Phosphorus or the like is injected into other regions at a high concentration to thereby metalize the semiconductor layer  123   b . The metalized portion of the semiconductor film  123   b  is denoted as  123   c.    
         [0053]     Though not shown in the figures, a photoresist process and an injection process are carried out according to a portion for injection of a second type of dope (boron or the like). Thereafter heat treatment is performed as required to activate doped elements.  
         [0054]     In  FIG. 9A , an interlayer insulating film  127  is formed by silicon nitride film, silicon oxide film or the like. Thereafter contact holes  128   a  and  128   b  are formed using means such as photolithography, wet etching or the like.  
         [0055]     In  FIG. 9B , a metal such as Al, Ta, W or the like is formed as data signal lines  129   a  and  129   b  and a connecting metal  130  for connection with a pixel electrode  132 . Thereafter the metal is removed using means such as photolithography, dry etching or the like, whereby a pattern is formed.  
         [0056]     Then, a second interlayer insulating film  131  is formed by silicon nitride film, silicon oxide film or the like. In order to provide this layer with a flattening effect, a photosensitive organic film, a photosensitive SOG (spin on glass) film or the like may be used. Also in this process, a contact hole for connection with the pixel electrode  132  is formed. Thereafter the pixel electrode  132  is formed using a transparent conductive film of ITO, IXO or the like.  
         [0057]     Then, though not shown in the figures, a corresponding color filter substrate prepared separately is superposed on the TFT substrate, an assembly process is carried out with a liquid crystal layer sandwiched between the color filter substrate and the TFT substrate, and further a polarizer and the like are attached, whereby the liquid crystal display device is completed.  
         [0058]     As shown in  FIGS. 8B and 8C , the above structure has the gate electrode  125   b  formed in a region for forming the auxiliary capacitance. Therefore no impurity can be injected into the semiconductor film  123   a  under the gate electrode  125   b , and the semiconductor film  123   a  under the gate electrode  125   b  cannot be metalized. In order to solve this problem within the scope of the conventional method, it is necessary to add processes of forming a mask, performing injection, and then removing the mask in an initial stage, or form an independent auxiliary capacitance line as shown in  FIG. 1 .  
       SUMMARY OF THE INVENTION  
       [0059]     The present invention has been made in view of the above problems, and it is an object of the present invention to provide a display device with a high aperture ratio and a large auxiliary capacitance and a method of manufacturing the display device without increasing manufacturing processes.  
         [0060]     According to an aspect of the present invention, there is provided a display device including: a display element; a first scanning line; a second scanning line; a data signal line; a switching element having a first terminal and a second terminal of a first conduction type, the first terminal being connected to the data signal line, for being held in a conducting state or a non-conducting state according to a voltage applied to the first scanning line; and a storage capacitance having a first electrode and a second electrode that shares the second scanning line; wherein the second terminal of the switching element is connected to the display element and connected to the first electrode of the storage capacitance including a semiconductor film of a second conduction type different from the second terminal.  
         [0061]     While the switching element is held in the conducting state, the storage capacitance is charged from the data signal line via the switching element, and while the switching element is held in the non-conducting state, the storage capacitance applies a voltage to the display element.  
         [0062]     The second terminal of the switching element and the first electrode of the storage capacitance are connected to each other by a conductive material.  
         [0063]     Specifically, the second terminal of the switching element and the first electrode of the storage capacitance are connected to each other by a conductive material deposited in a contact hole reaching the second terminal of the switching element and a contact hole reaching the first electrode of the storage capacitance.  
         [0064]     Alternatively, the second terminal of the switching element and the first electrode of the storage capacitance are connected to each other by a conductive material deposited in a contact hole reaching the second terminal of the switching element and the first electrode of the storage capacitance.  
         [0065]     Preferably, the conductive material for connecting the second terminal of the switching element and the first electrode of the storage capacitance to each other is identical with a conductive material used for the data signal line.  
         [0066]     Further, preferably, a part or a whole of the storage capacitance is formed between the data signal line and the second scanning line in a region where the data signal line and the second scanning line overlap each other.  
         [0067]     According to a second aspect of the present invention, there is provided a display device including: a plurality of first scanning lines and second scanning lines; a plurality of data signal lines; a plurality of pixels arranged in a form of a matrix; and a driving circuit for driving the plurality of pixels; wherein the driving circuit for driving each of the pixels includes: a switching element having a first terminal and a second terminal of a first conduction type, the first terminal being connected to the data signal line, for being held in a conducting state or a non-conducting state according to a voltage applied to the first scanning line; and a storage capacitance having a first electrode and a second electrode that shares the second scanning line; and the second terminal of the switching element is connected to a display element and connected to the first electrode of the storage capacitance including a semiconductor film of a second conduction type different from the second terminal.  
         [0068]     Preferably, the switching element is a thin film transistor having polycrystalline silicon as a semiconductor layer.  
         [0069]     According to a third aspect of the present invention, there is provided a method of manufacturing a display device, the method including the steps of: forming a conductive first scanning line and a conductive second scanning line arranged in parallel with each other, a first insulating film covering the first scanning line and the second scanning line, and a semiconductor film covering the first insulating film; forming a first protective mask and a second protective mask for protecting a first channel region and a second channel region, respectively, of the semiconductor film, the first channel region and the second channel region being opposed to the first scanning line and the second scanning line, respectively; and injecting an impurity of a first conduction type and an impurity of a second conduction type into a switching element region for forming a switching element including the first scanning line and a storage capacitance region for forming a storage capacitance including the second scanning line, respectively, in the semiconductor film, in a state in which the first protective mask and the second protective mask are formed.  
         [0070]     According to the present invention, there may be provided a method of manufacturing a display device, the method including the steps of: forming a second insulating film so as to cover the semiconductor film having the switching element region and the storage capacitance region injected with the impurity of the first conduction type and the impurity of the second conduction type, respectively, and the first protective mask and the second protective mask; forming, in the second insulating film, a first contact hole and a third contact hole reaching semiconductor regions of the first conduction type on both sides of the first scanning line; forming, in the second insulating film, a second contact hole reaching a semiconductor region of the second conduction type on one side of the second scanning line; connecting the semiconductor region of the first conduction type on one side of the first scanning line with the semiconductor region of the second conduction type on one side of the second scanning line by depositing a conductive material in the first contact hole and the second contact hole; and forming a data signal line by depositing a conductive material in the third contact hole.  
         [0071]     Preferably, the first contact hole is a same contact hole as the second contact hole.  
         [0072]     Further, preferably, the conductive material deposited in the first contact hole and the second contact hole is a same material as the conductive material deposited in the third contact hole.  
         [0073]     Further, according to a fourth aspect of the present invention, there is provided a method of manufacturing a display device, the method including the steps of: forming a semiconductor film, an insulating film covering the semiconductor film, and a conductive first scanning line and a conductive second scanning line arranged in parallel with each other on the insulating film; and injecting, from a side of the first scanning line and the second scanning line, an impurity of a first conduction type and an impurity of a second conduction type into a switching element region for forming a switching element including the first scanning line and a storage capacitance region for forming a storage capacitance including the second scanning line, respectively, in the semiconductor film.  
         [0074]     According to the present invention described above, in the semiconductor film, the conduction type of a source and drain region of a transistor (the conduction type of a channel between the source and drain, so to speak) is opposite to the conduction type of a semiconductor film region for forming an auxiliary capacitance. When the conduction type of the source and drain region of the transistor and the channel is an N-type, for example, the semiconductor film region for forming the auxiliary capacitance is made to be of a P-type. Thus, by applying a scanning line voltage (zero or negative) that turns off the N-channel transistor to the auxiliary capacitance of a P-type MOS structure, a large capacitance is obtained. Also, when the conduction type of the source and drain region of the transistor and the channel is the P-type, and when the semiconductor film region for forming the auxiliary capacitance is made to be of the N-type, the same effects are obtained.  
         [0075]     In addition, at the time of injection of impurities to form the above structure, it suffices only to inject different types of impurities, and therefore manufacturing processes are not increased.  
         [0076]     Further, since the above structure provides a sufficient capacitance, at the time of injection of an impurity into the region of the semiconductor film for forming the auxiliary capacitance, a mask for protecting a channel forming region of the semiconductor film which region is opposed to the scanning line does not need to be removed by etching. Thus defect causing factors are reduced.  
         [0077]     According to the present invention, a conventionally impossible structure of a switching transistor and an auxiliary capacitance is made possible, which increases the auxiliary capacitance and improves an aperture ratio.  
         [0078]     In manufacturing the display device according to the present invention, an effective auxiliary capacitance can be formed without an increase in the number of processes or with a small number of processes.  
         [0079]     In addition, since the number of times that the surface of the semiconductor layer is exposed to wet etching and dry etching is reduced, it is possible to reduce a rate of occurrence of defects such as a leakage current between the semiconductor layer and the gate electrode for the auxiliary capacitance and the like, thereby improving yield.  
         [0080]     The present invention reduces a non-transparent region, and can thereby increase the aperture ratio greatly. It is accordingly possible to reduce backlight brightness and thus reduce power consumption.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0081]      FIG. 1  shows an equivalent circuit of an example of a conventional display device;  
         [0082]      FIG. 2  is a plan view of a configuration of the conventional display device shown in  FIG. 1 ;  
         [0083]      FIG. 3A  shows an equivalent circuit of another example of a conventional display device having a bottom gate structure, and  FIG. 3B  is a plan view of a configuration of the display device;  
         [0084]      FIG. 4  is a graph showing a result of measurement of voltage-capacitance characteristics of an auxiliary capacitance in the conventional display device shown in  FIGS. 3A and 3B ;  
         [0085]      FIGS. 5A, 5B ,  5 C, and  5 D are sectional views of assistance in explaining a method of manufacturing the conventional display device shown in  FIGS. 3A and 3B ;  
         [0086]      FIGS. 6A, 6B , and  6 C are sectional views, continued from  FIGS. 5A, 5B ,  5 C, and  5 D, showing assistance in explaining the method of manufacturing the conventional display device shown in  FIGS. 3A and 3B ;  
         [0087]      FIGS. 7A and 7B  are sectional views, continuing from  FIGS. 6A, 6B , and  6 C, showing assistance in explaining the method of manufacturing the conventional display device shown in  FIGS. 3A and 3B ;  
         [0088]      FIGS. 8A, 8B , and  8 C are sectional views of assistance in explaining a method of manufacturing a conventional display device having a top gate structure;  
         [0089]      FIGS. 9A and 9B  are sectional views, continuing from  FIGS. 8A, 8B , and  8 C, showing assistance in explaining the method of manufacturing the conventional display device having the top gate structure;  
         [0090]      FIG. 10A  shows an equivalent circuit of a display device according to a first embodiment of the present invention, and  FIG. 10B  is a plan view of a configuration of the display device;  
         [0091]      FIG. 11  is a sectional view of a structure of the display device according to the first embodiment of the present invention;  
         [0092]      FIGS. 12A, 12B , and  12 C are timing charts of scanning line signals and pixel potential in the display device according to the first embodiment of the present invention;  
         [0093]      FIG. 13  is a graph showing a result of measurement of voltage-capacitance characteristics of an auxiliary capacitance in the display device according to the first embodiment of the present invention;  
         [0094]      FIGS. 14A and 14B  are sectional views of assistance in explaining a method of manufacturing the display device according to the first embodiment of the present invention;  
         [0095]      FIG. 15  is a diagram showing a driving circuit in the display device according to the first embodiment of the present invention;  
         [0096]      FIG. 16  is a plan view of a configuration of a display device according to a second embodiment of the present invention;  
         [0097]      FIG. 17  is a sectional view of the configuration of the display device according to the second embodiment of the present invention;  
         [0098]      FIG. 18  is a plan view of a configuration of a display device according to a third embodiment of the present invention;  
         [0099]      FIG. 19  is a sectional view of a configuration of a display device according to a fourth embodiment of the present invention; and  
         [0100]      FIGS. 20A and 20B  are sectional views of assistance in explaining a method of manufacturing the display device according to the fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0101]     Preferred embodiments of a display device and a method of manufacturing the same according to the present invention will hereinafter be described with reference to the accompanying drawings by taking a liquid crystal display device as an example.  
       First Embodiment  
       [0102]      FIGS. 10A and 10B  are diagrams showing an equivalent circuit and a configuration of a liquid crystal display device according to a first embodiment. The circuit arrangement diagram of  FIG. 10A  is similar to that of the conventional example shown in  FIG. 3A . However, in  FIG. 10A , a conduction type of semiconductor film possessed by auxiliary capacitances PCS 1 , PCS 2 , PCS 4 , and PCS 5  is different from a conduction type of transistors NTr 1 , NTr 2 , NTr 4 , and NTr 5 . In this example, the transistors NTr 1 , NTr 2 , NTr 4 , and NTr 5  are formed by an N-channel TFT, and the auxiliary MOS capacitances are formed by a P-type MOS structure. That is, the auxiliary MOS capacitances are formed by a scanning line (metal), a gate insulating film, and a P-type semiconductor film.  
         [0103]     The conduction types may be changed to form the transistors by a P-channel type TFT and the auxiliary capacitances by an N-type MOS structure.  
         [0104]      FIG. 10A  shows an equivalent circuit of 2×2 pixels. A plurality of scanning lines WLn−1, WLn, and WLn+1 in  FIG. 10A  are arranged in parallel with each other, and each connected to gate electrodes of the transistors NTr 1  and NTr 4  or the transistors NTr 2  and NTr 5  formed by N-channel TFTs, for example. The scanning lines WLn−1, WLn, and WLn+1 effect ON/OFF control of each of the transistors and thereby select pixels to be operated.  
         [0105]     The auxiliary capacitances PCS 1 , PCS 2 , PCS 4 , and PCS 5  of the P-type MOS structure each formed by a scanning line, a gate insulating film, and a P-type semiconductor film are each directly connected to the scanning line WLn or WLn+1.  
         [0106]     Data signal lines BLn−1, BLn, and BLn+1 arranged in parallel with each other for applying a voltage corresponding to a video signal to each pixel are connected to one impurity region, for example a source region of the transistors NTr 1  and NTr 2  or the transistors NTr 4  and NTr 5 . The data signal lines BLn−1, BLn, and BLn+1 apply voltage to electrodes of liquid crystal elements in pixels selected by the scanning line WLn−1, WLn, or WLn+1 while charging the auxiliary capacitances PCS of the P-type MOS structure, whereby light entering the liquid crystals is modulated to display an image.  
         [0107]      FIG. 10B  is a plan view of the configuration of scanning lines, data signal lines, and one pixel formed on a transparent substrate. In  FIG. 10B , the P-type MOS auxiliary capacitance PCS 1  shown in  FIG. 10A  is formed on the scanning line WLn with an intermediate gate insulating film not shown in the figure.  
         [0108]     One impurity region, for example a source region of the N-channel type transistor NTr 1  is connected to the data signal line BLn−1 via a conductive material deposited in a contact hole H 1 . Another impurity region, for example a drain region of the N-channel type transistor NTr 1  is connected to the semiconductor film of the auxiliary capacitance PCS 1  and an ITO electrode of an upper layer not shown in the figure via conductive materials deposited in contact holes H 2  and H 3 .  
         [0109]      FIG. 11  is a schematic sectional view of the pixel structure shown in  FIG. 10B .  FIG. 10B  is a sectional view taken along a semiconductor layer pattern from the data signal line to the auxiliary capacitance. Because of limited space, however, the TFT transistor portion is shown having a single gate structure rather than a double gate structure in which transistors are arranged in series.  
         [0110]     In  FIG. 11 , scanning lines  1   a  and  1   b  (WLn−1 and WLn) are formed on the transparent substrate not shown in the figure, a gate insulating film  2  is formed so as to cover the scanning lines  1   a  and  1   b , and semiconductor films  3 ,  4 ,  5 , and  6  are formed on the gate insulating film  2 , whereby the TFT transistor and the auxiliary capacitance are formed.  
         [0111]     In the semiconductor film, reference numeral  3  denotes an N +  semiconductor region having a high concentration of phosphorus (P), for example, injected therein, and reference numeral  4  denotes a P +  semiconductor region having a high concentration of boron (B), for example, injected therein. A center of the semiconductor film  5  is a so-called i-type semiconductor film without an impurity injected therein, and both sides of the semiconductor film  5  are an LDD region having a low concentration of phosphorus (P), for example, injected therein. The semiconductor film  5  forms a channel region of the TFT transistor. In the example of  FIG. 11 , this semiconductor film forms an N-channel region. Reference numeral  6  also denotes a so-called i-type semiconductor film without an impurity injected therein.  
         [0112]     References  7   a  and  7   b  denote stopper films formed so as to prevent impurity injection into the i-type semiconductor films  5  and  6  under the stopper films. Reference numeral  8  denotes an interlayer insulating layer.  
         [0113]     Contact holes are formed in the interlayer insulating layer  8  on the N +  semiconductor region  3  and the P +  semiconductor region  4 . A conductive material in the contact holes forms a connecting electrode  10  for connecting the N +  semiconductor region  3  and the P +  semiconductor region  4  to each other. The conductive material also forms a data signal line  9 .  
         [0114]     The gate electrode  1   a , the gate insulating film  2 , and the semiconductor films  4  and  5  form the N-channel type TFT transistor. On the other hand, the gate electrode  1   b , the gate insulating film  2 , and the semiconductor films  4  and  6  form a P-channel type transistor. A capacitance of the P-channel type transistor is used as the auxiliary capacitance.  
         [0115]     As to conduction of the N +  semiconductor region  3  and the P +  semiconductor region  4 , when the N +  semiconductor region  3  and the P +  semiconductor region  4  are directly connected to each other, a PN junction occurs between the N +  semiconductor region  3  and the P +  semiconductor region  4 , thus causing a potential loss. It is accordingly desirable to make a connection from the N +  semiconductor region  3  to the P +  semiconductor region  4  via a metal. In the first embodiment, the metal is deposited in the contact hole for connection to the N +  type semiconductor  3  and the contact hole for connection to the P +  type semiconductor  4 , whereby the connecting electrode  10  is formed to connect the N +  semiconductor region  3  and the P +  semiconductor region  4  to each other.  
         [0116]     It is desirable that a material for the connecting electrode  10  be a material used for the data signal line  9 . The use of the same metal as that of the data signal line  9  eliminates the need for a special process for the connection, and thus enables manufacturing at a lower cost.  
         [0117]     As another metal for the connection, a pixel electrode ( FIG. 7B  and  FIG. 9B ) may be used.  
         [0118]     However, the contact holes are not necessarily required; a metallic layer may be formed directly on the N +  type semiconductor  3  and the P +  type semiconductor  4 .  
         [0119]      FIGS. 12A, 12B , and  12 C are timing charts of scanning line voltage applied to the scanning lines WLn−1, WLn, and WLn+1 in the liquid crystal display device according to the first embodiment as shown by  FIG. 10A . Vdd and Vssg in  FIG. 12A  denote voltages for bringing the TFT transistor of each pixel into an ON state and an OFF state, respectively. As an example, Vdd=13 V and Vssg=−2 V.  
         [0120]     In  FIG. 12B , a broken line indicates a potential of a common electrode, and an irregular line indicates timing of change in a pixel potential.  
         [0121]     As shown in  FIGS. 12A, 12B , and  12 C, in displaying an image, the scanning lines WLn−1, WLn, WLn+1, . . . sequentially output a high-level voltage signal (Vdd) to the transistors NTr 1 , NTr 4 , NTr 2 , NTr 5  of the pixels, and thereby turn on the transistors to operate the pixels.  
         [0122]     For display of one screen, each pixel is operated only once. Therefore a period when the scanning line voltage is Vdd is far shorter than a period when the scanning line voltage is Vssg, and each transistor is maintained in an OFF state during most of a period of display of one screen. That is, the voltage of −2 V is applied to the scanning lines WLn−1, WLn, and WLn+1 during most of the display period.  
         [0123]     Thus, the voltage of −2 V is applied to the metallic electrode (scanning line) of the P-type MOS auxiliary capacitance PCS 1 , for example, shown in  FIGS. 10A and 10B  and  FIG. 11  during most of the period.  
         [0124]     As for another electrode including the P-type semiconductor film of the auxiliary capacitance PCS 1 , on the other hand, when the transistor NTr 1  is in an ON state, a high-level signal from the data signal line BLn−1 applies a voltage to electrodes on both sides of the liquid crystal while charging the auxiliary capacitance PCS 1  via the source and drain of the transistor NTr 1 . Since the auxiliary capacitance PCS 1  is charged, a potential of the semiconductor film electrode of the auxiliary capacitance PCS 1  is higher than Vssg. When the transistor NTr 1  is in an OFF state, the source and drain of the transistor NTr 1  are disconnected from each other, and therefore a signal from the data signal line BLn−1 does not supply a voltage to the liquid crystal and the auxiliary capacitance PCS 1 . The auxiliary capacitance PCS 1  supplies a voltage to the electrodes on both sides of the liquid crystal.  
         [0125]     As shown in the graph of the pixel potential of  FIG. 12B , while the potential of the semiconductor film of the auxiliary capacitance PCS 1  (the same as the pixel potential) is gradually lowered and raised, the potential is higher than Vssg at all times. Then, a voltage Vg from the metal side (scanning line side) to the semiconductor film of the auxiliary capacitance PCS 1  is negative at all times.  
         [0126]     As already described with reference to the graph of  FIG. 4 , when such a voltage Vg is applied to an N-type MOS capacitance comprising a scanning line (metal), a gate insulating film, and an N-type semiconductor film, since majority carriers of the N-type semiconductor are electrons, the negative scanning line voltage (or the voltage Vg) causes the majority carriers to be repelled from a surface of the semiconductor film and a depletion layer (and/or an inversion layer) to be formed, which corresponds to an increase in thickness of the insulating layer of the auxiliary capacitance. Thus, as shown in  FIG. 4 , a resulting capacitance is small.  
         [0127]      FIG. 13  is a graph showing capacitance-voltage characteristics of a P-type MOS structure.  
         [0128]     In the P-type MOS capacitance comprising a scanning line (metal), a gate insulating film, and a P-type semiconductor film, since majority carriers of the P-type semiconductor are holes, the negative scanning line voltage (or the voltage Vg) does not cause a depletion layer to be formed but instead causes the majority carriers to be gathered on a surface of the P-type semiconductor film. Thereby, as shown in  FIG. 13 , a large capacitance is obtained.  
         [0129]     Thus, with the first embodiment, a sufficient capacitance is formed in a use range (the period when the scanning line voltage is Vssg) in normal driving conditions.  
         [0130]     Thus, in general, when an N-channel type pixel transistor is formed, the auxiliary capacitance needs to be formed by a P-type MOS capacitance. When a P-channel type pixel transistor is formed, it is desirable that the auxiliary capacitance be formed by an N-type MOS capacitance.  
         [0131]     While in the first embodiment, the auxiliary capacitance is formed with the scanning line (gate line) WLn in the next stage, the auxiliary capacitance may be formed with a scanning line (gate line) WLn−2 in a preceding stage.  
         [0132]     Conventionally, when the scanning line WLn forming the auxiliary capacitance is raised to a high level, the pixel potential is substantially shifted. With the P-type MOS capacitance as in the first embodiment, when the scanning line WLn is raised to a high level, the P-type MOS capacitance is effectively decreased, and an amount of shift is decreased, as shown in  FIG. 12B . Thereby display quality is improved.  
         [0133]      FIGS. 14A and 14B  illustrate a method of manufacturing the liquid crystal display device according to the first embodiment. The manufacturing method according to the first embodiment is obtained by changing the conventional process shown in  FIG. 6A  in the conventional manufacturing method shown in  FIGS. 5A  to  5 D,  FIGS. 6A  to  6 C, and  FIGS. 7A and 7B .  
         [0134]     Following the process of  FIG. 5D , in  FIG. 14A , phosphorus is injected at a high concentration in the vicinity of the TFT transistor to thereby form the N +  semiconductor region  3  and thus metalize the semiconductor layer. At this time, a resist  11   b  is formed so as to prevent the high-concentration injection of phosphorus in the vicinity of the auxiliary capacitance. Thus, as in the process of  FIG. 5D , an N −  type semiconductor region  4   a  is formed around the auxiliary capacitance after the high-concentration injection of phosphorus.  
         [0135]     In addition, the conventional process of removing the protective insulating film on the auxiliary capacitance is not required.  
         [0136]     In  FIG. 14B , the resist pattern  11   b  in the vicinity of the auxiliary capacitance is removed, and boron is injected at a high concentration around the auxiliary capacitance to thereby form a P +  type semiconductor region  4   b . At this time, a resist  11   c  is formed so as to prevent the high-concentration injection of boron in the vicinity of the TFT transistor.  
         [0137]     Thereafter heat treatment is performed as required to activate doped elements.  
         [0138]     As described above, the liquid crystal display device according to the first embodiment includes an element of a first conduction type and a MOS structure of a second conduction type. The use of such elements of the two conduction types allows a CMOS type driving circuit or logical circuit to be formed in a display pixel region, a region outside of the display pixel region, or both the regions.  
         [0139]      FIG. 15  shows an example of configuration of a display device in which each pixel is driven by such CMOS. In  FIG. 15 , a plurality of scanning lines arranged in parallel with each other and a plurality of data signal lines arranged in parallel with each other are driven by a scanning line driving circuit and a data signal line driving circuit, respectively, and each of pixels arranged in a form of a matrix is driven by a driving circuit formed by an N-channel TFT and a P-type MOS capacitance, for example.  
         [0140]     A liquid crystal display device having such a circuit can be formed by a method as in the first embodiment without addition of a special process, and is therefore a most suitable configuration example. For example, it is desirable to use the first embodiment in a polysilicon transistor liquid crystal display device using a polysilicon film with a high mobility as a semiconductor, or the like.  
         [0141]     According to the first embodiment, a sufficient auxiliary capacitance is obtained in a use range (the period when the scanning line voltage is Vssg) in normal driving conditions. In addition, since the auxiliary capacitance can be formed by the CS-on-gate structure, a high aperture ratio is obtained.  
         [0142]     Further, with the method of manufacturing the liquid crystal device according to the first embodiment, processes in which the semiconductor layer is exposed to etching are reduced, and thereby defects and the like are decreased.  
       Second Embodiment  
       [0143]     In a second embodiment, another example of configuration of the liquid crystal display device according to the present invention will be shown.  
         [0144]      FIG. 16  and  FIG. 17  are a plan view and a schematic sectional view of the configuration of the liquid crystal display device according to the second embodiment.  
         [0145]     The liquid crystal display device shown in  FIG. 16  and  FIG. 17  are basically of the same configuration as shown in  FIG. 10B  and  FIG. 11 . Therefore repeated description in the second embodiment will be omitted where appropriate, and in  FIG. 16  and  FIG. 17 , the same references are used for the same components as in  FIG. 10B  and  FIG. 11 .  
         [0146]     There is a difference between  FIG. 16  and  FIG. 10B , and between  FIG. 17  and  FIG. 11  in that the contact holes H 2  and H 3  shown in  FIG. 10B  for connecting the N +  type semiconductor  3  and the P +  type semiconductor  4  to each other are combined into one contact hole H 4  in  FIG. 16 . The connecting electrode  10  shown in  FIG. 11  formed by depositing a conductive material in the two contact holes is converted into a connecting electrode  30  in  FIG. 17  formed by a conductive material deposited in the same contact hole.  
         [0147]     It is desirable that contacts for connecting the N +  type semiconductor  3  and the P +  type semiconductor  4  to each other are to be the same contact hole extending over both the conduction types. By combining the contact holes into one, an area within the pixel can be utilized effectively, thus improving the aperture ratio.  
       Third Embodiment  
       [0148]     In a third embodiment, another example of configuration of the liquid crystal display device according to the present invention will be shown.  
         [0149]      FIG. 18  is a plan view of the configuration of the liquid crystal display device according to the third embodiment.  
         [0150]     In  FIG. 18 , the same references are used for the same components as in  FIG. 16  and  FIG. 11 .  
         [0151]     There is a difference between  FIG. 18  and  FIG. 11  and  FIG. 16  in that a part of an auxiliary capacitance PCS 1  in  FIG. 18  is disposed under a data signal line BLn−1.  
         [0152]     In this case, a region required for forming the necessary auxiliary capacitance is formed in a region where light does not pass through originally, for example a metal region (data signal line in this case). Therefore a loss in transmittance becomes smaller, and thus a high aperture ratio can be secured.  
         [0153]     Either of a bottom gate type transistor structure and a top gate type transistor structure may be formed in this case.  
         [0154]     When the auxiliary capacitance PCS 1  is formed under the data signal line BLn−1 in the bottom gate structure shown in  FIG. 18 , a protective insulating film is left on the semiconductor layer, so that a coupling capacitance between the signal line and the semiconductor film is decreased. This improves display quality in terms of crosstalk and the like.  
         [0155]     In addition, since the coupling capacitance between the signal line and the electrode under the signal line is decreased, a total signal line capacitance is decreased, and thus rounding of signal line potential is reduced, thereby improving display quality.  
       Fourth Embodiment  
       [0156]     While the bottom gate type transistor structure has been described as an example, the present invention is also applicable to the top gate type transistor structure.  
         [0157]      FIG. 19  is a sectional view of an example of configuration of a liquid crystal display device of the top gate structure according to a fourth embodiment.  
         [0158]     In the liquid crystal display device of  FIG. 19 , semiconductor films  43 ,  44 ,  45 , and  46  are formed on a base layer not shown in the figure formed on a transparent substrate not shown in the figure, a gate insulating film  42  is formed on the semiconductor films, and further scanning lines  41   a  and  41   b  (WLn−1 and WLn) and an interlayer insulating film  48  are formed on the gate insulating film  42 . Thereby a TFT transistor and an auxiliary capacitance are formed.  
         [0159]     Reference numeral  43  denotes an N +  semiconductor region, and reference numeral  44  denotes a P +  semiconductor region. A center of the semiconductor film  45  is an i-type semiconductor film, and both sides of the semiconductor film  45  are an LDD region. The semiconductor film  45  forms a channel region of the TFT transistor. In the example of  FIG. 19 , this semiconductor film forms an N-channel region. Reference numeral  46  also denotes an i-type semiconductor film.  
         [0160]     Reference numeral  46  also denotes a so-called i-type semiconductor film without an impurity injected therein. Contact holes are formed in the interlayer insulating layer  48  on the N +  semiconductor region  43  and the P +  semiconductor region  44 . A conductive material in the contact holes forms a connecting electrode  50  for connecting the N +  semiconductor region  43  and the P +  semiconductor region  44  to each other. The conductive material also forms a data signal line  49 .  
         [0161]     The gate electrode  41   a , the gate insulating film  42 , and the semiconductor films  44  and  45  form the N-channel type TFT transistor. On the other hand, the gate electrode  41   b , the gate insulating film  42 , and the semiconductor films  44  and  46  form a P-channel type transistor. A capacitance of the P-channel type transistor is used as the auxiliary capacitance.  
         [0162]     The schematic sectional view of the structure shown above corresponds to the structure shown in  FIG. 10B . The structure shown above may correspond to that of  FIG. 16 .  
         [0163]     Also, the pixel transistor may have an N-channel or a P-channel.  
         [0164]      FIGS. 20A and 20B  illustrate a method of manufacturing the liquid crystal display device having the top gate structure according to the fourth embodiment. The manufacturing method according to the fourth embodiment is obtained by changing the conventional process shown in  FIG. 8C  in the conventional manufacturing method shown in  FIGS. 8A  to  8 C and  FIGS. 9A and 9B .  
         [0165]     Following the process of  FIG. 8B , in  FIG. 20A , a resist  47   a  having such a shape as to cover the LDD region on both sides of the channel region  45  is formed in the TFT transistor region. Then, phosphorus is injected at a high concentration in the vicinity of the TFT transistor to thereby form the N +  type semiconductor region  43  and thus metalize the semiconductor layer.  
         [0166]     At this time, a resist  47   b  is formed so as to prevent the high-concentration injection of phosphorus in the vicinity of the auxiliary capacitance. Thus, as in  FIG. 8B , an N −  type semiconductor region  44   a  is formed around the auxiliary capacitance after the high-concentration injection of phosphorus.  
         [0167]     In  FIG. 20B , the resist  47   b  in the vicinity of the auxiliary capacitance is removed, and boron is injected at a high concentration around the auxiliary capacitance to thereby form a P +  type semiconductor region  44   b . At this time, a resist  47   c  is formed so as to prevent the high-concentration injection of boron in the vicinity of the TFT transistor.  
         [0168]     Thereafter heat treatment is performed as required to activate doped elements.  
         [0169]     The fourth embodiment has the same effects as the first and second embodiments.  
         [0170]     While the present invention has been described above on the basis of the preferred embodiments, the present invention is not limited to the embodiments described above and is susceptible of various modifications without departing from the spirit of the present invention.  
         [0171]     While the foregoing embodiments have been described by taking a liquid crystal display device as an example, the present invention is applicable to other display devices having a similar driving method.  
         [0172]     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.