Abstract:
The present invention concerns a liquid crystal display device including: a substrate; a plurality of thin film transistors each including a gate electrode, a source electrode, and a drain electrode; a gate line and a data line electrically connecting with the gate electrode and the source electrode, respectively; and a pixel electrode electrically connecting with the drain electrode, wherein overlapping widths between the gate electrode and the source and drain electrodes vary in accordance with the position of the thin film transistor.

Description:
This application is a continuation of U.S. patent application No. 11/028,225, filed on Jan. 4, 2005 now U.S. Pat. No. 7,136,118, which is a divisional of U.S. Application Ser No. 09/910,058, filed on Jul. 23, 2001, now U.S. Pat. No. 6,853.406, issued on Feb. 8, 2005, which claim the benefit of Korean Patent Application No. 2000-42533, filed on Jul. 24, 2000, which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate. 
     2. Discussion of the Related Art 
     A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the aligrnment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed. 
     Liquid crystal is classified into positive liquid crystal and negative liquid crystal, depending on the electrical properties of the liquid crystal. The positive liquid crystal has a positive dielectric anisotropy such that long axes of liquid crystal molecules are aligned parallel to an electric field. Whereas, the negative liquid crystal has a negative dielectric anisotropy such that long axes of liquid crystal molecules are aligned perpendicular to an electric field. 
     By now, active matrix LCDs, in which the thin film transistors and the pixel electrodes are arranged in the form of a matrix, are widely used because of their high resolution and superiority in displaying moving video data. 
       FIG. 1  is a cross-sectional view illustrating a typical twisted nematic (TN) LCD panel. As shown in  FIG. 1 , the TN LCD panel has lower and upper substrates  2  and  4  and an interposed liquid crystal layer  10 . The lower substrate  2  includes a first transparent substrate  1   a  and a thin film transistor (“TFT”) “S”. The TFT “S” is used as a switching element to change orientation of the liquid crystal molecules. The lower substrate  2  further includes a pixel electrode  15  that applies an electric field to the liquid crystal layer  10  in accordance with signals applied by the TFT “S”. The upper substrate  4  has a second transparent substrate  1   b , a color filter  8  on the second transparent substrate  1   b , and a common electrode  14  on the color filter  8 . The color filter  8  implements color for the LCD panel. The common electrode  14  serves as another electrode for applying a voltage to the liquid crystal layer  10 . The pixel electrode  15  is arranged over a pixel region “P,” i.e., a display area. A transparent conductive material like indium tin oxide (ITO) having superior light transmittance is used for the pixel electrode  15 . Further, to prevent leakage of the liquid crystal layer  10  between the lower and upper substrates  2  and  4 , those substrates are sealed by a sealant  6 . 
     As described above, because the pixel and common electrodes  15  and  14  of the conventional TN LCD panel are positioned on the lower and upper substrates  2  and  4 , respectively, the electric field induced therebetween is perpendicular to the lower and upper substrates  2  and  4 . The above-mentioned liquid crystal display device has advantages of high transmittance and aperture ratio, and further, since the common electrode on the upper substrate serves as an electrical ground, the liquid crystal is protected from a static electricity. 
     However, the above-mentioned operation mode of the TN LCD panel has a disadvantage of a narrow viewing angle. To overcome the above-mentioned problem, an in-plane switching (IPS) LCD panel was developed. The IPS LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN or STN (super twisted nematic) LCD panel. A detailed explanation about operation modes of a typical IPS LCD panel will be provided with reference to  FIGS. 2 ,  3 A,  3 B,  4 A and  4 B. 
     As shown in  FIG. 2 , first and second substrates  1   a  and  1   b  are spaced apart from each other, and a liquid crystal “LC” is interposed therebetween. The first and second substrates  1   a  and  1   b are called an array substrate and a color filter substrate, respectively. Pixel and common electrodes  15  and  14  are disposed on the first substrate  1   a . The pixel and common electrodes  15  and  14  are parallel with and spaced apart from each other. On a surface of the second substrate  1   b , a color filter  25  is disposed opposing the first substrate  1   a . The pixel and common electrodes  15  and  14  apply an electric field “E” to the liquid crystal “LC”, then it is aligned parallel to the electric field “E”. 
       FIGS. 3A and 3B  conceptually illustrate “off state” operation modes for a typical IPS LCD device. In an off state, the long axes of the LC molecules “LC” maintain a definite angle with respect to a line that is perpendicular to the pixel and common electrodes  15  and  14 . The pixel and common electrode  15  and  14  are parallel with each other. Herein, the angle difference is 45 degrees, for example. 
       FIGS. 4A and 4B  conceptually illustrate “on state” operation modes for the typical IPS LCD device. In an on state, an in-plane electric field “E”, which is parallel with the surface of the first substrate  1   a , is generated between the pixel and common electrodes  15  and  14 . The reason is that the pixel electrode  15  and common electrode  14  are formed together on the first substrate  1   a . Then, the LC molecules “LC” are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the LC molecules “LC” are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes  15  and  14 . 
     By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS LCD device displays images. The IPS LCD device has wide viewing angle and low color dispersion. Specifically, the viewing angle of the IPS LCD device is about 70 degrees in direction of up, down, right, and left. In addition, the fabricating processes of this IPS LCD device are simpler than other various LCD devices. However, because the pixel and common electrodes are disposed on the same plane of the lower substrate, the transmittance and aperture ratio are low. In addition, the IPS LCD device has disadvantages of a relatively slow response time and a relatively small alignment margin. Because of the small alignment margin, the IPS LCD device needs a uniform cell gap. 
     The IPS LCD device has the above-mentioned advantages and disadvantages. Users may or may not select an IPS LCD device depending on the intended use. 
     Now, with reference to  FIGS. 5 , and  6 A to  6 D, a fabricating process for a conventional IPS LCD device is provided.  FIG. 5  is a plan view illustrating a unit pixel region “P” of a conventional IPS LCD device. As shown, a gate line  50  and a common line  54  are arranged parallel to each other, and a data line  60  is arranged perpendicular to the gate and common lines  50  and  54 . Near a cross point of the gate and data lines  50  and  60 , a gate electrode  52  and a source electrode  62  are disposed. The gate and source electrodes  52  and  62  integrally communicate with the gate line  50  and the data line  60 , respectively. The source electrode  62  overlaps a portion of the gate electrode  52 . In addition, a drain electrode  64  is disposed opposite to the source electrode  62  with an interval therebetween. 
     A plurality of common electrodes  54   a  are disposed perpendicular to the common line  54  and connected thereto. The plurality of common electrode  54   a  are spaced apart from each other with an equal interval therebetween. A first connecting line  66  integrally communicates with the drain electrode  64 , and a plurality of pixel electrodes  66   a  are disposed perpendicular to the first connecting line  66 . First ends of the pixel electrodes  66   a  are connected with the first connecting line  66 , and the second ends of the pixel electrodes  66   a  are connected with a second connecting line  68  that is disposed over the common line  54 . The plurality of common electrodes  54   a  and pixel electrodes  66   a  are spaced apart from each other and arranged in an alternating pattern. Therefore, each common electrode  54   a  is parallel to an adjacent pixel electrode  66   a.    
       FIGS. 6A to 6D  illustrate a sequence of fabricating processes for an array substrate  1  of the above-mentioned IPS LCD device. 
     As shown in  FIG. 6A , on the array substrate  1 , a first metal layer is deposited and patterned to form the gate electrode  52  and the plurality of common electrodes  54   a . The first metal layer is selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), antimony (Sb), and alloys thereof. 
     As shown in  FIG. 6B , a gate-insulating layer  70  is formed on the array substrate  1  to cover the gate and common electrodes  52  and  54   a , and on the gate-insulating layer  70 , an active layer  72  is formed over gate electrode  52 . The gate-insulating layer  70  is silicon nitride (SiNx) or silicon oxide (SiO 2 ), while the active layer  72  includes an amorphous silicon layer (not shown) and a doped amorphous silicon layer (not shown). 
     As shown in  FIG. 6C , a second metal layer is deposited and patterned to form the source and drain electrodes  62  and  64  on the active layer  72  and the pixel electrodes  66   a  on the gate-insulating layer  70 . The pixel electrodes  66   a  are spaced apart from the adjacent common electrode  54   a  by a desired distance. 
     As shown in  FIG. 6D , a passivation layer  74  is formed to cover the source, drain, and pixel electrodes  62 ,  64 , and  66   a . The passivation layer  74  serves to protect the source, drain, and pixel electrodes  62 ,  64 , and  66   a  from exterior humidity or contaminants. 
     As described above, the plurality of common electrodes  54   a  of the IPS LCD device are connected with the common line  54 . Though the IPS LCD device has an advantage of wide viewing angle because of the above-mentioned structure including the common line  54 , the common line  54  of the IPS LCD device is much higher in resistance than the common electrode of the TN LCD device. 
     That is to say, the common electrode  14  of the TN LCD device in  FIG. 1  is uniformly disposed on the overall surface of the upper substrate opposing the pixel electrode. Therefore, the common electrode  14  of the TN LCD device has the shape of a surface and has a relatively low resistance. In contrast, the common line  54  of the IPS LCD device in  FIG. 4  has the shape of a line and has a relatively high resistance. 
     A common voltage signal is applied to the common line  54  of the IPS LCD device or the common electrode  14  of the TN LCD device. In case of the IPS LCD device, because the common line has a relatively high resistance, it is associated with a relatively high voltage drop. Because the common voltage signal is applied from both ends of the common line, the voltage drop is highest at the center of the IPS LCD panel. That is to say, the common voltage signal is decreasing as it transmits from the sides of the IPS LCD panel to the center thereof. 
     Even if the common voltage signal is adjusted or compensated at an exterior drive circuit, the above-mentioned voltage drop cannot be avoided. The voltage variation due to the voltage drop of the common voltage signal causes fatal defects, for example, a flicker to display quality of the conventional IPS LCD device. 
     Together with the above-mentioned voltage variation of the common voltage signal, a conventional structure of an inverted-staggered type TFT itself is associated with the flicker problem. The inverted-staggered type TFT is widely selected as a switching element for the LCD devices including the EPS LCD device. The inverted-staggered type TFT has a simple structure to be easily fabricated but exhibits superior electrical qualities. To achieve such superior electrical qualities, a source electrode and a drain electrode of the inverted-staggered type TFT overlap a gate electrode thereof. Specifically, the overlapping width of the drain electrode over the gate electrode is associated with the flicker problem. A detailed explanation will be provided with reference to  FIGS. 7 and 8 . 
     In  FIG. 7 , a first pixel region “A” and a third pixel region “C” are arranged at a left side and at a right side, respectively, with a second pixel region “B” disposed therebetween. In the first to third pixel regions “A” to “C”, drain electrodes  64   a  to  64   c  overlap gate electrodes  52   a  to  52   c , respectively. At this point, a first overlapping width “L A ”, a second overlapping width “L B ”, and a third overlapping width “L C ” conventionally have the same value ( LA =L B =L C ). Within the first to third overlapping widths “L A ” to “L C ”, a parasitic capacitance “Cgs” exists between the drain electrode  64   a  ( 64   b ,  64   c ) and the gate electrode  52   a  ( 52   b ,  52   c ). 
     In  FIG. 8 , when a gate voltage signal “Vg” is applied to the TFT (“S” in  FIG. 5 ), the TFT switches a drain voltage signal “Vd” such that a pixel voltage signal “Vp” is applied to the pixel electrode. At this point, a kickback voltage “ΔVp” is induced by the parasitic capacitance “Cgs”, thereby decreasing the pixel voltage signal “Vp” by the kickback voltage “ΔVp”. 
     The above-mentioned kickback voltage “ΔVp” is conventionally compensated for by adjusting the common voltage signal. However, in case of the IPS LCD device, because the common line has a high resistance, the common voltage signal varies along the common line. Accordingly, the kickback voltage “ΔVp” cannot be compensated for sufficiently, and, thus, the flicker problem occurs in the IPS LCD device. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an IPS LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     In order to achieve the above object, the present invention provides a liquid crystal display device. The liquid crystal display device includes: a substrate; a plurality of thin film transistors each including a gate electrode, a source electrode, and a drain electrode; a gate line and a data line electrically connecting with the gate electrode and the source electrode; and a pixel electrode electrically connecting with the drain electrode, wherein overlapping widths between the gate electrode and the source and drain electrodes vary in accordance with the position of the thin film transistor. 
     A distance between the source electrode and the drain electrode is constant. 
     In another aspect, the present invention provides an array substrate for a liquid crystal display device, the array substrate including: a substrate including a first pixel region, a second pixel region, and a third pixel region, the first pixel region and the third pixel region being disposed at both sides of the substrate with the second pixel region disposed therebetween; a plurality of gate lines and data lines on the substrate; a plurality of thin film transistors each formed on the first to third pixel regions, the thin film transistor including a gate electrode, a source electrode, and a drain electrode; and a pixel electrode on the substrate, wherein overlapping widths between the drain electrode and the gate electrode vary among the first to third pixel regions. 
     The overlapping width of the drain electrode is the highest in the first pixel region, and is the larger in the third pixel region than in the second pixel region. 
     The array substrate further includes a common electrode formed on the substrate and being parallel to the pixel electrode. 
     A distance between the source electrode and the drain electrode is constant in the first to third pixel regions. 
     In another aspect, the present invention provides a liquid crystal display device, which includes: first and second substrates; a gate line and a data line on the first substrate, the gate and data lines being formed on a plurality of pixel regions; a common line on the first substrate; a plurality of common and pixel electrodes on the first substrate; a thin film transistor having a gate electrode, a source electrode, and a drain electrode at a crossing point of the gate and data lines, wherein a capacitance between the gate electrode and the drain electrode varying in accordance with a position of the corresponding pixel region; and a liquid crystal layer between the first and second substrates. 
     The capacitance is larger in a center pixel region centered among the plurality of pixel regions than in the other pixel regions. 
     The pixel electrode is selected from a group consisting of indium tin oxide (ITO) and indium zinc oxide (IZO). The common electrode is selected from a group consisting of chromium (Cr), aluminum (Al), aluminum alloy (Al alloy), molybdenum (Mo), tantalum (Ta), tungsten (W), antimony (Sb), and alloys thereof. 
     The common and pixel electrodes are different layers. 
     The common electrode is electrically connected with the common line 
     The device further includes a first connecting line being electrically connected with the drain electrode, and the first connecting line is electrically connected with the pixel electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is a cross-sectional view illustrating a typical liquid crystal display device; 
         FIG. 2  is a cross-sectional view illustrating a typical IPS LCD device; 
         FIGS. 3A and 4A  are perspective views illustrating operation modes of the typical IPS LCD device of  FIG. 2 ; 
         FIGS. 3B and 4B  are plan views illustrating the operation modes of the typical IPS LCD device of  FIGS. 3A and 4A , respectively; 
         FIG. 5  is a plan view illustrating a unit pixel region of a conventional IPS LCD device; 
         FIGS. 6A to 6D  are sequential cross-sectional views taken along a line “VI-VI” of  FIG. 5 ; 
         FIG. 7  is a plan view showing first to third pixel regions of the conventional IPS LCD device; 
         FIG. 8  is a graph showing electric signals applied to electrodes of the conventional IPS LCD device; and 
         FIG. 9  is a plan view illustrating first to third pixel regions of an IPS LCD device according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     In an IPS LCD device according to the preferred embodiment, an overlapping width of a drain electrode varies in accordance with a position of a corresponding pixel region. Thereby, variation of a kickback voltage “ΔVp “(delta_Vp)” with respect to the position of the pixel region is minimized and a flicker problem of the IPS LCD device is avoided. 
     In  FIG. 9 , a first pixel region “A” and a third pixel region “C” are arranged at a left side and at a right side, respectively, with a second pixel region “B” disposed therebetween. In the pixel region “A” (“B”, “C”), a gate line  150  and a common line  154  are arranged parallel to each other, and a data line  160  is arranged perpendicular to the gate and common lines  150  and  154 . Near a cross point of the gate and data lines  150  and  160 , a gate electrode  152   a  ( 152   b ,  152   c ) and a source electrode  162  are disposed. The gate and source electrodes  152   a  ( 152   b ,  152   c ) and  162  integrally communicate with the gate line  150  and the data line  160 , respectively. The source electrode  162  overlaps a portion of the gate electrode  152   a  ( 152   b ,  152   c ). In addition, a drain electrode  164   a  ( 164   b ,  164   c ) is disposed opposite to the source electrode  162  with an interval therebetween. 
     A plurality of common electrodes  154   a  are disposed perpendicular to the common line  154  and connected thereto. The plurality of common electrodes  154   a  are spaced apart from each other with an equal interval therebetween. A first connecting line  166  integrally communicates with the drain electrode  164   a  ( 164   b ,  164   c ), and a plurality of pixel electrodes  166   a  are disposed perpendicular to the first connecting line  166  and connected thereto. The plurality of common electrodes  154   a  and the pixel electrodes  166   a  are spaced apart from each other and arranged in an alternating pattern. Therefore, each common electrode  154   a  is parallel to an adjacent pixel electrode  166   a  with a predetermined interval therebetween. 
     The first pixel region “A” is adjacent a gate pad (not shown) applying a gate voltage signal. The gate voltage signal is transmitted through the gate line  150  in a direction “Z”, which is from the first pixel region “A”. to the third pixel region “C”. When the gate voltage signal is transmitted from the first pixel region “A” to the third pixel region “C”, a voltage drop occurs along the gate line  150 . Because of the direction “Z” of the gate voltage signal, the voltage drop is the highest at the third pixel region “C”, the farthest pixel region away from the gate pad (not shown). 
     In another aspect, a common voltage signal is applied to both ends of the common line  154  from both sides of the IPS LCD device. Therefore, the voltage drop of the common voltage signal is the highest in the second pixel region “B”. The first and third pixel regions “A” and “C” are associated with almost the same voltage drop lower than that of the second pixel region “B”. When the above-mentioned variation of the common voltage signal is involved with a kickback voltage “ΔVp”, the flicker problem of the LCD device occurs. 
     The kickback voltage “ΔVp” is induced in a pixel voltage signal “Vp” when a drain voltage signal is transmitted to the pixel electrode  166   a . The kickback voltage “ΔVp” is determined by a parasitic capacitance “Cgs”, a liquid crystal capacitance “C LC ”, a storage capacitance “C ST ”, and a gate voltage signal “Vg” (see a following equation 1). 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Vp 
                   
                   = 
                   
                     
                       
                         C 
                         
                           g 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           5 
                         
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       g 
                     
                     
                       
                         C 
                         
                           g 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           5 
                         
                       
                       + 
                       
                         C 
                         LC 
                       
                       + 
                       
                         C 
                         ST 
                       
                     
                   
                 
               
               
                 
                   equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     Referring now to the equation 1, variation of the kickback voltage “ΔVp” and a design for compensating the variation thereof is explained with respect to the first to third pixel regions “A” to “C”. At this point, the liquid crystal capacitance “C LC ” and the storage capacitance “C ST ” are assumed constant throughout the overall pixel regions. 
     In the first pixel region “A” where the voltage drop of the gate voltage signal “Vg” is the lowest, because the gate voltage signal “Vg” is the highest, the kickback voltage “ΔVp” is relatively high. In the third pixel region “C” where the voltage drop of the gate voltage signal “Vg” is the highest, however, because the gate voltage signal “Vg” is the lowest, the kickback voltage “ΔVp” is relatively low. In addition, the kickback voltage “ΔVp” exhibit an intermediate value in the second pixel region “B” disposed between the first and third regions “A” and “C”. 
     To compensate for the variation of the kickback voltage “ΔVp” between the first and third pixel regions “A” and “C”, the parasitic capacitance “Cgs” of the first pixel region “A” is designed to have a relatively low value. In addition, the parasitic capacitance “Cgs” of the third pixel region “C” is designed to have a relatively high value. For the second pixel region “B”, however, the parasitic capacitance “Cgs” is designed to have the highest value, even though the kickback voltage “ΔVp” of the second pixel region “B” has the intermediate value between those of the first and third pixel regions “A” and “C”. Because the voltage drop of the common voltage signal is the highest in the second pixel region “B”, more compensation is needed for the parasitic capacitance “Cgs” of the second pixel region “B”. That is to say, in case of the second pixel region “B”, the voltage drop of the common voltage signal as well as the gate voltage signal affects the kickback voltage “ΔVp”. Therefore, the parasitic capacitance “Cgs” must have the highest value for the second pixel region “B”. 
     To achieve the optimum parasitic capacitance “Cgs”, first to third overlapping widths “L A ”, “L B ”, and “L C ” of the drain electrode  164   a ,  164   b , and  164   c  respectively adjusted in accordance with the first to third pixel regions “A” to “C”. In case of considering the voltage drop of the gate voltage signal “Vg” only, the third overlapping width “L C ” is designed to be the largest. The second overlapping width “L B ” is designed to be larger than the first overlapping width “L A ”. 
     Further, in case of considering the voltage drop of the common voltage signal “Vc” as well as the gate voltage signal “Vg”, the second overlapping width “L B ” is designed to be the largest. At this point, the third overlapping width “L C ” is designed to be larger than the first overlapping width “L A ”. 
     For the above-explained configuration, a distance between the source electrode  162  and the drain electrode  164   a  ( 164   b ,  164   c ) is constant regardless of the different overlapping width of the drain electrodes  164   a ,  164   b , and  164   c . That is to say, the source electrodes  162  also exhibit various overlapping widths over the gate electrode  152   a  ( 152   b ,  152   c ). However, the overlapping width of the source electrode  162  little affects the operation quality of the IPS LCD device according to the preferred embodiment. 
     As explained above, for the preferred embodiment of the present invention, the overlapping width of the drain electrode is variously adjusted in accordance with the relative position of the corresponding pixel region. That is to say, the parasitic capacitance “Cgs”, which is an overlapping capacitance induced between the drain electrode and the gate electrode, is adjusted in accordance with the position of the corresponding pixel region, thereby compensating for the kickback voltage “ΔVp” throughout the overall pixel regions of the IPS LCD device. Accordingly, the flicker problem of the conventional EPS LCD device is avoided, and an improved display quality can be achieved. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the method of manufacturing a thin film transistor of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.