Abstract:
A sensor TFT includes a substrate, a gate electrode formed on the substrate, a semiconductor layer patterned on the insulating layer to generate an optical current using received light, source and drain electrodes formed on the semiconductor layer, the source and drain electrodes being spaced apart from each other, and a conductive channel defined by an area between the source and drain electrodes, wherein the conductive channel is not rectangular-shaped, such that the channel width is increased for a fixed channel length.

Description:
This application claims the benefit of Korean Patent Application No. 1998-54096, filed on Dec. 10, 1998, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thin film transistor (TFT) and, more particularly, to a sensor TFT used in an optical detecting sensor that can detect light reflected from an object. 
     2. Discussion of the Related Art 
     A thin film transistor type optical detecting sensor can be used as an image reader of an image detecting system such as a character recognition system, a fingerprint recognition system and a telecopy machine. 
     As shown in FIG. 1A, such a TFT type optical detecting sensor comprises a window through which light is transmitted, a sensor TFT (ST) for forming an optical current using light reflected from an object, a current charging part, or a storage capacitor (SC) for charging current flowing through the sensor TFT (ST), and a switching part, or a switch (SW) for selectively discharging the current charged in the current charging part (SC). 
     In operation, when light reflected from the object to be read is transmitted to an active area formed between drain and source electrodes of the sensor TFT (ST), an optical current flows along the active area. The optical current is then transmitted to an external circuit through the current charging part (SC) and the switching part (SW). At this point, the optical current corresponds to information on an image of the object. That is, the amount of the optical current varies according to the strength of the reflected light. In addition, the amount of the optical current further depends on the length and width of the active area to which the reflected light is introduced. 
     For example, when the length of the active area is fixed, the amount of the optical current is increased as the width of the active area is increased. 
     As is well known, as the amount of the optical current is increased, the image information becomes more accurate. Accordingly, by enlarging the width of the active area relative to the length, the amount of the optical current can be increased. However, when the width is increased, since the sensor TFT occupies much space, it is very difficult to improve the degree of integration of the sensor. 
     To solve the above problems, a method has been developed for increasing the current ratio as a function of light intensity by reducing an off current flowing along a semiconductor layer of the sensor TFT. To realize this, a second sensor gate electrode is provided between a first sensor gate electrode and a semiconductor layer. 
     FIG. 1B shows a conventional sensor TFT. 
     The conventional sensor TFT comprises a first gate electrode  23  for performing an On/Off operation of a transistor by receiving a voltage from a gate wiring; second gate electrodes  27   a  and  27   b  disposed on the first gate electrode, the second gate electrodes  27   a  and  27   b  spaced away from each other in parallel; a semiconductor layer  31  formed on the second gate electrodes  27   a  and  27   b ; and source and drain electrodes  33  and  35  disposed on the second gate electrodes  27   a  and  27   b , respectively. 
     An exposed portion of the semiconductor layer  31  between the source and drain electrodes  33  and  35  is an active area or conducting channel which has a length L and a width W. That is, the length L becomes a channel length of the semiconductor layer along which the optical current flows, and the width W becomes a channel width of the semiconductor layer. 
     FIG. 2 is a sectional view taken along line II—II of FIG. 1B for illustrating a manufacturing process of the sensor TFT. 
     A metal conductive layer is first deposited on a glass substrate  21 , then patterned into the first gate electrode  23 . A first insulating layer  25  is formed on the substrate, covering the first gate electrode  23 . 
     The second gate electrodes  27   a  and  27   b  are formed on the first insulating layer  25 , then a second insulating layer  29  is formed on the substrate while covering the second gate electrodes  27   a  and  27   b.    
     An amorphous silicon layer is deposited on the second insulating layer  29 , then patterned into the semiconductor layer  31 . 
     A contact hole  28  is formed on the second insulating layer  29  so that the drain electrode  35  can be electrically connected to the second gate electrode  27   a.    
     Next, the source and drain electrodes  33  and  35  are formed on the second insulating layer  29  while respectively covering both edges of the semiconductor layer  31 . 
     Finally, a protecting layer  37  is formed covering the semiconductor layer  31 , and the source and drain electrodes  33  and  35 . 
     In the above described sensor TFT, the first gate electrode  23  is always applied with a negative voltage as the sensor TFT operates with an optical current created by light in an Off state. The optical current created by the light reflected from an object flows along the semiconductor layer  31 . At this point, a hole is generated at a portion of the semiconductor layer  31  contacting the second insulating layer  29  by the negative voltage applied to the first gate electrode  23 . A current generated by the hole is called an Off current. In this state, when the light is radiated, electron-hole pairs are formed on the semiconductor layer  31  by the light energy. 
     The holes of the electron-hole pairs are directed to the source electrode  33  along the hole channel formed by a gate voltage, and the electrons are directed to the drain electrode  35  to produce optical current. 
     Since there is a limit to an amount of the optical current generated in a TFT having a predetermined ratio between the width and length of the channel, if the amount of the off-current is too much, the display quality will be not good. 
     Accordingly, to increase an optical current ratio by reducing the amount of the off-current, the second gate electrode  27   a  is provided between the semiconductor layer  31  and the first gate electrode  23 . That is, to apply a positive voltage to the source and drain electrodes  33  and  35 , the drain electrode  35  is connected to the second gate electrode  27   a  so that an equi-potential can be generated on the semiconductor layer  31  disposed between the drain electrode  35  and the second gate electrode  27   a . The equi-potential characteristic suppresses the generation of holes, reducing the amount of the off-current flowing along the semiconductor layer. 
     However, in the above-described conventional sensor TFT, since an additional process for forming the second gate electrodes  27   a  and  27   b  is further required, the manufacturing process is complicated. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a thin film transistor type photo detecting sensor, a thin film transistor and a method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a sensor TFT that can increase the amount of the optical current flowing along the semiconductor layer without using the second gate electrodes. 
     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. 
     To achieve the above-need, the present invention provides a sensor TFT comprising a substrate, a gate electrode formed on the substrate; an insulating layer covering the substrate and gate electrode a semiconductor layer patterned on the insulating layer to generate an optical current using received light, source and drain electrodes formed on the semiconductor layer, the source and drain electrodes being spaced apart from each, and a conductive channel defined by a space between the source and drain electrodes, the conductive channel being non-rectangularly-shaped. 
     The source and drain electrodes are preferably made of a transparent material selected from the group consisting of ITO, TiO 2  and SnO 2 . 
     The gate electrode is rectangular-shaped having first, second, third and fourth sides, the first side being connected to a gate wire. 
     The source electrode comprises a plurality of horizontal portions disposed parallel to the first side of the gate electrode and a vertical portion interconnecting ends of the horizontal portions proximal to the second sides of the gate electrode, the vertical portion being connected to a source electrode wiring; and the drain electrode comprises a plurality of horizontal portions which are disposed in parallel between the horizontal portions of the source electrode and a vertical portion interconnecting ends of the horizontal portions proximal to the third side of the gate electrode. 
     The source electrode comprises a vertical portion connected to a source electrode wiring and disposed parallel to the second side of the gate electrode, a first horizontal portion extending from one end of the vertical portion proximal to the first side to be parallel to the first side of the gate electrode, a second horizontal portion extending from the other end of the vertical portion proximal to the fourth side of the gate electrode to be parallel to the fourth side of the gate electrode, and a plurality of vertical branch portions alternately branched off from the first and second horizontal portions to be parallel to the vertical portion, and the drain electrode at a predetermined distance from each portion of the source electrode. 
     The conductive channel is S-shaped or saw-tooth-shaped. 
     According to another aspect of the present invention, a combination comprises a gate electrode formed on a substrate, an insulating layer formed on the gate electrode, a semiconductor layer formed on the insulating layer, and source and drain electrodes source and drain electrodes formed on the semiconductor layer, the source and drain electrodes being spaced away from each to define a conductive channel and the conductive channel having at least one curve or bend. 
     According to still another aspect of the present invention, a TFT type optical sensor comprises a sensor part comprising a gate electrode, a semiconductor layer, and source and drain electrode spaced away from each other; a storage capacitor connected to the drain electrode to store charges discharged from the drain electrode; and a switching part for selectively outputting the charges stored in the storage capacitor to an external system. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     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. 1A is a diagram of a conventional TFT type optical detecting sensor; 
     FIG. 1B is a plane view of a conventional sensor TFT; 
     FIG. 2 is a sectional view taken along a line II—II of FIG. 1B; 
     FIG. 3 is a plane view of a sensor TFT according to a first embodiment of the present invention; 
     FIG. 4 is a sectional view taken a line IV—IV of FIG. 3; 
     FIG. 5 is a plane view of a sensor TFT according to a second embodiment of the present invention; 
     FIG. 6 is a sectional view taken along a line VI—VI of FIG. 5; 
     FIG. 7 a  is a plane view of a sensor TFT according to a third embodiment of the present invention; 
     FIG. 7 b  is a comparative example view for illustrating a difference in a channel width; 
     FIG. 8 a  is a plane view of a sensor TFT according to a fourth embodiment of the present invention; and 
     FIG. 8 b  is an enlarged view of a portion A of FIG. 8 a.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiment of the present invention, example of which is illustrated in the accompanying drawings. 
     FIG. 3 shows a plane view of a sensor TFT according to a first embodiment of the present invention, and FIG. 4 shows a sectional view taken along a line IV—IV of FIG.  3 . 
     A gate electrode  243  for turning On/Off a transistor by receiving a voltage from a gate wiring  111  is formed on a substrate  241 . An insulating layer  245  is formed on the substrate  241  while covering the gate electrode  243 . The gate electrode  243  is rectangular-shaped having first, second, third and fourth sides  243   a ,  243   b ,  243   c  and  243   d . Formed on the gate electrode  243  is an island semiconductor layer  247 . Source and drain electrodes  251  and  253  are formed on the insulating layer  245  while covering a portion of the semiconductor layer  247 . The source electrode  251  comprises a plurality of horizontal portions  251   c  disposed parallel to the first side  243   a  of the gate electrode  243  and a vertical portion  251   a  interconnecting ends of the horizontal portions  251   c  proximal to the second sides  243   b  of the gate electrode  243 . The vertical portion  251   a  of the source electrode  251  is connected to a source electrode wiring  251   b . The drain electrode  253  comprises a plurality of horizontal portions  253   c , which are disposed in parallel between the horizontal portions  251   c  of the source electrode  251 , and a vertical portion  253   a  interconnecting ends of the horizontal portions  251   c  proximal to the third side  243   c  of the gate electrode  243 . 
     Accordingly, the length L of the channel of the semiconductor layer becomes short, while the width W of the channel of the semiconductor layer becomes long. Accordingly, the W/L ratio is substantially increased. 
     A process for manufacturing the above-described sensor TFT will be described hereinafter with reference to FIG.  4 . 
     The gate electrode  243  is first formed on the substrate  241  in a predetermined pattern. The gate electrode is made of a material selected from the group consisting of Al, Mo, Cr, W, and W—Mo. 
     Next, the insulating layer  245  is formed on the substrate while covering the gate electrode  243 , then the semiconductor layer  247  is deposited on the insulating layer  245 . The insulating layer  245  is made of SiNx or SiOx, and the semiconductor layer  247  is made of a-Si:H. 
     Next, ohmic contact layers  249   a  and  249   b  are formed on edges of the semiconductor layer  247  to reduce a contact resistance with the source and drain electrodes  251  and  253  which will be formed on the ohmic contact layers  249   a  and  249   b , respectively. 
     The source and drain electrodes  251  and  253  are preferably formed of a transparent material selected from the group consisting of ITO, TiO, and SnO 2 . 
     The drain electrode  253  is used to apply a signal voltage to the semiconductor layer  247 , and the source electrode  251  is used to discharge current flowing along the semiconductor layer  247  to a storage device. 
     At this point, the charges discharged to the storage device is output as display information by being transmitted to a circuit portion by a switching portion. 
     Finally, a protecting layer  255  is formed on the substrate while covering the semiconductor layer  247  and the source and drain electrodes  251  and  253 . Preferably, the protecting layer  255  is formed of a transparent organic material such as acryl, polyamid and benzocyclobutene. 
     FIG. 5 shows a plane view of a sensor TFT according to a second embodiment of the present invention, and FIG. 6 shows a sectional view taken along a line VI—VI of FIG.  5 . 
     A gate electrode  459  for turning On/Off the transistor by receiving a voltage from a gate wiring  457  is formed on a substrate  461 . An insulating layer  465  is formed on the substrate  461  while covering the gate electrode  459 . The gate electrode  459  is rectangular-shaped having first, second, third and fourth sides  459   a ,  459   b ,  459   c , and  459   d . An insulating layer  465  is formed on the substrate  461  while covering the gate electrode  459 . Source and drain electrodes  473  and  471  are formed on the insulating layer  465  while covering a portion of the semiconductor layer  467 . The source electrode  473  comprises a vertical portion  473   a  connected to a source electrode wiring  473   f  and disposed parallel to the second side  459   b  of the gate electrode  459 , a first horizontal portion  473   c  extending from one end of the vertical portion  473   a  proximal to the first side  459   a  to be parallel to the first side  459   a  of the gate electrode  459 , a second horizontal portion  473   b  extending from the other end of the vertical portion  473   a  proximal to the fourth side  459   d  of the gate electrode  459  to be parallel to the fourth side  459   d , and a plurality of vertical branch portions  473   d  alternately branched off from the first and second horizontal portions  473   c  and  473   b  to be parallel to the vertical portion  473   a . The drain electrode  471  is disposed at a predetermined distance from each portion of the source electrode  473 . 
     In the above-described sensor TFT, the ratio of the width W of the channel of the semiconductor layer  467  with respect to the length L of the semiconductor layer  462 , which is defined by the shortest horizontal distance between the source and drain electrodes  473  and  471 , can be increased. As a result, since the amount of off-current can be reduced, the display characteristic can be improved. 
     Since a process for manufacturing the above-described sensor TFT is identical to the first embodiment, the detailed description thereof will be omitted herein. 
     FIG. 7 a  shows a sensor TFT according to a third embodiment of the present invention. 
     As shown in the drawing, a gate electrode  509  is formed on an insulating layer (not shown) on which a semiconductor layer  515  is formed. Source and drain electrodes  511  and  513  are formed on the semiconductor layer  515 . The source electrode  511  comprises a concave semi-circular portion  511   a  and a convex semi-circular portion  511   b  extending from the concave semi-circular portion  511   a . The drain electrode  513  comprises a convex semi-circular portion  513   a  spaced apart from and disposed complementary to the concave semi-circular portion  511   a  of the source electrode  511 , and a concave semi-circular portion  513   b  extending from the convex semi-circular portion  513   a  and spaced apart from and complementary to the convex semi-circular portion  511   b  of the source electrode  511   a . Therefore, a portion of the semiconductor layer  515  exposed between the source and drain electrodes  511  and  513  becomes S-shaped. This S-shaped portion allows the width of a semiconductor channel to be increased. That is, the width of the channel becomes a sum of the circumferences of the concave and convex semi-circular portions. Accordingly, the width W of the channel can be calculated by the following formula when a radius of each semicircular portion is R: 
     
       
           W= (½)*2π R* 2=2π R   
       
     
     According to the above formula, the width W of the channel of this embodiment becomes 6.28R. 
     If the facing ends of the source and drain electrodes  501  and  503  are formed in a straight line as shown in FIG. 7 b , the width W′ of the channel becomes 4R. 
     Accordingly, the width of the channel in this embodiment is increased by 2.28R when compared with the width of the conventional channel shown in FIG. 7 b.    
     FIG. 8 a  shows a sensor TFT according to a fourth embodiment of the present invention. 
     As shown in the drawing, a gate electrode  609  is formed on an insulating layer (not shown) on which a semiconductor layer  615  is formed. Source and drain electrodes  611  and  613  are formed on the semiconductor layer  615 . 
     The source and drain electrodes  611  and  613  face each other at a predetermined distance. 
     Facing ends of the source and drain electrodes  611  and  613  are saw-toothed complementary to each other. Accordingly, a width of the semiconductor channel of this embodiment becomes a sum of lengths of lines a, b, c and d taken along the saw-toothed ends. When the lengths of the lines a, b, c and d are identical to each other, the length of one line can be calculated according to the following formulas: 
     
       
           R=a  sin  K   (1) 
       
     
     
       
           a=R /sin  K   (2) 
       
     
     wherein K is half of an angle of a valley of the saw-toothed portion, and R is one-fourth the straight-line width of the semiconductor layer. 
     Accordingly, when assuming that K is 45 degrees, the length of the line “a” becomes 2R. Therefore, the width of the semiconductor channel of this embodiment becomes 5.65R. This shows that the width of this embodiment is longer than the straight-line width by 1.56R. 
     In the above embodiments, since a second gate electrode, which is formed in the conventional art, is not used in the present invention, the manufacturing process can be simplified. In addition, by varying the shape of the source and drain electrodes, semiconductor layer having a very wide active area can be obtained, increasing the ratio between the width and length. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the thin film transistor type photo detecting sensor, thin film transistor and the method for fabricating of same 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.