Abstract:
A thin-film transistor (TFT) is described to have a gate layer, an insulating layer, a semiconductor layer, and a source/drain layer formed on a flexible substrate. The source and the drain layers are separated by a channel with a special shape. This does not only increase the aspect ratio of the channel per unit area, the source and the drain also have multiple directions for transmitting carriers. The carrier mobility of the TFT is thus enhanced with uniform and stable circuit properties.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The invention relates to a TFT and, in particular, to a TFT with a special structure.  
         [0003]     2. Related Art  
         [0004]     The active layer of the TFT is made of semiconductor materials to increase the carrier mobility. Therefore, they have been widely used in circuits of various functions. However, the active layer has grains of different sizes. Such intrinsic defects will reduce the carrier mobility. Moreover, the TFT itself requires a higher working voltage. For example, the carrier mobility of an α-Si TFT is between 0.5 cm 2 /V.S and 1 cm 2 /V.S, whereas that of a poly-Si TFT is between 30 cm 2 /V.S and 300 cm 2 /V.S  
         [0005]     Under the restriction of lower carrier mobility due to the above-mentioned intrinsic defects, it is necessary to have a sufficiently large driving current to charge pixel capacities. This can only be achieved by increasing the aspect ratio, W/L, of the channel. However, one then faces such problems as increasing area and lower aperture rate. The gate-drain and gate-source interfaces of the TFT are working under a huge electric field. Therefore, the kink effect is likely to occur. This in turn will result in the problems of a shorter lifetime and functioning instability.  
         [0006]     There are two solutions to improve the intrinsic defects of the TFT. One is to improve the manufacturing process. This is a big engineering problem that requires a huge amount of manpower, time, and capital. The other is to change the structure of the TFT. As shown in  FIG. 1 , the conventional TFT has a structure with a gate  10 , a source  20 , and a drain  30 . A rectangular channel  40  is formed between the source  20  and the drain  30 . It occupies a larger area when the channel aspect ratio is fixed. This needs to be improved. Moreover, as shown in  FIGS. 2A and 2B , the conventional TFT has a low architecture deflection in the vertical and horizontal directions. Therefore, it is not suitable for flexible circuits. Also, as shown in  FIGS. 3A  to  3 D, the process control migration is small. Once there is any deviation in the process, the electrical performance will be bad. As shown in  FIG. 4 , the structure of the conventional TFT is likely to be locally over-heated; that is, heat concern of hot spots a-d will be generated. In the future, the substrate in the TFT process can be changed from the current rigid substrate to the flexible substrate, so that it is more convenient to carry and use. Therefore, the TFT itself has to be flexible too, and the element characters are not to be seriously changed or damaged by the deflection of the substrate. The conventional TFT structure is totally unsuitable for the above purposes. Therefore, it is imperative to provide a new TFT to solve these problems.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the foregoing, an object of the invention is to provide a TFT which, through a special structure design, can avoid the undesired effects due to its intrinsic defects and the electrical property changes due to the deflection of the substrate.  
         [0008]     To achieve the above object, the disclosed TFT is formed with a source/drain layer, a gate layer, an insulating layer, a semiconductor layer, and a flexible substrate. The source/drain layer, the gate layer, the insulating layer, and the semiconductor layer are formed on the flexible substrate. The source/drain layer contains a source, a drain, and a channel. The channel encloses and defines a peninsula region with one open end. One of the source and the drain is located inside the peninsula region, while the other is outside the channel. The source and the drain have two or more transmission directions. The gate layer is provided in the direction perpendicular to the channel of the source/drain layer. The insulating layer is then used to separate the source/drain layer and the gate layer. The semiconductor layer is connected to the source/drain layer and the insulating layer.  
         [0009]     Moreover, another TFT disclosed herein is formed with a source/drain layer, a semiconductor layer, an insulating layer, a gate layer, and a flexible substrate. The source/drain layer, the gate layer, the insulating layer, and the semiconductor layer are formed on the flexible substrate. The source/drain layer contains a source, a drain, and a channel. The channel encloses and defines an island region, which is a closed region. One of the source and the drain is located inside the island region, while the other is outside the channel. The source and the drain have two or more transmission directions. The gate layer is provided in the direction perpendicular to the channel of the source/drain layer. The insulating layer is then used to separate the source/drain layer and the gate layer. The semiconductor layer is connected to the source/drain layer and the insulating layer.  
         [0010]     The disclosed TFT with the above-mentioned structure does not only have a higher channel area per unit area, such a channel design also increases the transmission directions of the carriers between the source and the drain. Therefore, the disclosed TFT has such advantages as a lower grain boundary trap effect, higher carrier mobility, a more uniform current, a higher driving capability, and reducing the field and kink effects.  
         [0011]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0013]      FIG. 1  is a schematic top view of the conventional TFT structure;  
         [0014]      FIGS. 2A and 2B  show respectively the deflection of the conventional TFT in the vertical and horizontal directions;  
         [0015]      FIGS. 3A  to  3 D show respectively the situations that a conventional TFT is deflected to the left, right, up, and down;  
         [0016]      FIG. 4  shows that a conventional TFT is locally over-heated;  
         [0017]      FIGS. 5A and 5B  are schematic cross-sectional and top views of the TFT in a first embodiment of the invention;  
         [0018]      FIGS. 6A  to  6 C schematically show the source/drain layer with different shapes of channels according to the first embodiment;  
         [0019]      FIGS. 7A and 7B  show respectively the TFT with different areas of gate layers in the first embodiment, where each gate layer has an opening region;  
         [0020]      FIG. 8  is a schematic top view of the TFT in a second embodiment of the invention;  
         [0021]      FIGS. 9A and 9B  are schematic views of the source/drain layer with different shapes of channels in the second embodiment;  
         [0022]      FIGS. 10A and 10B  are schematic views of the TFT&#39;s with different areas of gate layers in the second embodiment, where each gate layer has an opening region;  
         [0023]      FIG. 11  is a schematic view of using the TFT in  FIG. 7B  as the switch of pixels in the panel;  
         [0024]      FIGS. 12A and 12B  show that the TFT of  FIG. 7B  is deflected in the vertical and horizontal directions, respectively;  
         [0025]      FIGS. 13A  to  13 D show that the TFT in  FIG. 7B  is deviated respectively to the left, right, up and down;  
         [0026]      FIG. 14  shows the current distribution in the TFT of  FIG. 7B ; and  
         [0027]      FIGS. 15A  to  15 C show the cross-sectional views of coplanar, inverted coplanar, and staggered TFT&#39;s in  FIG. 7B . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIGS. 5A and 5B  are the cross-sectional and top views of the TFT according to a first embodiment of the invention. The TFT is an inverter staggered TFT of the bottom gate type. The flexible substrate  100  is formed with a gate layer  120 . An insulating layer  110  is formed on the gate layer  120  to provide insulation. The α-Si semiconductor layer  130  is formed on the gate layer  120  and the insulating layer  110 . The source/drain layer  140  is formed on the semiconductor layer  130 . Besides, the disclosed TFT further contains an Ohmic contact layer between the semiconductor layer and the source/drain layer in practice. The Ohmic contact layer is the adhesive layer between the semiconductor layer and the source/drain layer, forming an Ohmic contact in between. The source/drain layer  140  contains a source  141 , a drain  142 , and a channel  143 . The source  141  and the drain  142  are formed inside and outside a channel  143 . The channel  143  is comprised of an annular band and two non-annular regions. A peninsula region is enclosed and defined on the inner side. The peninsula region is a half-closed region with one open end. The source  141  is located inside the peninsula region. Therefore, the structure has a round head and a neck. The gate layer  120  has a shape similar to the source  141 , also with a round head and a neck. However, its area is larger than the source  141 . The drain  142  is provided along the outer side of the channel  143 . Since the carrier transmission between the source  141  and the drain  142  uses the path of the semiconductor layer  130  under the channel  143 , there are multiple carrier transmission directions between the source  141  and the drain  142  in this embodiment.  
         [0029]     In this embodiment, the channel  143  includes an annular band and two non-annular regions so that the source  141  has the shapes of a round head and a neck. The drain  142  has concave arcs. The shape of the gate layer  120  is similar to the source  141 . However, the invention is not limited to this. Moreover, the source  141  and the drain  142  can be provided respectively along the inner and outer sides of the channel  143  or along the outer and inner sides, respectively. Since the source  141  and the drain  142  are separated by the channel  143 , the shapes of the source  141  and the drain  142  need to match the shape of the channel  143 . In this embodiment, the shape of the channel  143  is so to enclose a peninsula region. The shape of the source  141  also has a peninsula shape. As shown in  FIGS. 6A  to  6 C, the peninsula regions defined by the channels  143   a ,  143   b ,  143   c  are roughly in the shapes of a U, a rectangle, and a polygon.  
         [0030]     The profile of the gate layer  120  corresponds to that of the peninsula region. The area of the gate layer  120  can be either smaller or bigger than the peninsula region. Alternatively, as shown in  FIGS. 7A and 7B , the gate layer  120  has an opening region  121   a  or  121   b . Their shapes correspond to the peninsula region. The area of the opening region  121   a  or  121   b  can be smaller ( FIG. 7A ) or bigger ( FIG. 7B ) than the peninsula region.  
         [0031]     As shown in  FIG. 8 , the channel  243  in the second embodiment of the invention has an annular band, whose inner side defines a closed island region. The source  241  is circular, and so is the gate layer  220 . This structure enables more transmission directions between the source  241  and the drain  242 , achieving almost omni-directional. A higher current stability is achieved. In particular, the source  241  is provided with a wire  250  for electrically connecting to outside.  
         [0032]     The shape of the channel  243  is not limited to annular, and the shapes of the source  241  and the drain  242  only need to match with that of the channel. The source  241  has the same shape as the island region. This is illustrated in  FIGS. 9A and 9B . The island regions defined by the channels  243   a ,  243   b , respectively, are roughly rectangular and polygonal.  
         [0033]     The profile of the gate layer  220  corresponds to that of the island region. The area of the gate layer  220  can be either smaller or bigger than the island region. Alternatively, as shown in  FIGS. 10A and 10B , the gate layer  220  has an opening region  221   a  or  221   b . Their shapes correspond to the island region. The area of the opening region  221   a  or  221   b  can be smaller ( FIG. 10A ) or bigger ( FIG. 10B ) than the peninsula region.  
         [0034]     In the following, we use the TFT in  FIG. 7B  as an example to explain the features and advantages of the invention.  
         [0035]     As shown in  FIG. 11 , when using the TFT as the switch of pixels in a panel, it is formed at a corner of the cross of a gate line  300  and a data line  310 . The drain  142  extends from the data line  310 . The gate layer  120  extends from the gate line  300 . The source  141  is connected to a capacitor  320 . In comparison with the conventional TFT, the invention has a smaller area for the same channel aspect ratio. The invention reduces the area occupied by the pixels. In other words, the disclosed TFT has a wider channel for the same total area. This can increase the carrier mobility, so that the charging/discharge speed of the pixel is increased for a better display quality.  
         [0036]     Moreover, the channel design in the disclosed TFT enables multiple transmission directions between the source  141  and the drain  142 , unlike the conventional TFT that has only one transmission direction with worse electrical performance. In contrast, not only can the TFT in this embodiment reduce the grain boundary trap effect and increase the carrier mobility, current homogeneity, and driving capability, it further has the advantages of reducing field and kink effects.  
         [0037]     As illustrated in  FIGS. 12A and 12B , the TFT according to this embodiment is very flexible. In practice, the disclosed TFT is deflectable in almost all directions. Thus, it is very suitable for a flexible circuit. In comparison with the prior art, the driving current of the TFT is more uniform and therefore more stable.  
         [0038]     As depicted in  FIGS. 13A  to  13 D, when any deviation happens during the manufacturing process of the TFT (to the left, right, up or down), the symmetric structure of the disclosed TFT renders a smaller deviation. Therefore, the TFT has the same gate-drain capacitance (Cgd) and gate-source capacitance (Cgs). Moreover, the channel aspect ratio is fixed. This increases the yield and lowers the process cost.  
         [0039]     As shown in  FIG. 14 , due to the symmetric structure of the disclosed TFT, the current is more uniform and thus avoids the hot spot problem. It is not over-heated and has a uniform electric field.  
         [0040]     The TFT of the current embodiment can be of the bottom contact type, the top contact type, the bottom gate type, or the top gate type.  FIGS. 15A  to  15 C show the coplanar, inverted coplanar, and staggered TFT&#39;s.  
         [0041]     Therefore, the invention develops a new special structure for the TFT without changing the process conditions. In addition to obtaining a larger channel aspect ratio within a smaller area, the invention also overcomes the electrical performance problem due to its intrinsic defects. It can be used in the flexible display technology to reduce possible abrupt changes in its electrical properties and to avoid the problem of lower display quality when the TFT experiences deflections in any direction.  
         [0042]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.