Patent Publication Number: US-9905627-B2

Title: Thin film transistor substrate and organic light-emitting diode display including the same

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2015-0082567, filed on Jun. 11, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     Field 
     The described technology generally relates to a thin film transistor substrate and an organic light-emitting diode display including the same. 
     Description of the Related Technology 
     There is a demand for display devices to be thin and lightweight. As the demand for display devices having high resolution and a wide display area has increased, there is also a need for high-quality thin film transistors that are small and not influenced by unintended voltage drops. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect relates to a thin film transistor substrate and an organic light-emitting diode (OLED) display including the same. 
     Another aspect is a thin film transistor substrate that includes a substrate; and a thin film transistor located on the substrate, wherein the thin film transistor includes: a lower gate electrode; a first insulating layer covering the lower gate electrode; an oxide semiconductor layer located on the first insulating layer; an inner electrode located on the oxide semiconductor layer and in an island shape; an outer electrode located on the oxide semiconductor layer and surrounding the inner electrode; a second insulating layer covering the oxide semiconductor layer; and an upper gate electrode located on the second insulating layer, wherein the oxide semiconductor layer includes a center region and an outer region surrounding the center region and an intermediate region between the center region and the outer region, wherein the inner electrode is located on the center region of the oxide semiconductor layer, wherein the outer electrode is located on the outer region of the oxide semiconductor layer, and wherein the lower gate electrode and the upper gate electrode are disposed to face each other with the intermediate region of the oxide semiconductor layer disposed between the lower gate electrode and the upper gate electrode. 
     The inner electrode may include a source electrode, and the outer electrode may include a drain electrode. 
     The lower gate electrode may include a first opening corresponding to the inner electrode, and the upper gate electrode may include a second opening corresponding to the first opening. 
     A size of the inner electrode may be greater than a size of the first opening and smaller than a size of the second opening. 
     The lower gate electrode may have a first width from an inner end portion adjacent to the first opening to an outer end portion of the lower gate electrode, and the upper gate electrode may have a second width from an inner end portion adjacent to the second opening to an outer end portion of the upper gate electrode. 
     The first width may be greater than the second width. 
     The first width may be greater than a width of an intermediate region of the oxide semiconductor layer, and at least one of the inner end portion and the outer end portion of the lower gate electrode may partially overlap the inner electrode or the outer electrode. 
     The second width may be less than a width of the intermediate region of the oxide semiconductor layer, wherein the inner end portion of the upper gate electrode is spaced apart from the outer end portion of the inner electrode in a direction toward the outer electrode from the inner electrode, and wherein the outer end portion of the upper gate electrode is spaced apart from the inner end portion of the outer electrode in a direction toward the inner electrode from the outer electrode. 
     The inner electrode and the outer electrode may be located on the same layer. 
     The lower gate electrode and the upper gate electrode may have the same voltage level. 
     The thin film transistor substrate may further include: a connection wiring located on the second insulating layer and electrically connected to the inner electrode. 
     Another aspect is an OLED display that includes a plurality of pixels arranged on a substrate, wherein each of the plurality of pixels includes: a switching thin film transistor located on the substrate; a driving thin film transistor located on the substrate and electrically connected to the switching thin film transistor; a pixel electrode connected to the driving thin film transistor; an emissive layer located on the pixel electrode; and a counter electrode facing the pixel electrode with the emissive layer disposed between the counter electrode and the pixel electrode, wherein the driving thin film transistor includes: an oxide semiconductor layer; an inner electrode located on a center region of the oxide semiconductor layer in an island shape; an outer electrode located on an outer region of the oxide semiconductor layer, wherein the outer region is spaced apart from the center region and surrounds the center region; and lower and upper gate electrodes disposed to face each other with the oxide semiconductor layer disposed between the lower and upper gate electrodes. 
     The inner electrode may include a source electrode, and the outer electrode may include a drain electrode. 
     The lower gate electrode may include a first opening corresponding to the inner electrode, and the upper gate electrode may include a second opening corresponding to the first opening. 
     The first opening, the second opening, and the inner electrode may be disposed on the same axial line. 
     An inner end portion of the lower gate electrode adjacent to the first opening may partially overlap the inner electrode. 
     A size of the second opening of the upper gate electrode may be greater than a size of the inner electrode. 
     The inner electrode and the outer electrode may be located on the same layer. 
     The lower gate electrode and the upper gate electrode may have the same voltage level. 
     The upper and lower gate electrodes of the driving thin film transistor may control a grayscale of each of the plurality of pixels including the driving thin film transistor. 
     Another aspect is a thin film transistor (TFT) substrate, comprising: a substrate and a TFT located on the substrate. The TFT comprising: a lower gate electrode; a first insulating layer covering the lower gate electrode; an oxide semiconductor layer located on the first insulating layer; a first electrode located on the oxide semiconductor layer and having an island shape; a second electrode located on the oxide semiconductor layer and surrounding the first electrode; a second insulating layer at least partially covering the oxide semiconductor layer; and an upper gate electrode located on the second insulating layer. The oxide semiconductor layer includes a first region, a second region surrounding the first region, and a third region interposed between the first and second regions, wherein the first electrode is located in the first region of the oxide semiconductor layer, wherein the second electrode is located in the second region of the oxide semiconductor layer, wherein the lower and upper gate electrodes face each other, and wherein the third region of the oxide semiconductor layer is interposed between the lower and upper gate electrodes. 
     In the above TFT substrate, the first electrode includes a source electrode, wherein the second electrode includes a drain electrode. 
     In the above TFT substrate, the lower gate electrode has a first opening corresponding to the first electrode, wherein the upper gate electrode has a second opening corresponding to the first opening. 
     In the above TFT substrate, the first electrode is larger than the first opening and smaller than the second opening. 
     In the above TFT substrate, the lower gate electrode has a first width defined from an inner end portion, adjacent to the first opening, to an outer end portion of the lower gate electrode, wherein the upper gate electrode has a second width defined from an inner end portion, adjacent to the second opening, to an outer end portion of the upper gate electrode. 
     In the above TFT substrate, the first width is greater than the second width. 
     In the above TFT substrate, the first width is greater than a width of the third region of the oxide semiconductor layer, wherein at least one of the inner and outer end portions of the lower gate electrode partially overlaps the first electrode or the second electrode. 
     In the above TFT substrate, the second width is less than a width of the third region of the oxide semiconductor layer, wherein the inner end portion of the upper gate electrode is spaced apart from the outer end portion of the first electrode in a direction extending from the first electrode toward the second electrode, and wherein the outer end portion of the upper gate electrode is spaced apart from the inner end portion of the second electrode in a direction extending from the second electrode toward the first electrode. 
     In the above TFT substrate, the first and second electrodes are located on the same layer. 
     In the above TFT substrate, the lower and upper gate electrodes are configured to have the same voltage level. 
     The above TFT substrate further comprises a connection wiring located on the second insulating layer and electrically connected to the first electrode. 
     Another aspect is an organic light-emitting diode (OLED) display, comprising: a plurality of pixels disposed on a substrate. Each of the pixels comprises: a switching thin film transistor (TFT) located on the substrate; a driving TFT located on the substrate and electrically connected to the switching TFT; a pixel electrode electrically connected to the driving TFT; an emissive layer located on the pixel electrode; and a counter electrode facing the pixel electrode with the emissive layer interposed between the counter electrode and the pixel electrode. The driving TFT comprises: an oxide semiconductor layer; a first electrode located on a first region of the oxide semiconductor layer and having an island shape; a second electrode located on an second region of the oxide semiconductor layer, wherein the second region is spaced apart from the first region and surrounds the first region; and lower and upper gate electrodes facing each other with the oxide semiconductor layer disposed between the lower and upper gate electrodes. 
     In the above OLED display, the first electrode includes a source electrode, wherein the second electrode includes a drain electrode. 
     In the above OLED display, the lower gate electrode has a first opening corresponding to the first electrode, wherein the upper gate electrode has a second opening corresponding to the first opening. 
     In the above OLED display, the first opening, the second opening, and the first electrode are disposed on the same axial line. 
     In the above OLED display, an inner end portion of the lower gate electrode adjacent to the first opening partially overlaps the first electrode. 
     In the above OLED display, the second opening of the upper gate electrode is larger than the first electrode. 
     In the above OLED display, the first and second electrodes are located on the same layer. 
     In the above OLED display, the lower and upper gate electrodes are configured to have the same voltage level. 
     In the above OLED display, the upper and lower gate electrodes of the driving TFT are configured to control a grayscale of a selected pixel including the driving TFT. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1, 3, 5, and 7  are cross-sectional views illustrating stages in an embodiment of a manufacturing method of a thin film transistor substrate. 
         FIGS. 2, 4, 6, and 8  are plan views illustrating stages in an embodiment of a manufacturing method of a thin film transistor substrate. 
         FIG. 9  is a graph showing characteristics of thin film transistors according to an exemplary embodiment and a comparative example 1. 
         FIG. 10  is a graph showing characteristics of thin film transistors according to comparative examples 2 and 3. 
         FIG. 11  is a graph showing a characteristic of a thin film transistor according to a comparative example 4. 
         FIG. 12  is a graph showing a characteristic of a thin film transistor according to a comparative example 5. 
         FIG. 13  is a block diagram schematically showing an OLED display according to an exemplary embodiment. 
         FIG. 14  is an equivalent circuit diagram of one pixel of the OLED display of  FIG. 13 . 
         FIG. 15  is a cross-sectional view of a driving thin film transistor and an OLED of the OLED display of  FIG. 14 . 
         FIG. 16  is a diagram of a plurality of pixels that are included in one column of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS 
     Since the described technology may have various modifications and several embodiments, exemplary embodiments are shown in the drawings and will be described in detail. Advantages, features, and a method of achieving the same will be specified with reference to the exemplary embodiments described below in detail together with the attached drawings. However, the exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. 
     Exemplary embodiments of the described technology will be described below in more detail with reference to the accompanying drawings. Those components that are the same or are in correspondence are rendered the same reference numeral in the drawings, and redundant explanations are omitted. 
     It will be understood that although the terms “first,” “second,” etc. may be used herein to describe various components, these components should not be limited by these terms. These components are only used to distinguish one component from another. 
     As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In the exemplary embodiments below, it will be further understood that the terms “comprise” and/or “have” used herein specify the presence of stated features or components, but do not preclude the presence or addition of one or more other features or components. 
     In the exemplary embodiments below, it will be understood when a portion such as a layer, an area, or an element is referred to as being “on” or “above” another portion, it can be directly on or above the other portion, or intervening portion may also be present. 
     Also, in the drawings, for convenience of description, sizes of elements may be exaggerated or contracted. In other words, since sizes and thicknesses of components in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments are not limited thereto. 
     When an exemplary embodiment is implementable in another manner, a predetermined process order may be different from a described one. For example, two processes that are consecutively described may be substantially simultaneously performed or may be performed in an opposite order to the described order. 
     It will be understood when a portion such as a layer, an area, or an element is referred to as being connected, it can be connected directly, or intervening portion may also be present. For example, it will be understood that when a portion such as a layer, an area, or an element is referred to as being electrically connected, it can be electrically connected directly, or intervening portion may also be present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. In this disclosure, the term “substantially” includes the meanings of completely, almost completely or to any significant degree under some applications and in accordance with those skilled in the art. The term “connected” can include an electrical connection. 
       FIGS. 1, 3, 5, and 7  are cross-sectional views illustrating stages in an embodiment of a manufacturing method of a thin film transistor substrate.  FIGS. 2, 4, 6, and 8  are plan views illustrating each stages in an embodiment of a manufacturing method of a thin film transistor substrate.  FIGS. 1, 3, 5, and 7  respectively correspond to cross-sectional views taken along lines A-A′ of  FIGS. 2, 4, 6, and 8 . 
     Referring to  FIGS. 1 and 2 , a lower gate electrode  110  formed on a substrate  100 . 
     The substrate  100  may be formed of a glass material, a metal material, or a plastic material such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide, etc. A buffer layer (not shown) having an insulating property such as silicon oxide and a silicon nitride may be formed on the substrate  100  before forming the lower gate electrode  110  on the substrate  100 . 
     The lower gate electrode  110  may be formed on the substrate  100  by forming a metal layer (not shown) and patterning the metal layer. The lower gate electrode  110  may include a first opening OP 1  in a center region thereof and may have a first width W 1  from an inner end portion adjacent to the first opening OP 1  to an outer end portion. At least a part of the lower gate electrode  110  may approximately have a ring shape. The first opening OP 1  of the lower gate electrode  110  may overlap an inner electrode  130  formed during a process that will be described later, thereby preventing generation of parasitic capacitance between the inner electrode  130  and the lower gate electrode  110 . 
     The lower gate electrode  110  may be formed to have a single or multi-layer structure by using one or more materials among, for example, aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium (Cr), lithium (Li), calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), copper (Cu), or an alloy thereof, by taking into account an adhesive force with respect to an adjacent layer, a degree of surface planarization of a layer to be stacked, and process ability. 
     Referring to  FIGS. 3 and 4 , an oxide semiconductor layer  120  is formed after forming a first insulating layer  103  on the lower gate electrode  110 . 
     The first insulating layer  103  may be formed to completely cover the substrate  100  and may include an insulating inorganic material such as silicon oxide and silicon nitride or an insulating organic material. 
     The oxide semiconductor layer  120  may be located on the first insulating layer  103  and may be patterned such that a part of the oxide semiconductor layer has an island shape and overlaps the lower gate electrode  110 . The size of the oxide semiconductor layer  120  may be greater than that of the lower gate electrode  110  so that the oxide semiconductor layer  120  may completely cover the lower gate electrode  110 . For example, a diameter of the oxide semiconductor layer  120  is greater than the diameter of the lower gate electrode  110 , i.e. the diameter of an outer end portion of the lower gate electrode  110 , so that the oxide semiconductor layer  120  may completely cover the lower gate electrode  110 . 
     The oxide semiconductor layer  120  may be formed of one or more elements of indium (In), gallium (Ga), stannum (Sn), zirconium (Zr), vanadium (V), hafnium (Hf), cadmium (Cd), germanium (Ge), and zinc (Zn). For example, the oxide semiconductor layer  120  includes indium gallium zinc oxide (IGZO) or/and indium tin zinc oxide (ITZO). As another example, the oxide semiconductor layer  120  includes at least one of SnO 2 , In 2 O 3 , ZnO, CdO, Cd 2 SnO 4 , TiO 2 , etc. 
     Referring to  FIGS. 5 and 6 , the inner electrode  130  and an outer electrode  140  are formed on the oxide semiconductor layer  120 . The inner electrode  130  may be located on a center region of the oxide semiconductor layer  120 . The outer electrode  140  may be located on an outer region of the oxide semiconductor layer  120 . 
     The inner electrode  130  and the outer electrode  140  may be formed on the oxide semiconductor layer  120  and may be electrically connected to the oxide semiconductor layer  120 . The inner electrode  130  may be in an island shape and may be electrically connected to the center region of the oxide semiconductor layer  120 . The outer electrode  140  may be electrically connected to the outer region of the oxide semiconductor layer  120 . The inner electrode  130  may be a source electrode. The outer electrode  140  may be a drain electrode. In this case, the center region of the oxide semiconductor layer  120  may correspond to the source area, and the outer region thereof may correspond to the drain area. 
     Hereinafter, the center region of the oxide semiconductor layer  120 , in which the inner electrode  130  is formed, is referred to as a first area  120   a , the outer region of the oxide semiconductor layer  120 , in which the outer electrode  140  is formed, is referred to as a second area  120   b , and an area of the oxide semiconductor layer  120  exposed between the inner electrode  130  and the outer electrode  140 , i.e. an intermediate region between the first area  10   a  and the second area  120   b  is referred to as a third area  120   c.    
     The outer electrode  140  may include a third opening OP 3  therein. The inner electrode  130  may be disposed in the third opening OP 3 . The size of the inner electrode  130  may be less than that of the third opening OP 3 . For example, the diameter of the inner electrode  130  is less than that of the third opening OP 3  so that the inner electrode  130  may be surrounded by the outer electrode  140  while being spaced apart from the outer electrode  140  by a predetermined distance W 0 . The spaced distance W 0  between the outer electrode  140  and the inner electrode  130 , i.e. a distance between an inner end portion of the outer electrode  140  and an outer end portion of the inner electrode  130 , may correspond to a width of the third area  120   c  of the oxide semiconductor layer  120 . 
     The inner electrode  130  and the outer electrode  140  may include the same material on the same layer. For example, the inner electrode  130  and the outer electrode  140  are formed by forming and patterning a metal layer (not shown) on the oxide semiconductor layer  120 . The inner electrode  130  and the outer electrode  140  may be formed to have a single or multi-layer structure by using one or more materials among, for example, aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium (Cr), lithium (Li), calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), copper (Cu), or an alloy thereof, by taking into account conductivity, etc. 
     The size of the inner electrode  130  may be greater than that of the first opening OP 1  of the lower gate electrode  110 . For example, a diameter of the inner electrode  130  is greater than that of the first opening OP 1  so that the inner electrode  130  partially overlaps the lower gate electrode  110 . In some exemplary embodiments, an outer end portion of the inner electrode  130  partially overlaps an inner end portion of the lower gate electrode  110  adjacent to the first opening OP 1 . OL 1  of  FIG. 5  denotes an area of the inner electrode  130  that partially overlaps the lower gate electrode  110 . 
     The size of the third opening OP 3  formed in the outer electrode  140  may be smaller than that of the lower gate electrode  110 . For example, the diameter of the third opening OP 3  is less than the diameter of the outer end portion of the lower gate electrode  110  so that the outer electrode  140  partially overlaps the lower gate electrode  110 . For example, an inner end portion of the outer electrode  140  partially overlaps the outer end portion of the lower gate electrode  110 . OL 2  of  FIG. 5  denotes an area of the outer electrode  140  that partially overlaps the lower gate electrode  110 . 
     When the third area  120   c  of the oxide semiconductor layer  120  and the lower gate electrode  110  corresponding to the spaced distance W 0  between the outer electrode  140  and the inner electrode  130  have the same shape and area, the electrodes  110 ,  130 , and  140  may be misaligned, and a channel characteristic formed in the third area  120   c  of the oxide semiconductor layer  120  may deteriorate. However, in the exemplary embodiments, since the outer electrode  140  and the inner electrode  130  are formed to partially overlap the lower gate electrode  110 , such problems may be prevented. 
     Referring to  FIGS. 7 and 8 , an upper gate electrode  150  and a connection wiring  160  are formed after forming a second insulating layer  105 . 
     The second insulating layer  105  may be formed to completely cover the substrate  100  and may be formed of an insulating inorganic material such as silicon oxide and silicon nitride or an insulating organic material. 
     The upper gate electrode  150  and the connection wiring  160  may be formed by forming a metal layer (not shown) on the second insulating layer  105  and patterning the metal layer. The upper gate electrode  150  may be formed to have a single or multi-layer structure by using one or more materials among, for example, aluminum (Al), platinum (Pt), palladium (Pd), silver (Ag), magnesium (Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium (Cr), lithium (Li), calcium (Ca), molybdenum (Mo), titanium (Ti), tungsten (W), copper (Cu), or an alloy thereof, by taking into account an adhesive force with respect to an adjacent layer, a degree of surface planarization of a layer to be stacked, and processability. 
     The upper gate electrode  150  may be disposed to face the lower gate electrode  110  with the third area  120   c  of the oxide semiconductor layer  120  disposed between the upper gate electrode  150  and the lower gate electrode  110 . When light of a visible light band is received in the oxide semiconductor layer  120 , a threshold voltage of a thin film transistor or a drain current may unexpectedly increase due to electrons excited by photons. However, according to the exemplary embodiments, the upper and lower gate electrodes  110  and  150  may be used to form a strong field and respectively cover the oxide semiconductor layer  120  above and below, and thus light may be blocked from being received in the oxide semiconductor layer  120 , thereby preventing the threshold voltage or/and the drain current from increasing due to the light. 
     The upper gate electrode  150  may include a second opening OP 2  in a center region thereof. The inner electrode  130  and the second opening OP 2  of the upper gate electrode  150  may be formed to partially overlap each other on the same axial line, thereby preventing generation of parasitic capacitance between the inner electrode  130  and the upper gate electrode  150 . 
     The upper gate electrode  150  may have a second width W 2  from an inner end portion adjacent to the second opening OP 2  to an outer end portion thereof. The second width W 2  of the upper gate electrode  150  may be less than a width of the third area  120   c  of the oxide semiconductor layer  120 , i.e. the spaced distance W 0  between the outer electrode  140  and the inner electrode  130 . Thus, in the plan view of the thin film transistor shown in  FIG. 8 , the upper gate electrode  150  may be spaced apart from the outer electrode  140  and the inner electrode  130  by a predetermined distance between the outer electrode  140  and the inner electrode  130 . For example, an inner end portion of the upper gate electrode  150  is spaced apart from an outer end portion of the inner electrode  130  in a direction toward the outer electrode  140  from the inner electrode  130 , and an outer end portion of the upper gate electrode  150  is spaced apart from an inner end portion of the outer electrode  140  in a direction toward the inner electrode  130  from the outer electrode  140 . 
     If the second width W 2  of the upper gate electrode  150  is the same as the first width W 1  of the lower gate electrode  110 , the upper gate electrode  150  and the inner electrode  130  may partially overlap each other, and thus parasitic capacitance may be generated therebetween, Also, the upper gate electrode  150  and the outer electrode  140  may partially overlap each other, and thus parasitic capacitance may be generated therebetween. However, according to some exemplary embodiments, as described above, the first width W 1  of the lower gate electrode  110  is greater than the spaced distance W 0  between the outer electrode  140  and the inner electrode  130 , and the second width W 2  of the upper gate electrode  150  is less than the spaced distance W 0  between the outer electrode  140  and the inner electrode  130 , thereby preventing deterioration of a channel characteristic due to misalignment and simultaneously minimizing an influence caused by parasitic capacitance. Thus, a kickback voltage and a leakage current may be reduced. 
     The upper gate electrode  150  and the lower gate electrode  110  may receive the same signal and may have the same voltage level. Since the same signal is applied to the lower gate electrode  110  and the upper gate electrode  150 , a channel may be formed in the third area  120   c  of the oxide semiconductor layer  120 . 
     The connection wiring  160  may be electrically connected to the inner electrode  130  through a contact hole formed in the second insulating layer  105 . A signal may be applied to the inner electrode  130  patterned in an island shape through the connection wiring  160 . The connection wiring  160  may be formed of the same material as the upper gate electrode  150  on the same layer as the upper gate electrode  150 . In this case, the upper gate electrode  150  may have a partially open C shape, as shown in  FIG. 8 , such that the connection wiring  160  may extend in one direction and may be connected to another device or wiring. 
     The connection wiring  160  may include the same material as the upper gate electrode  150  on the same layer as the upper gate electrode  150  in the present exemplary embodiment but is not limited thereto. As another exemplary embodiment, the connection wiring  160  may be formed on the upper gate electrode  150  with an insulating layer disposed between the connection wiring  160  and the upper gate electrode  150 . In this case, unlike  FIG. 8 , the upper gate electrode  150  may have a ring shape. 
     A function of the thin film transistor according to exemplary embodiments will now be described in more detail with reference to  FIG. 9  below. 
       FIG. 9  is a graph showing characteristics of thin film transistors according to an exemplary embodiment and a comparative example 1.  FIG. 10  is a graph showing characteristics of thin film transistors according to comparative examples 2 and 3.  FIG. 11  is a graph showing a characteristic of a thin film transistor according to a comparative example 4.  FIG. 12  is a graph showing a characteristic of a thin film transistor according to a comparative example 5. 
     In  FIG. 9 , a thin film transistor of the Embodiment has a structure described with reference to  FIGS. 1 through 8 . A thin film transistor of the Comparative example 1 includes only the lower gate electrode  110  among the upper and lower gate electrodes  110  and  150  included in the thin film transistor of the exemplary embodiment. In the thin film transistors of the Embodiment and the Comparative example 1, the inner electrode  130  is a source electrode, and the outer electrode  140  is a drain electrode. 
     In  FIG. 10 , a thin film transistor of the Comparative example 2 has the same structure as described in the exemplary embodiment, however, the inner electrode  130  of the Comparative example 2 is a drain electrode, and the outer electrode  140  of the Comparative example 2 is a source electrode. A thin film transistor of the Comparative example 3 includes only the lower gate electrode  110  among the upper and lower gate electrodes  110  and  150  included in the thin film transistor of the Comparative example 2. 
     In  FIG. 11 , a thin film transistor of the Comparative example 4 includes only the upper gate electrode  150  among the upper and lower gate electrodes  110  and  150  included in the thin film transistor of the exemplary embodiment. A thin film transistor includes of the Comparative example 5 includes a bar type oxide semiconductor layer and a source electrode and a drain electrode at both end portions of the bar type oxide semiconductor layer, and includes upper and lower gate electrodes disposed to face each other with the oxide semiconductor layer disposed between the upper and lower gate electrodes. 
     In  FIGS. 9 through 12 , V GS  denotes upper and lower gate electrodes (Embodiment, and Comparative examples 2 and 5), the lower gate electrode (Comparative examples 1 and 3), or the upper gate electrode (Comparative example 4). 
     Referring to  FIG. 9 , with regard to the thin film transistor of the Embodiment, a drain current has a substantially uniform value in an area when at least the threshold voltage is applied. Such a characteristic may be used to inhibit a brightness difference for each pixel due to a voltage drop in a large size OLED display as will be described later with reference to  FIG. 16 . 
     With regard to the thin film transistor of the Comparative example 1, a drain current is maintained to be substantially uniform in an area when at least the threshold voltage is applied, similar to the thin film transistor of the exemplary embodiment. However, an increase rate of the drain current with respect to an increase in a gate voltage is reduced more than about 2 times by the thin film transistor of the exemplary embodiment. For example, in the thin film transistor of the Embodiment, unlike the Comparative example 1, the increase rate of the drain current with respect to the increase of the gate voltage is relatively greater than that of the Comparative example 1, which shows that the drain current is slightly adjusted by the gate voltage and/or an operation characteristic is excellent even though the thin film transistor is small. Such characteristics may be used to represent abundant and diverse grayscales in the OLED display as will be described later with reference to  FIG. 16  and implement high resolution. 
     Referring to  FIGS. 10 and 12 , in the thin film transistors of the Comparative examples 2, 3, and 5, a drain current changes according to a change in a drain voltage in an area after a threshold voltage, and thus it is difficult to compensate for a voltage drop in a large display apparatus, unlike the thin film transistor of the Embodiment. 
     Referring to  FIG. 11 , in the thin film transistor of the Comparative example 4, since a drain current is very small such as about several 10 −8  A or so, it is impossible to actually drive the thin film transistor. 
     An OLED display including a thin film transistor according to an exemplary embodiment will now be described below. 
       FIG. 13  is a block diagram schematically showing an OLED display according to an exemplary embodiment. 
     Referring to  FIG. 13 , the OLED display includes a display unit  10  including a plurality of pixels  1 , a scanning driver  20 , a data driver  30 , and a controller  40 . 
     The display unit  10  may include the pixels  1  arranged at intersections of a plurality of scanning lines SL 1  through SLn and a plurality of data lines DL 1  through DLm. The scanning lines SL 1  through SLn may extend in a first direction. The data lines DL 1  through DLm may extend in a second direction. Each of the pixels  1  is connected to one of the scanning lines SL 1  through SLn in  FIG. 12  but is not limited thereto. In some exemplary embodiments, each of the pixels  1  is connected to two or more of the scanning lines SL 1  through SLn according to the number and functions of transistors included in the pixels  1 . 
     The scanning driver  20  may generate and transmit a scanning signal to each pixel  1  through each of the scanning lines SL 1  through SLn. The data driver  30  may transmit a data signal to each pixel  1  through the data lines DL 1  through DLm. The controller  40  may change a plurality of image signals R, G, and B received from an external source to a plurality of image data signals DR, DG, and DB, and transmit the image data signals DR, DG, and DB to the data driver  30 . The controller  40  may generate control signals for controlling driving of the scanning driver  20  and the data driver  30  upon receiving a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, and a clock signal MCLK, and transmit the control signals to each of the scanning driver  20  and the data driver  30 . The controller  40  may generate and transmit a scanning driving control signal SCS for controlling the scanning driver  20  and a data driving control signal DCS for controlling the data driver  30 . 
     Each pixel  1  may receive a first power voltage ELVDD and a second power voltage ELVSS (see  FIG. 13 ) from an external source. The first power voltage ELVDD may be a predetermined high level voltage. The second power voltage ELVSS may be a voltage lower than the first power voltage ELVDD or a ground voltage. 
       FIG. 14  is an equivalent circuit diagram of one pixel  1  of the OLED display of  FIG. 13 .  FIG. 15  is a cross-sectional view of a driving thin film transistor T 2  and an OLED of the OLED display of  FIG. 14 .  FIG. 16  is a diagram of a plurality of pixels that are included in one column of  FIG. 13 . The equivalent circuit diagram of  FIG. 14  is merely an example but the exemplary embodiments are not limited thereto. 
     Referring to  FIG. 14 , each pixel  1  includes at least one switching thin film transistor T 1 , the driving thin film transistor T 2 , a storage capacitor Cst and the OLED that emits light of a predetermined brightness by a driving current bled. 
     A gate electrode of the switching thin film transistor T 1  may be connected to the scanning line SLn. One of a source electrode and a drain electrode may be connected to the data line DLm, and the other one may be connected to one end of the storage capacitor Cst. The switching thin film transistor T 1  may be turned on when a scanning signal is provided from the scanning line SLn and then provide the storage capacitor Cst with a data signal provided from the data line DLm. In this regard, the storage capacitor Cst may be charged with a voltage corresponding to the data signal. 
     Referring to  FIGS. 14 and 15 , the driving thin film transistor T 2  has the same structure as that of the thin film transistors TFT described with reference to  FIGS. 1 through 8  above. The lower gate electrode  110  and the upper gate electrode  150  of the driving thin film transistor T 2  may be electrically connected to the switching thin film transistor T 1 . The outer electrode  140  that is a drain electrode may be electrically connected to one end of the storage capacitor Cst and a driving voltage line PL. The inner electrode  130  that is a source electrode may be connected to a pixel electrode  210  of the OLED. The inner electrode  130  of the driving thin film transistor T 2  may be pixel electrode  210  of the OLED with a third insulating layer  107  disposed between the inner electrode  130  and the pixel electrode  210 . An emissive layer  220  of the OLED may be disposed between the pixel electrode  210  and a counter electrode  230  and formed on a pixel defining layer  108  that exposes the pixel electrode  210 . 
     The driving thin film transistor T 2  may be turned on by the switching thin film transistor T 1  and may control the driving current Ioled that flows from the driving voltage line PL to the counter electrode  230  in correspondence with a voltage value stored in the storage capacitor Cst. The emissive layer  220  of the OLED may emit light having a predetermined brightness by the driving current Ioled. 
     The first power voltage ELVDD may be applied to a plurality of pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n  through the driving voltage line PL as shown in  FIG. 16 . Although the first power voltage ELVDD applied to the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n  needs to be the same ideally, since a voltage drop occurs due to resistance by a length of the driving voltage line PL and/or resistance of a circuit included in each of the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n  actually, a voltage applied to each of the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n  through the driving voltage line PL may be gradually reduced. When such a voltage drop occurs, since the driving current Ioled that flows in the OLED is changed for each of the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n , a brightness deviation may occur for each of the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   n . For example, a voltage supplied to the driving thin film transistor T 2  of the nth pixel  1   n  through the driving voltage line PL, i.e. a voltage in an N point, may be different from a voltage supplied to the first pixel  1   a  by resistance of the driving voltage line PL and a voltage drop caused by the first pixel  1   a  through an n−1th pixel  1   n −1. 
     However, according to the exemplary embodiments, the driving thin film transistor T 2  is a thin film transistor described with reference to  FIGS. 1 through 8 , and thus the driving current Ioled (see  FIG. 13 ) that flows in the OLED of each of the pixels  1   a ,  1   b , . . . ,  1   n −1, and  1   nh  may have substantially the same value. 
     For example, referring to  FIGS. 9, 14, and 15 , when the gate voltage V GS  of the driving thin film transistor T 2  is about 10V and a voltage (drain voltage) supplied to the first pixel  1   a  is about 15V, a drain current Ids of the driving thin film transistor T 2  and the driving current Ioled of the OLED are about 18 μA. Although a voltage (drain voltage) applied to the driving thin film transistor T 2  of the nth pixel In by the voltage drop drops to about 10 V, since the driving thin film transistor T 2  has the characteristic as shown in  FIG. 9 , the drain current Ids of the driving thin film transistor T 2  of the nth pixel In and the driving current Ioled of the OLED of the first pixel  1   a  may be the same such as about 18 μA. Thus, although the voltage drop occurs in the OLED display, an influence on each of the pixels  1   a ,  1   b , . . . ,  1   n −1 may be minimized or removed. 
     In the driving thin film transistor T 2  of each of the pixels  1   a ,  1   b , . . . ,  1   n −1, as described with reference to  FIG. 9  above, an increase rate of a drain current with respect to an increase of a gate voltage is relatively great, thereby representing abundant and diverse grayscales using the gate voltage applied to the driving thin film transistor T 2 . 
     A case where the voltage drop occurs is described with reference to  FIG. 16  but the exemplary embodiments are not limited thereto. For example, the thin film transistor according to the exemplary embodiments maintains a substantially uniform current (drain current) not only when the voltage drop occurs as shown in  FIG. 9 , but also when a voltage rise occurs. 
     A thin film transistor substrate shown in  FIGS. 1 through 8  means in the above-described exemplary embodiments that the upper gate electrode  150  is formed but the exemplary embodiments are not limited thereto. In the present specification, the thin film transistor substrate means a state where a thin film transistor is formed on the substrate  100 . As shown in  FIG. 7 , the thin film transistor substrate may mean a state where a thin film transistor TFT is formed on the substrate  100 . Alternatively, as shown in  FIG. 15 , the thin film transistor substrate may mean a state where the third insulating layer  107  is formed over the thin film transistor (e.g. the driving thin film transistor T 2 ). Alternatively, as shown in  FIG. 15 , the thin film transistor substrate may mean a state where the pixel electrode  210  is formed over the thin film transistor (e.g. the driving thin film transistor T 2 ). 
     According to the exemplary embodiments, a film transistor may remove the influence due to a voltage drop or a voltage increase, may be small in size, and may have excellent electrical characteristics, and thus a display quality of a high resolution and wide area display apparatus may be enhanced. 
     It should be understood that exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. 
     While the inventive technology has been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.