Abstract:
A high-speed flat panel display having a long lifetime. Thin film transistors in a pixel portion having a plurality of pixels are contacted differently from thin film transistors in driving circuit portions for driving the pixels, thereby enhancing luminance uniformity and reducing power consumption. The thin film transistors each have a channel region and a body contact region for applying a predetermined voltage to the channel region. At least one thin film transistor in the pixel portion is a source-body contact thin film transistor having the body contact region connected to one of source and drain electrodes so that the predetermined voltage can be provided to the channel region. Each thin film transistor in the driving circuit portion is a gate-body contact thin film transistor having the body contact region connected to the gate electrode so that a predetermined voltage can be provided to the channel region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/938,000, filed Sep. 10, 2004 now U.S. Pat. No. 7,450,100 which claims priority to and the benefit of Korean Patent Application No. 2003-64895, filed Sep. 18, 2003 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flat panel display and, more particularly, to a high-speed flat panel display having a long lifetime in which thin film transistors of a pixel portion and driving circuit portions have different body contact structures. 
     2. Description of the Related Art 
     Generally, an active matrix organic light emitting diode (AMOLED) display includes a pixel portion in which an array of thin film transistors is arranged, and a data driving circuit portion and a gate driving circuit portion for driving the array of thin film transistors in the pixel portion. 
     In a conventional AMOLED display, the thin film transistors in the pixel portion and thin film transistors in the data or gate driving circuit portion are all composed of polysilicon thin film transistors. Therefore, in a 180 ppi or higher resolution AMOLED display, if the pixel portion and the driving circuit portions are composed of the polysilicon thin film transistors, a high-speed operating characteristic of the driving circuit portions could be obtained because of high mobility of the thin film transistors. However, because an on-current is very high, the amount of a current flowing through EL elements in the pixel portion exceeds a limit value, and thus luminance per unit area increases, which shortens the lifetime of the EL elements. 
     Meanwhile, when the pixel portion and the driving portions are composed of thin film transistors with low mobility to maintain a suitable on-current characteristic, it results in a relatively low on-current and thus causes appropriate luminance, which overcomes the shortened lifetime problem of the EL element, but it cannot support a high-speed operating characteristic of the driving circuit portions. 
     SUMMARY OF THE INVENTION 
     Accordingly, in exemplary embodiments of the present invention, a high-speed flat panel display having a long lifetime is provided. The flat panel display has a pixel portion and driving circuit portions that are composed of thin film transistors having different body contacts, thereby enhancing luminance uniformity and reducing power consumption. Such a flat panel display may be suitable for small size displays having high resolution. 
     In an exemplary embodiment of the present invention, a flat panel display including a pixel portion in which a plurality of pixels are arranged; and driving circuit portions for driving the pixels, is provided. The pixel portion and the driving circuit portions each include thin film transistors each having a channel region and a body contact region for applying a predetermined voltage to the channel region. The body contact region of at least one of the thin film transistors in the pixel portion is contacted differently from the body contact region of the thin film transistors in the driving circuit portions. 
     The at least one of the thin film transistors in the pixel portion may be a source-body contact thin film transistor and may further include a gate electrode, a source electrode, a drain electrode and source and drain regions connected, respectively, to the source and drain electrodes. The body contact region of the at least one of the thin film transistors may be connected to one of the source and drain electrodes so that the predetermined voltage from the connected electrode can be provided to the channel region. The body contact region may include an impurity region which is doped with an impurity of an opposite conductivity type from impurities in the source and drain regions. 
     Each of the thin film transistors in the driving circuit portions may be a gate-body contact thin film transistor, and may further include a gate electrode, a source electrode, a drain electrode, and source and drain regions connected, respectively, to the source and drain electrodes. The body contact region of each of the thin film transistors in the driving circuit portions may be connected to the gate electrode so that the predetermined voltage from the gate electrode can be provided to the channel region. The body contact region may include an impurity region which is doped with an impurity of opposite conductivity type from impurities in the source and drain regions. 
     The thin film transistors in the pixel portion may include a first thin film transistor which is switched by a gate driving signal to deliver a data signal, and a second thin film transistor for driving an EL element according to the data signal delivered via the first thin film transistor. The body contact region of at least one of the first thin film transistor and the second thin film transistor may be contacted differently from the body contact region of the thin film transistors in the driving circuit portions. 
     The thin film transistors in the pixel portion may be NMOS or PMOS thin film transistors and the thin film transistors in the driving circuit portions may be PMOS or NMOS thin film transistors. Alternatively, each of the thin film transistors in the pixel portion may be either an NMOS thin film transistor or a PMOS thin film transistor, and each of the thin film transistors in the driving circuit portions may be a PMOS thin film transistor or an NMOS thin film transistor based on a CMOS technology. 
     In another exemplary embodiment of the present invention, there is provided a flat panel display including a pixel portion in which a plurality of pixels are arranged, and driving circuit portions for driving the pixels. The pixel portion includes thin film transistors each having a substantially uniform output current over a predetermined range of input voltages, and the driving circuit portions include thin film transistors each having a suitable ON/OFF characteristic at a low input voltage. 
     Each of the thin film transistors in the pixel portion may be a source-body contact thin film transistor and may include an active layer having a channel region and a body contact region for providing a predetermined voltage to the channel region, a gate electrode, a source electrode and a drain electrode. The body contact region may be connected to one of the source electrode and the drain electrode so that the predetermined voltage from the connected electrode can be provided to the channel region. A drain current outputted via the drain electrode may be substantially uniform with respect to an input voltage applied to the drain electrode. 
     Each of the thin film transistors in the driving circuit portions may be a gate-body contact thin film transistor, and may include an active layer having a channel region and a body contact region for providing a predetermined voltage to the channel region, a gate electrode, a source electrode and a drain electrode. The body contact region may be connected to the gate electrode so that the predetermined voltage from the gate electrode can be provided to the channel region. A drain current outputted via the drain electrode may have a suitable ON/OFF characteristic with respect to an input voltage applied to the gate electrode. 
     In yet another exemplary embodiment of the present invention, a flat panel display includes a pixel portion including a plurality of pixel circuits, each comprising at least one first thin film transistor having a first body contact region, a first channel region connected to the first body contact region, a source electrode and a drain electrode, wherein the first body contact region is connected to the source electrode or the drain electrode, such that a first predetermined voltage applied at the connected electrode is applied to the first channel region through the first body contact region; and a driving circuit portion for driving the pixel circuits, the driving circuit portion including a plurality of second thin film transistors, each having a second body contact region, a second channel region connected to the second body contact region, and a gate electrode, wherein the second body contact region is connected to the gate electrode, such that a second predetermined voltage applied at the gate electrode is applied to the second channel region through the second body contact region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features in exemplary embodiments of the present invention will be described in reference to certain exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a block diagram of an organic light emitting diode (OLED) display device according to an exemplary embodiment of the present invention; 
         FIG. 2  is a plan view of a thin film transistor in a pixel portion in an organic light emitting diode according to an exemplary embodiment of the present invention; 
         FIG. 3A  is a cross-sectional view of the thin film transistor in the pixel portion taken along line  2 A- 2 A′ of  FIG. 2 ; 
         FIG. 3B  is a cross-sectional view of the thin film transistor in the pixel portion taken along line  2 B- 2 B′ of  FIG. 2 ; 
         FIGS. 4A and 4B  are graphs illustrating a relationship between a drain voltage and a drain current in the thin film transistor of the pixel portion shown in  FIG. 2 ; 
         FIG. 5  is a plan view of a thin film transistor of a driving circuit portion in an organic light emitting diode according to an embodiment of the present invention; and 
         FIGS. 6A and 6B  are graphs illustrating a relationship between a gate voltage and a drain current in the thin film transistor of the driving circuit portion shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an OLED display device includes a pixel portion  2  in which a plurality of pixels are arranged in a matrix form, and driving circuit portions for driving the pixel portion  2  on an insulating substrate  1 . Although the pixel portion  2  is not shown in detail in  FIG. 1 , a plurality of gate lines, a plurality of data lines, a plurality of power lines, and a plurality of pixels connected to the lines are arranged in a matrix form. Each pixel is basically composed of an electroluminescent (EL) element, a driving transistor for supplying a driving current according to a data signal from a data line to the EL element, a switching transistor for delivering the data signal to the driving transistor in response to a scan signal applied to a gate line, a capacitor for storing the data signal, and the like. 
     The driving circuit portions for driving the pixels in the pixel portion  2  include a gate driving circuit portion  4  for providing a scan signal for driving the gate lines in the pixel portion  2 , and a data driving circuit portion  3  for supplying the data signals to data lines in the pixel portion  2 . 
     In an exemplary embodiment of the present invention, each of the thin film transistors in the pixel portion  2  is a source-body contact thin film transistor, and each of the thin film transistors of the driving circuit portions  3  and  4  is a gate-body contact thin film transistor. The source-body contact thin film transistor has a body contact structure including a body contact region separately formed from the source and drain regions on an active layer. The body contact region is connected to either the source region or the drain region, for example, the source region. The gate-body contact thin film transistor has a body contact structure including a body contact region separately formed from the source and drain regions on the active layer. The body contact region is connected to the gate electrode. 
     Since identical drain currents can be obtained over a wide range of drain voltages in the source-body contact thin film transistor by reducing a kink effect, the source-body contact thin film transistor is suitable for thin film transistors making up the pixel portion  2 . On the other hand, since the gate-body contact thin film transistor can implement an ON/OFF characteristic at a low gate voltage, it is suitable for thin film transistors making up the driving circuit portions  3  and  4 . 
     An exemplary source-body contact thin film transistor that can be used as the thin film transistors in the pixel portion  2  is described in Korean Patent Application No. 2003-0027339, the entire content of which is incorporated by reference herein. Further, an exemplary gate-body contact thin film transistor that can be used as the thin film transistors of the driving circuit portions  3  and  4  is described in Korean Patent Application No. 2003-0056594. 
       FIG. 2  is a plan view of a source-body contact thin film transistor making up the pixel portion in the OLED display device according to an exemplary embodiment of the present invention as shown in  FIG. 1 , and  FIGS. 3A and 3B  show cross-sectional structures taken along lines  2 A- 2 A′ and  2 B- 2 B′ of  FIG. 2 , respectively. 
     Referring to  FIG. 2  and  FIGS. 3A and 3B , a source-body contact thin film transistor used in an exemplary embodiment of the present invention includes an active layer  30 , a gate electrode  50 , and source and drain electrodes  71  and  73 . The source-body contact thin film transistor is formed on an insulating substrate  10 , and an insulation layer  60  separates the gate electrode  50  from the source and drain electrodes  71  and  73 . The active layer  30  includes source and drain regions  31  and  33  with a channel region  35  formed therebetween, and a body contact region  37  separately formed from the source and drain regions  31  and  33 . 
     The gate electrode  50  is formed corresponding to the channel region  35  of the active layer  30 . The source electrode  71  is formed corresponding to the source region  31  and is electrically connected to the impurity region for the source  31  via a contact  61 . The drain electrode  73  is formed corresponding to the drain region  33 , and is electrically connected to the impurity region for the drain  33  via a contact  63 . Meanwhile, a connection wiring  77  is formed corresponding to the body contact region  37 , and it electrically connects the body contact region  37  to the source electrode  71  via a contact  67 . 
     Further, although the connection wiring  77  for applying a power to the body contact region  37  is formed integrally with the source electrode  71  in the described exemplary embodiment, it may be separated from the source electrode  71 , and the same power as that applied to the source electrode  71  may be applied to it. Further, although the connection wiring  77  is formed to connect to the source electrode  71 , it may be formed instead to connect to the drain electrode  73 . 
     In the source-body contact thin film transistor having the above-stated structure according to the exemplary embodiment of the present invention, hot carriers generated during normal operation at an interface between the drain region  33  and the channel region  35  by a lateral electric field in the drain region, are forced to go out through the body contact region  37 . As a result, the hot carriers are prevented from moving into the source region  31 , and thus a kink effect is suppressed. 
       FIGS. 4A and 4B  are graphs that show operating characteristics of a conventional floating body thin film transistor (TFT) and a TFT having a body contact region used in the described exemplary embodiment.  FIG. 4A  shows an I D -V D  characteristic of the present invention and the prior art in an n type thin film transistor in which W/L=4 μm/4 μm and the width of a Lightly Doped Drain (LDD) region is 1 μm.  FIG. 4B  shows an I D -V D  characteristic of the described exemplary embodiment and the prior art in a p type thin film transistor in which W/L=4 μm/4 μm. 
     Referring to  FIGS. 4A and 4B , it can be seen that the source-body contact thin film transistor used in the described exemplary embodiment has a better kink free characteristic as compared to a conventional TFT in which the active layer thereof is floated. At this time, a difference in the I D -V D  characteristic between the n type TFT and the p type TFT results because an impact ionization characteristic of holes is less than that of electrons. 
       FIG. 5  is a plan view of a gate-body contact thin film transistor used in a driving circuit portion in an OLED display device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the gate-body contact thin film transistor used in an exemplary embodiment of the present invention includes an active layer  130 , a gate electrode  150 , and source and drain electrodes  171  and  173 . The active layer  130  includes source and drain regions  131  and  133  with a channel region  135  formed therebetween, and a body contact region  137  separately formed from the source and drain regions  131  and  133 . 
     The gate electrode  150  is formed corresponding to the channel region  135  of the active layer  130 . The source electrode  171  is formed corresponding to the source region  131  and is electrically connected to the impurity region for the source  131  via a contact  161 . The drain electrode  173  is formed corresponding to the drain region  133 , and is electrically connected to the impurity region for the drain  133  via a contact  163 . 
     Further, a connection wiring  180  is formed corresponding to the body contact region  137 . The connection wiring  180  couples the body contact region  137  to the gate electrode  150  through a contact  165  formed on the gate electrode  150  and a contact  167  formed on the body contact region  137 . The connection wiring  180  is formed of the same material as that of the source or drain electrode  171  or  173 , and has an island shape connection pattern. 
     Further, in the exemplary embodiment of the present invention, the connection wiring  180  for applying power to the body contact region  137  is electrically connected to the gate electrode  150  via the contact  165 , which enables a low voltage drive, thereby reducing the swing width of a threshold voltage and deriving a high drain current at a low gate voltage. 
       FIGS. 6A and 6B  are graphs that show an operating characteristic of a conventional floating body thin film transistor and a gate-body contact thin film transistor used in the present invention.  FIG. 6A  illustrates a drain current I D  with respect to a gate voltage V G  in the case where each of the floating body thin film transistor and the gate-body contact thin film transistor is an NMOS transistor, and  FIG. 6B  illustrates a drain current I D  with respect to a gate voltage V G  in the case where each of the floating body thin film transistor and the gate-body contact thin film transistor is a PMOS transistor. 
     Referring to  FIGS. 6A and 6B , since a threshold voltage of the gate-body contact thin film transistor has a steeper slope than that of the floating body thin film transistor, an ON/OFF characteristic can be obtained at a low gate voltage. 
     Accordingly, in the exemplary embodiment of the present invention, the driving circuit portions  3  and  4  are composed of the gate-body contact thin film transistor shown in  FIG. 4 , and the pixel portion  2  is composed of the source-body contact thin film transistor as shown in  FIG. 2  and  FIGS. 3   a  and  3   b , thereby obtaining a high-speed operating characteristic, while allowing a low voltage drive at the same time. Further, since a substantially uniform current flows through the EL element in the pixel portion, it is possible to obtain a substantially uniform luminance characteristic and to expand the lifetime thereof. 
     In the exemplary embodiment of the present invention, the source and drain regions and the body contact region are of different conductive types. For example, if the source and drain regions are composed of a high concentration n-type impurity region, the body contact region is composed of a high concentration p-type impurity region. On the other hand, if the source and drain regions are composed of the high concentration p-type impurity region, the body contact region is composed of the high concentration n-type impurity region. In the described exemplary embodiment, the channel region in the active layer is an intrinsic region in which first or second conductive type impurities are not doped. 
     Further, although forming the body contact region in the thin film transistor in which the source and drain regions are composed of high concentration impurity regions has been described in reference to the exemplary embodiments of the present invention, the principles of the present invention are applicable to a thin film transistor in which source and drain regions of the transistor have an LDD structure of a high concentration impurity region and a low concentration impurity region. 
     In addition, the source-body contact thin film transistor and the gate-body contact thin film transistor in exemplary embodiment of the present invention are not limited to the illustrated structures and may have either or both a structure in which the source and the body contact region are interconnected and a structure in which the gate and the body contact region are interconnected. 
     In the exemplary embodiment of the present invention, it is possible to use the source-body contact thin film transistor as both a switching transistor and a driving transistor of the pixel portion. Alternatively, it is possible to use the source-body contact as either a switching transistor or a driving thin film transistor, but not both. By way of example, the driving transistor may be a source-body contact thin film transistor, while the switching transistor is a conventional floating body thin film transistor. 
     According to the above-described embodiments of the present invention, the thin film transistor in the pixel portion is a source-body contact thin film transistor having an excellent drain current characteristic, and the thin film transistor in the driving circuit portion is a gate-body contact thin film transistor having an excellent ON/OFF characteristic at a low voltage, thereby maintaining a substantially uniform current flowing through an EL element to obtain a substantially uniform luminance characteristic as well as obtaining a high-speed operating characteristic, and extending the lifetime of the EL element. 
     Although the present invention has been described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various modifications and variations can be made to the described exemplary embodiments without departing from the spirit or scope of the present invention defined in the following claims and equivalents thereof.