Abstract:
A display panel includes: data lines adapted to transmit a data signal; scan lines adapted to transmit a select signal; and pixels, each pixel being coupled to one of the scan lines and one of the data lines, and each pixel including: light emitting elements adapted to emit light corresponding to a current supplied thereto; a pixel driver adapted to input the data signal while the select signal is being supplied and to output a first current corresponding to the data signal; and switching units adapted to transmit the first current to the light emitting elements, each of the switching units including first transistors respectively coupled between the pixel driver and the light emitting elements, and having different type channels.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for LIGHT EMITTING DISPLAY, AND DISPLAY PANEL AND PIXEL CIRCUIT THEREOF earlier filed in the Korean Intellectual Property Office on 28 Jul. 2004 and there duly assigned Serial No. 10-2004-0059213. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Light Emitting Display (LED), and a display panel and a pixel circuit thereof, and more particularly to an Organic Light Emitting Diode (OLED) display and pixel circuit thereof. 
     2. Description of the Related Art 
     In general, an OLED display, which is a kind of LED for emitting light by electrically exciting a fluorescent organic compound, displays images by driving N×M organic light emitting pixels using a voltage programming method or a current programming method. An organic light emitting pixel has a multi-layered structure including an anode layer, an organic thin film layer, and a cathode layer. The organic thin film also has a multi-layered structure including an EMitting Layer (EML), an Electron Transport Layer (ETL), and a Hole Transport Layer (HTL) in order to enhance light emission efficiency by balancing electrons and holes. The organic thin film further include a separate Electron Injecting Layer (EIL) and a separate Hole Injecting Layer (HIL). 
     Methods for driving the organic light emitting pixels are generally classified into a passive matrix method and an active matrix method using Thin Film Transistors (TFTs). In the passive matrix method, anodes are perpendicular to cathodes and lines are selected and driven, while in the active matrix method, TFTs are coupled to respective pixel electrodes and the TFTs are driven by voltages maintained by capacitors coupled to gates of the TFTs. The active matrix method is classified into a voltage programming method and a current programming method based on the form of a signal which programs a voltage into a capacitor and maintains the programmed voltage. 
     In an OLED display, one pixel is composed of a plurality of sub pixels having respective colors, so that a color can be represented in various ways by combining colors generated by the plurality of sub pixels. In general, one pixel is composed of a sub pixel representing Red (R), a sub pixel representing Green (G), and a sub color representing Blue (B), and various colors can be represented by combinations of the red, green and blue. 
     In order to drive these sub pixels, a driving transistor for driving an OLED element for each sub pixel, a switching transistor, and a capacitor are required. In addition to this, a data line for transmitting a data signal and a power line for transmitting an operating voltage are further required. Therefore, there arises an increase in the number of transistors, capacitors, and lines required to form one pixel. Difficulties are encountered in arranging them inside the pixel. In addition, there arises a problem in that an aperture ratio corresponding to a light emitting area of the pixel is reduced. 
     SUMMARY OF THE INVENTION 
     In accordance with an exemplary embodiment of the present invention, a light emitting display with an improved aperture ratio is provided. 
     In accordance with another exemplary embodiment of the present invention, a light emitting display with a simplified configuration and interconnection of devices included in a pixel is provided. 
     In accordance with one aspect of the present invention, a display panel is provided comprising: a plurality of data lines adapted to transmit a data signal; a plurality of scan lines adapted to transmit a select signal; and a plurality of pixels, each pixel being coupled to one of the plurality of scan lines and one of the plurality of data lines, and each pixel including: a plurality of light emitting elements adapted to emit light corresponding to a current supplied thereto; a pixel driver adapted to input the data signal while the select signal is being supplied and to output a first current corresponding to the data signal; and a plurality of switching units adapted to transmit the first current to the plurality of light emitting elements, each of the plurality of switching units including a plurality of first transistors respectively coupled between the pixel driver and the plurality of light emitting elements, the plurality of first transistors having different respective types of channels. 
     The pixel driver preferably comprises: a second transistor having first, second, and third electrodes and adapted to output a current to the third electrode, the current corresponding to a voltage supplied between the first and second electrodes; a first capacitor coupled between the first and second electrodes of the second transistor; and a switch adapted to transmit the data signal to the first capacitor in response to the select signal. 
     The display panel preferably further comprises a first power source coupled to the second electrode of the second transistor; the pixel driver preferably further includes: a second capacitor coupled between the first electrode of the second transistor and the first capacitor; a fourth switch adapted to diode-couple the second transistor in response to a first control signal; and a fifth switch adapted to supply a voltage of the first power source to one electrode of the first capacitor coupled to one electrode of the second capacitor in response to a second control signal. 
     The first control signal preferably corresponds to the second control signal. 
     The first control signal preferably comprises a select signal of a previous scan line supplied immediately before a current select signal is supplied. 
     The plurality of the light emitting elements each preferably comprises first and second light emitting elements adapted to emit light of different respective colors corresponding to a current supplied thereto. 
     The plurality of switching units each preferably comprises a first switching unit adapted to transmit the first current to the first light emitting element and a second switching unit adapted to transmit the first current to the second light emitting element, each of the first and second switching units preferably respectively including a PMOS transistor and an NMOS transistor coupled in series. 
     A first emit signal is preferably supplied to a gate electrode of the NMOS transistor in the first switching unit, and an emit signal corresponding to the first emit signal is preferably supplied to a gate electrode of the PMOS transistor in the second switching unit; and a second emit signal is preferably supplied to a gate electrode of the PMOS transistor in the first switching unit, and an emit signal corresponding to the second emit signal is preferably supplied to a gate electrode of the NMOS transistor in the second switching unit. 
     Each of the pixels preferably comprises first, second and third light emitting elements adapted to respectively emit light of different colors corresponding to a current supplied thereto. 
     Each of the plurality of switching units preferably comprises first, second and third switching unit adapted to respectively transmit the first current to the first, second and third light emitting elements, each of the first, second and third switching units preferably including three second transistors coupled in series. 
     In accordance with another aspect of the present invention, a display is provided comprising a display unit including: a plurality of data lines adapted to transmit a data signal; a plurality of scan lines adapted to transmit a select signal; and a plurality of pixels, each pixel being coupled to one of the plurality of scan lines and one of the plurality of data lines a data driver adapted to time-divide the plurality of data signals for one field and to supply the time-divided data signals to the plurality of data lines; and a scan driver adapted to supply the select signal sequentially to the plurality of scan lines; wherein each of the pixels includes: a plurality of light emitting elements adapted to emit light corresponding to a current supplied thereto; a pixel driver adapted to input the data signal while the select signal is being supplied and to output a first current corresponding to the data signal; and a plurality of switching units adapted to respectively transmit the first current to the light emitting elements, each of the switching units including a plurality of transistors respectively coupled in series between the pixel driver and the light emitting elements, and having different type channels. 
     The one field is preferably divided into a plurality of subfields and the scan driver is preferably adapted to supply the select signal to the plurality of scan lines for each subfield. 
     The plurality of light emitting elements is preferably adapted to respectively emit light of different colors corresponding to the current supplied thereto, and the data driver is preferably adapted to sequentially supply the data signals corresponding to the plurality of light emitting elements. 
     The plurality of light emitting elements each preferably comprise first and second light emitting elements adapted to respectively emit light of different colors corresponding to the current supplied thereto, and the plurality of switching units each preferably comprise first and second switching units adapted to respectively transmit the first current to the first and second light emitting elements. 
     The one field is preferably divided into first and second subfields, the first switching unit adapted to transmit the first current to one of the first and second light emitting elements for a first period of time, and the second switching unit adapted to transmit the first current to one of the first and second light emitting elements for a second period of time. 
     The data driver and the scan driver are preferably arranged on a display panel on which the display unit is arranged. 
     In accordance with still another aspect of the present invention, a pixel circuit is provided comprising: a plurality of light emitting elements adapted to emit light corresponding to a current supplied thereto; a driving circuit adapted to input a data signal and to output a first current corresponding to the data signal; a first switching circuit adapted to transmit the first current to one of at least two of the plurality of light emitting elements for a first period of time; and a second switching circuit adapted to transmit the first current to one of the at least two light of the plurality of emitting elements for a second period of time; wherein at least one of the first and second switching circuits includes two transistors having different type channels. 
     The driving circuit preferably comprises: a transistor having first, second, and third electrodes adapted to output a current to the third electrode, the current corresponding to a voltage supplied between the first and second electrodes; a first capacitor coupled between the first and second electrode of a transistor; and a switch adapted to transmit the data signal to the first capacitor in response to the select signal. 
     Each of the plurality of light emitting elements is preferably adapted to respectively emit light of different colors corresponding to the current supplied thereto, and each of the first and second switching circuits preferably include two transistors coupled in series. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention, and many of the attendant advantages thereof, will be readily apparent as the present invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a schematic plan view of an OLED display according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a schematic conceptual diagram of a pixel in the OLED display of  FIG. 1 ; 
         FIG. 3  is a circuit diagram of a pixel in the OLED display according to the first exemplary embodiment of the present invention; 
         FIG. 4  is a driving timing diagram of the OLED display according to the first exemplary embodiment of the present invention; 
         FIG. 5  is a circuit diagram of a pixel in an OLED display according to a second exemplary embodiment of the present invention; 
         FIG. 6  is a circuit diagram of a pixel in an OLED display according to a third exemplary embodiment of the present invention; 
         FIG. 7  is a circuit diagram of a pixel in an OLED display according to a fourth exemplary embodiment of the present invention; and 
         FIG. 8  is a circuit diagram of a pixel in an OLED display according to a fifth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments can be modified in various ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, rather than restrictive. 
     In the drawings, illustrations of elements having no relation with the present invention have been omitted in order to prevent the subject matter of the present invention from being unclear. In the specification, the same or similar elements are denoted by the same reference numerals even if depicted in different drawings. Also, a coupling between one element and another element includes an indirect coupling with a different element interposed therebetween, as well as a direct coupling therebetween. 
     A light emitting display and a driving method thereof according to exemplary embodiments of the present invention are described below in detail with reference to the drawings. 
       FIG. 1  is a schematic plan view of an OLED display according to a first exemplary embodiment of the present invention, and  FIG. 2  is a schematic conceptual diagram of a pixel in the OLED display of  FIG. 1 . 
     As shown in  FIG. 1 , the OLED display according to the first exemplary embodiment of the present invention includes a display panel  100 , a select scan driver  200 , an emit scan driver  300 , and a data driver  400 . The display panel  100  includes a plurality of scan lines S 1  to Sn and E 1  to En extending in a row direction, a plurality of data lines D 1  to Dm extending in a column direction, and a plurality of pixels  110 . Each pixel  110  is formed in a pixel area defined by two adjacent scan lines S 1  to Sn and two adjacent data lines D 1  to Dm. Referring to  FIG. 2 , each pixel  110  includes OLED elements OLED 1  and OLED 2  for emitting light of different colors, and a pixel driver  111  for driving the OLED elements OLED 1  and OLED 2 . These OLED elements emit light with a brightness corresponding to the amount of current supplied thereto. 
     The select scan driver  200  sequentially supplies a select signal to the plurality of scan lines S 1  to Sn so that the data signal is programmed into a pixel coupled to a corresponding scan line, and the emit scan driver  300  sequentially supplies an emit signal to a plurality of emit scan lines E 1  to En in order to control the light emission of the OLED elements OLED 1  and OLED 2 . Also, the data driver  400  supplies the data signal to the data lines D 1  to Dm, the data signal corresponding to the pixel of the scan line to which the select signal is supplied, every time the select signal is sequentially supplied. 
     The select and emit scan drivers  200  and  300  and the data driver  400  are coupled to a substrate on which the display panel  100  is formed. Alternatively, the scan drivers  200  and  300  and/or the data driver  400  can be directly mounted on a glass substrate of the display panel  100 , or can be replaced by a driving circuit formed with the same layer as the scan lines, the data lines and transistors. Alternatively, the scan drivers  200  and  300  and/or the data driver  400  can be mounted on a Tape Carrier Package (TCP), a Flexible Printed Circuit (FPC), or a Tape Automatic Bonding (TAB), which is conductively bonded to the substrate of the display panel  100 , in the form of a chip. 
     In the first exemplary embodiment of the present invention, one field is divided into two subfields in each of which data corresponding to respective OLED elements OLED 1  and OLED 2  is programmed for light emission. To this end, the select scan driver  200  supplies the select signal to the selection scan lines S 1  to Sn sequentially for each subfield, and the emit scan driver  300  supplies the emit signal to the emit scan lines E 1  to En so that the OLED element emits light with a respective color in a respective subfield. In addition, the data driver  400  supplies data signals corresponding to the OLED elements OLED 1  and OLED 2 , respectively, in two subfields. 
     Hereinafter, the operation of the OLED display according to the first exemplary embodiment of the present invention is described in detail with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a circuit diagram of the pixel in the OLED display according to the first exemplary embodiment of the present invention, and  FIG. 4  is a driving timing diagram of the OLED display according to the first exemplary embodiment of the present invention. 
       FIG. 3  shows the pixel employing the voltage programming method in which a selection scan line Sn is coupled to a data line Dm. Transistors used are shown as a p-channel transistors in  FIG. 3 . Other pixels in the OLED display have the same configuration as the pixel of  FIG. 3 , and therefore, an explanation thereof has been omitted. 
     As shown in  FIG. 3 , the pixel circuit according to the first exemplary embodiment of the present invention includes a driving transistor M 1 , a switching transistor M 2 , two OLED element OLED 1  and OLED 2 , and two light emitting transistors M 31  and M 32  for controlling light emission of the OLED elements OLED 1  and OLED 2 . One light emitting scan line En is composed of two emit signal lines Ena and Enb. Although not shown in  FIG. 3 , each of the other emit scan lines E 1  to E(n−1) is also composed of two emit signal lines. The light emitting transistors M 31  and M 32  and the emit signal lines Ena and Enb form a switching unit for selectively transmitting a current from the driving transistor M 1  to the OLED elements OLED 1  and OLED 2 . 
     In more detail, the switching transistor M 2  with a gate electrode coupled to the selection scan line Sn and a source electrode coupled to the data line Dm transmits a data voltage from the data line Dm in response to the select signal from the selection scan line Sn. The driving transistor M 1  has a source electrode coupled to a power line for supplying an operation voltage VDD and a gate electrode coupled to a drain electrode of the switching transistor M 2 . A capacitor Cst is coupled between the source electrode and the gate electrode of the driving transistor M 1 . Source electrodes of the light emitting transistors M 31  and M 32  are coupled to a drain gate of the driving transistor M 1 , and the emit signal lines Ena and Enb are coupled to gate electrodes of the transistors M 31  and M 32 . Anodes of the OLED elements OLED 1  and OLED 2  are respectively coupled to drain electrodes of the light emitting transistors M 31  and M 32 , and an operation voltage VSS lower than the operation voltage VDD is supplied to cathodes of the OLED elements OLED 1  and OLED 2 . A negative voltage or a ground voltage can be used as the operation voltage VSS. 
     The switching transistor M 2  transmits the data voltage from the data line Dm to the gate electrode of the driving transistor M 1  in response to a select signal of a low level from the selection scan line Sn, and a difference voltage between the data voltage transmitted to the gate electrode of the transistor M 1  and the operation voltage VDD is stored in the capacitor Cst. When the light emitting transistor M 31  is turned on in response to the emit signal of a low level from the emit signal line Ena, a current corresponding to the voltage stored in the capacitor Cst flows into the OLED element OLED 1  through the driving transistor M 1 . Accordingly, the OLED element OLED 1  emits light. 
     Similarly, when the light emitting transistor M 32  is turned on in response to the emit signal of a low level from the emit signal line Enb, a current corresponding to the voltage stored in the capacitor Cst flows into the OLED element OLED 2  through the driving transistor M 1 . Accordingly, the OLED element OLED 2  emits light. 
     The two emit signals are supplied to the two emit signal lines such that one pixel can represent different colors having respective low level periods of time during which the two emit signals do not overlap with each other for one field. 
     Hereinafter, a driving method of the OLED display according to the first exemplary embodiment of the present invention is described in detail with reference to  FIG. 4 . As shown in  FIG. 4 , according to the first exemplary embodiment of the present invention, one field  1  TV is composed of two subfields  1 SF and  2 SF. In the subfields  1 SF and  2 SF, signals for driving the OLED elements OLED 1  and OLED 2  in the pixel are respectively supplied. Intervals of the subfields are shown to be equal in  FIG. 4 . 
     For the sake of convenience of explanation, it is hereinafter assumed that the OLED element OLED 1  represents a red color image and the OLED element OLED 2  represents a green color image. 
     In the subfield  1 SF, first, when the select signal of a low level is supplied to a selection scan line S 1  in a first row, a data voltage R corresponding to the OLED element OLED 1  in a pixel in the first row is supplied to the data lines D 1  to Dm. 
     In addition, the emit signal of a low level is supplied to an emit signal line E 1   a  in the first row. Then, the data voltage R is supplied to the capacitor Cst through the switching transistor M 2  of each pixel in the first row, and a voltage corresponding to the data voltage R is stored in the capacitor Cst. In addition, the light emitting transistor M 31  in the pixel in the first row is turned on, and a current corresponding to a gate-source voltage of the light emitting transistor M 31  stored in the capacitor Cst flows into the OLED element OLED  1  representing the red color image through the driving transistor Ml. Accordingly, the OLED element OLED  1  emits red light. 
     When the select signal of a low level is supplied to a selection scan line S 2  in a second row, a data voltage R corresponding to a red color image of a pixel in the second row is supplied to the data lines D 1  to Din. In addition, the emit signal of a low level is supplied to an emit signal line E 2   a  in the second row. Then, a current corresponding to the data voltage R from the data lines D 1  to Dm flows into the OLED element OLED 1  representing the red color image in the pixel in the second row. Accordingly, the OLED element OLED 1  emits red light. 
     The data voltage is subsequently supplied to pixels in the third to (n−1)-th rows so that the red OLED element OLED 1  emits red light. Finally, when the select signal of a low level is supplied to a selection scan line Sn in an n-th row, a data voltage R corresponding to a red color image of a pixel in the n-th row is supplied to the data lines D 1  to Dm, and an emit signal of a low level is supplied to a emit signal line Enr in the n-th row. Then, a current corresponding to the data voltage R from the data lines D 1  to Dm flows into the OLED element OLED 1  representing the red color image in the pixel in the n-th row. Accordingly, the OLED element OLED 1  emits red light. 
     In this way, in the subfield  1 SF, the data voltage R corresponding to the red color image is supplied to each pixel formed in the display panel  100 . In addition, the emit signal supplied to the emit signal lines E 1   a  to Ena is maintained at a low level for a certain time, and the OLED element OLED 1  coupled to the light emitting transistor M 31  to which the emit signal is supplied continues to emit light while the emit signal is maintained at the low level. This certain time is shown to be equal to the subfield  1 SF in  FIG. 4 . That is, the red OLED element OLED 1  in each pixel emits light with a brightness corresponding to the data voltage supplied for a time corresponding to the subfield  1 SF. 
     In the next subfield  2 SF, in a way similar to the previous subfield  1 SF, a select signal of a low level is sequentially supplied to selection scan line S 1  to Sn in a first row to an n-th row, respectively, and a data voltage corresponding to a green color image of a pixel in a corresponding row is supplied to the data lines D 1  to Din when the select signal is supplied to each selection scan line S 1  to Sn. Also, in synchronization with the sequential application of the select signal of the low level to the selection scan lines S 1  to Sn, an emit signal of a low level is sequentially supplied to emit signal lines E 1   b  to Enb. Then, a current corresponding to the supplied data voltage flows into the OLED element OLED 2  representing the green color image through the light emitting transistor M 32 . Accordingly, the OLED element OLED 2  emits green light. 
     In the subfield  2 SF, similarly, the emit signal supplied to the emit signal lines E 1   b  to Enb is maintained at a low level for a certain time, and the green OLED element OLED 2  coupled to the light emitting transistor M 32  to which the emit signal is supplied continues to emit light while the emit signal is maintained at the low level. This certain time is shown to be equal to the subfield  2 SF in  FIG. 4 . That is, the green OLED element OLED 2  in each pixel emits light with a brightness corresponding to the data voltage supplied for a time corresponding to the subfield  2 SF. 
     In this way, in the driving of the OLED display according to the first exemplary embodiment of the present invention, one field is divided into two subfields to be driven sequentially. In each subfield, only one OLED element representing one color in one pixel emits light. Two OLED elements representing different colors respectively emit light sequentially through two subfields. 
     Although the OLED display is shown to be driven by a progressive scan method in a single scan in  FIG. 4 , the present invention is not limited to this, and can use a dual scan method, an interlaced scan method or other scan methods. 
     In addition, although the pixel circuit employing the voltage programming method using only the switching transistor and the driving transistor has been described in the first exemplary embodiment of the present invention, the present invention can be used with a pixel circuit employing the voltage programming method using a transistor for compensating for a threshold voltage of the driving transistor or a transistor for compensating for a voltage drop, in addition to the switching transistor and the driving transistor, described later. 
     However, when the pixel circuit according to the first exemplary embodiment of the present invention is used, since the light emitting transistors M 31  and M 32  are PMOS transistors, gate-source voltages of the transistors M 31  and M 32  become large when an emit signal of a high level is supplied. This can cause a leakage current to flow into the OLED element. 
     More specifically, while the emit signal of the low level is supplied to the emit signal line Ena in the subfield  1 SF and a current from the transistor M 1  flows into the red OLED element OLED 1 , the emit signal of the high level is supplied to the emit signal line Enb, and accordingly, the current from the transistor M 1  is prevented from flowing into the green OLED element OLED 2 . 
     However, when the transistor M 32  is a PMOS transistor, as shown in  FIG. 3 , the gate-source voltage of the transistor M 32  become large when the emit signal of the high level is supplied to the emit signal line Enb. This causes a leakage current to flow into the OLED element OLED 2 . 
     Similarly, although the current from the driving transistor M 1  flows into the OLED element OLED 2  and must not flow into the OLED element OLED 1 , there arises a problem of a leakage current flowing into the OLED element OLED 2  due to the increased gate-source voltage of the transistor M 31 . 
     Therefore, a voltage stored in the capacitor Cst is divided into divided voltages and the divided voltages are respectively supplied to the OLED elements OLED 1  and OLED 2 . This leads to a display of images having undesired gray scales, thereby causing a deterioration of image quality. 
       FIG. 5  is a circuit diagram of a pixel in an OLED display according to a second exemplary embodiment of the present invention. 
     The pixel circuit of the second exemplary embodiment of the present invention is different from the pixel circuit of the first exemplary embodiment in that the light emitting transistors M 31  and M 32  are NMOS transistors, as shown in  FIG. 5 . 
     When the light emitting transistors M 31  and M 32  are NMOS transistors, the absolute value of the gate-source voltages of the light emitting transistors M 31  and M 32  is small so that a leakage current can be prevented from flowing into the OLED elements OLED 1  and OLED 2  even when a low level voltage is supplied to the emit scan lines Ena and Enb and the current from the transistor M 1  is interrupted. 
     However, in order to prevent the leakage current using the NMOS light emitting transistors M 31  and M 32 , the channel lengths of the transistors M 31  and M 32  must be disadvantageously long. 
     Accordingly, a third exemplary embodiment of the present invention is provided to overcome a disadvantage of the pixel circuits of the first and second exemplary embodiments by using an NMOS transistor and a PMOS transistor in series for the light emitting transistors. 
       FIG. 6  is a circuit diagram of a pixel in an OLED display according to the third exemplary embodiment of the present invention. 
     As shown in  FIG. 6 , transistors M 31   a  and M 31   b  are coupled in series between a transistor M 1  and an OLED element OLED 1 , and transistors M 32   a  and M 32   b  are coupled in series between the transistor M 1  and an OLED element OLED 2 . 
     The transistors M 31   a  and M 32   b  are PMOS transistors and the transistors M 32   a  and M 31   b  are NMOS transistors. Gate electrodes of the transistors M 31   a  and M 32   a  are coupled to a emit signal line Ena and gate electrodes of the transistors M 31   b  and M 32   b  are coupled to a emit signal line Enb. 
     Accordingly, in the subfield  1 SF, when a low level voltage is supplied to the emit signal line Ena and a high level voltage is supplied to the emit signal line Enb, the transistors M 31   a  and M 31   b  are turned on and accordingly a current from the transistor M 1  flows into the OLED element OLED 1 . Since the transistors M 32   a  and M 32   b  coupled to the OLED element OLED 2  are interrupted, a leakage current can be effectively prevented from flowing into the OLED element OLED 2 . 
     Similarly, in the subfield  2 SF, when a high level voltage is supplied to the emit signal line Ena and a low level voltage is supplied to the emit signal line Enb, the transistors M 32   a  and M 32   b  are turned on and accordingly a current from the transistor M 1  flows into the OLED element OLED 2 . Since the transistors M 31   a  and M 31   b  coupled to the OLED element OLED 1  are interrupted, a leakage current can be effectively prevented from flowing into the OLED element OLED 1 . 
     Accordingly, according to the third exemplary embodiment of the present invention, the leakage current flowing into the OLED elements in a non-light emission interval can be significantly reduced using the driving waveforms of  FIG. 4 . In addition, since two transistors are coupled to each other in series, a channel length of each of the transistors can be short. 
       FIG. 7  is a circuit diagram of a pixel in an OLED display according to a fourth exemplary embodiment of the present invention. 
     As shown in  FIG. 7 , the pixel circuit of the fourth exemplary embodiment of the present invention is different from the pixel circuit of the third exemplary embodiment in that three OLED elements OLED 1 , OLED 2  and OLED 3  are coupled to one driver, and three light emitting transistors are coupled in series between a driving transistor M 1  and the OLED elements OLED 1 , OLED 2  and OLED 3 , respectively. 
     When the three OLED elements OLED 1 , OLED 2  and OLED 3  are coupled to one driver, one field is divided into three subfields, and signals for driving the OLED elements OLED 1 , OLED 2  and OLED 3  are supplied in each subfield. 
     More specifically, in a first subfield, when a low level voltage is supplied to an emit signal line Ena and a high level voltage is supplied to emit signal lines Enb and Enc, transistors M 31   a  to M 31   c  are turned on and accordingly a current from the transistor M 1  flows into the OLED element OLED 1 . 
     In addition, an NMOS transistor M 32   a  and a PMOS transistor M 32   b  coupled to the OLED element OLED 2  are turned off and accordingly a current from the driving transistor M 1  is prevented from flowing into the OLED element OLED 2 . Also, an NMOS transistor M 33   a  and a PMOS transistor M 33   c  coupled to the OLED element OLED 3  are turned off and accordingly the current from the driving transistor M 1  is prevented from flowing into the OLED element OLED 3 . 
     Accordingly, in the first subfield, only the OLED element OLED 1  emits light with a gray scale corresponding to a data voltage, and the OLED elements OLED 2  and OLED 3  do not emit light since a current does not flow into them. 
     A leakage current can be prevented from flowing into the OLED elements OLED 2  and OLED 3  since the NMOS transistors and the PMOS transistors coupled to the OLED elements OLED 2  and OLED 3  interrupt the leakage current from the OLED elements OLED 2  and OLED 3 . 
     Similarly, in a second subfield, when a low level voltage is supplied to the emit signal line Enb and a high level voltage is supplied to the emit signal lines Ena and Enc, only the OLED element OLED 2  emits light and the remaining OLED elements OLED 1  and OLED 3  do not emit light. Similarly, in a third subfield, when a low level voltage is supplied to the emit signal line Enc and a high level voltage is supplied to the emit signal lines Ena and Enb, only the OLED element OLED 3  emits light. 
     Accordingly, when one driver drives three OLED elements by respectively coupling three light emitting transistors in series between the driving transistor and the OLED elements, a leakage current flowing into the OLED elements can be minimized, and, by interrupting a current from the OLED elements using an NMOS transistor and a PMOS transistor coupled to each other in series, a channel length of each transistor can be short. 
       FIG. 8  is a circuit diagram of a pixel in an OLED display according to a fifth exemplary embodiment of the present invention. 
     As shown in  FIG. 8 , the pixel circuit of the fifth exemplary embodiment of the present invention is different from the pixel circuit of the third exemplary embodiment in that a driver further includes transistors for compensating for a deviation of the threshold voltage of the driving transistor M 1 , and a capacitor Cvth. 
     In the pixel circuit of the third exemplary embodiment, the current flowing into the OLED elements is affected by the threshold voltage VTH of the driving transistor M 1 . Accordingly, if there is a deviation of the threshold voltage between thin film transistors due to a non-uniformity in a manufacturing process of the transistors, it is difficult to attain high gray scales. 
     Accordingly, in the fifth exemplary embodiment of the present invention, the threshold voltage V TH  of the driving transistor M 1  is compensated for such that a current flowing into the OLED elements is not affected by the threshold voltage V TH  of the driving transistor M 1   
     Hereinafter, the pixel circuit of the fifth exemplary embodiment of the present invention is described in detail. An explanation of portions overlapping with contents of the third exemplary embodiment have been omitted. A selection scan line through which a current select signal is transmitted is called a “current scan line” and a selection scan line through a select signal is transmitted immediately prior to the transmission of the current select signal is called a “just-prior scan line”. 
     A capacitor Cvth is coupled between a gate electrode of a transistor M 1  and a capacitor Cst. A transistor M 4  is coupled between the gate electrode and a drain electrode of the transistor M 1  and diode-couples the transistor M 1  in response to a select signal from a just-prior scan line Sn- 1 . In addition, a transistor M 5  is coupled in parallel to the capacitor Cst and supplies an operation voltage VDD to one electrode of the capacitor Cvth in response to the select signal from the just-prior scan line Sn- 1 . 
     When a low level voltage is supplied to the just-prior scan line Sn- 1 , the transistor M 4  is turned on and the transistor M 1  goes into a diode-coupling state. The transistor M 5  is turned on and the threshold voltage of the transistor M 1  is stored in the capacitor. Cvth. 
     Thereafter, when a low level voltage is supplied to a current scan line Sn, a transistor M 2  is turned on and a data voltage Vdata charges the capacitor Cst. Since the threshold voltage Vth of the transistor M 1  is stored in the capacitor Cvth, a voltage corresponding to the sum of the data voltage Vdata and the threshold voltage Vth of the transistor M 1  is supplied to the gate electrode of the transistor M 1 . 
     When a low level voltage is supplied to one of emit scan lines Ena and Enb and corresponding light emitting transistors M 31  and M 32  are turned on, OLED elements emit light based on a current flowing into the OLED elements. The current is expressed by the following Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           OLED 
                         
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 Vgs 
                                 - 
                                 Vth 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     Vdata 
                                     + 
                                     Vth 
                                     - 
                                     VDD 
                                   
                                   ) 
                                 
                                 - 
                                 Vth 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 VDD 
                                 - 
                                 Vdata 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   &lt;Equation    1&gt; 
                 
               
             
           
         
       
     
     wherein I OLED  is a current flowing into an OLED element, Vgs is a source-gate voltage of the transistor M 1 , Vth is a threshold voltage of the transistor M 1 , Vdata is a data voltage, and β is a constant value. 
     Since the current flowing into the OLED elements is not affected by the threshold voltage of the transistor M 1 , images with a desired gray scale can be displayed. 
     As apparent from the above description, by driving a plurality of OLED elements using a single driver, the present invention provides a light emitting display with an improved aperture ratio. 
     In addition, the present invention provides a light emitting display with a simplified configuration and interconnection of devices included in a pixel. 
     Furthermore, the present invention provides a light emitting display with an improved image quality by preventing a leakage current from flowing into OLED elements in a non-light emission interval. 
     While the present invention has been described in connection with the OLED display as certain exemplary embodiments, the present invention can be adapted to other displays requiring other power supplies. Therefore, it is to be understood that the present invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
     For example, although  FIG. 6  shows two light emitting transistors coupled in series between the driving transistor and the OLED elements, and  FIG. 7  shows three light emitting transistors coupled in series between the driving transistor and the OLED elements, the present invention is not limited thereto and the number of light emitting transistors can be varied. 
     In addition, although p-channel driving transistors have been described in the exemplary embodiments, n-channel driving transistors can also be used in other embodiments of the present invention. In other embodiments of the present invention, the driving transistors can be implemented using other active devices, instead of the MOS transistors, including first to third electrodes for controlling a current outputted from the third electrode in response to a voltage supplied between the first and second electrodes.