Abstract:
An electro-luminescent display includes a current mirror for supplying uniform drive currents to the electro-luminescent diodes of the electro-luminescent display. The current mirror is not effected by the various threshold voltages V TH  of the switching devices in the electro-luminescent display so that uniform current is output to the electro-luminescent diodes throughout the display. The electro-luminescent display includes a gate line, a data line intersecting the gate line, a first TFT for selecting an arbitrary pixel by a gate signal from the gate line, wherein a gate of the first TFT is connected to the gate line, and a current mirror for outputting a signal to an arbitrary pixel selected by the first TFT by receiving a data signal from the data line at the same time the current mirror is being driven by applied voltage. The current mirror includes a second TFT and a third TFT, and an electro-luminescent diode driven by the signal output from the current mirror.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electro-luminescent display (ELD) including a current mirror for uniform illumination throughout the whole display. 
     2. Discussion of the Related Art 
     An ELD, into which electrons and holes are injected, uses the recombination between electrons and holes to generate an electro-luminescence. An ELD is a next generation display technology and has the benefits of not requiring a back light, providing a thinner panel, and achieving reduced power consumption. 
     In an active ELD, a plurality of pixels are defined by a plurality of gate lines and data lines intersecting each other. In the respective pixels, power supply lines are arranged in the same direction as the data lines. A pixel has at least one switching device such as a thin film transistor (TFT), a storage capacitor, and an electro-luminescence (EL) portion. 
     When an ELD includes a pair of TFTs in each pixel, an EL exciting signal and a scanning signal are used. The EL portion is selected by a logic TFT and exciting power of the EL portion is controlled by the power of the other TFT. A storage capacitor is provided for maintaining the exciting power of the EL portion in the selected cell. 
     FIG. 1 is a schematic of an equivalent circuit of an ELD according to a related art. Referring to FIG. 1, a plurality of pixel regions are defined by a plurality of gate lines, for example, G 1 , G 2 , and data lines, for example, D 1 , D 2 , arranged to intersect each other. 
     First, TFTs M 1  are connected to the intersections of the gate lines and data lines, for example, G 1  and D 1  in a pixel. A storage capacitor C STO  and a gate of a second TFT M 2  are connected in parallel to a source of the first TFT Ml. An electro-luminescent diode EL, which is a light-emitting device, is connected to a source of the second TFT M 2 . A gate driver (not shown in the drawing) is connected to one stage of the gate lines and supplies each of the gate lines with a proper scanning signal. A data driver (not shown in the drawing) is connected to one stage of the data lines and supplies each of the data lines with a data voltage for driving a corresponding electroluminescent diode EL. 
     The following description explains the operation of the above-described ELD. After a first gate line G 1  has been turned on for selecting a specific pixel, a predetermined voltage from a data signal in the first data line D 1  is applied to a node A through the first TFT M 1 . Thereafter, the first gate line G 1  is turned off. Until the first gate line G 1  is turned on again, the storage capacitor C STO  maintains the voltage at the node A, while the second TFT M 2  functions as a drive switch for supplying the EL diode with a fixed current for emitting light. 
     In general, the drive switch is driven in the saturation region, and the drive current I depends on the following formula, I=½ Xμ n C o (W/L)(V GS −V TH ) 2 , where μ n  is the mobility of an electric field, C o  is the capacitance of a gate insulating layer, W is the channel width, L is the channel length, V GS  is the voltage at the gate and source electrodes, and V TH  is the threshold voltage. 
     Unfortunately, as the display size of the ELD of the related art increases, the deviation in the threshold voltage V TH  between the TFTs in each of the other pixels increases, especially on a large substrate. This occurs because the characteristics of the silicon film that constitute the TFTs are irregular throughout the whole pixel array. Specifically, when TFTs made of polycrystalline silicon are used as the switching devices, the irregularity in threshold voltage between the TFTs gets worse due to the difficulty in providing a polycrystalline film having silicon grains that are uniform throughout the whole surface of the substrate. 
     Therefore, the threshold voltage V TH  of the second TFT differs from the first TFT in each pixel even though the same V GS  is applied to the first and second TFTs. Thus, the brightness of the image throughout the display is not uniform as different amounts of current flows through the respective EL diodes that are driven by the switching devices in each of the pixels. 
     SUMMARY OF THE INVENTION 
     To overcome the problems described above, preferred embodiments of the present invention provide an ELD having uniform brightness throughout the whole display by supplying the respective EL diodes with uniform drive currents with the use of a current mirror, even though the V TH  of the switching devices in each pixel is not the same. 
     A preferred embodiment of the present invention includes a substrate, a gate line on the substrate, a data line crossing the gate line, a first TFT for selecting an arbitrary pixel by a gate signal, wherein a gate of the first TFT is connected to the gate line, a current mirror for outputting a signal to the arbitrary pixel selected by the first TFT by receiving a data signal from the data line at the same time that the current mirror is being voltage driven, the current mirror including a second TFT and a third TFT, and an electro-luminescent diode connected to the current mirror, wherein the diode is driven by a signal output from the current mirror. 
     In another preferred embodiment of the present invention, a method of manufacturing an ELD includes the steps of providing a substrate, forming a gate line on the substrate, forming a data line that crosses the gate line, forming a first TFT for selecting an arbitrary pixel by a gate signal, and connecting a gate of the first TFT with the gate line, providing a current mirror that receives a data signal from the data line as the current mirror is being voltage driven, and outputs a signal to the arbitrary pixel selected by the first TFT, and providing an electro-luminescent diode for receiving the outputted signal of the current mirror. 
     In preferred embodiments of the present invention, the voltage is continuously supplied to the current mirror such that the current mirror is being voltage driven at the same time that the current mirror receives the data signal from the data line. 
     Thus, in the present invention, the ELD has uniform luminescence throughout the whole display despite variations in the V TH  of the switching devices since the current that is output from the current mirror to the respective electro-luminescent diode is uniform throughout the whole display. 
     Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention and wherein: 
     FIG. 1 is a schematic view of an equivalent circuit of an ELD according to a related art; 
     FIG. 2 is a schematic view of an equivalent circuit of an ELD according to a first preferred embodiment of the present invention; 
     FIG. 3 is a schematic view of an equivalent circuit of an ELD according to a second preferred embodiment of the present invention; and 
     FIG. 4 is a schematic view of an equivalent circuit of an ELD according to a third preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 is a schematic view of an equivalent circuit of an ELD according to a first preferred embodiment of the present invention where a current mirror is provided for a general ELD. Referring to FIG. 2, a plurality of pixel regions are defined by a plurality of gate lines, for example, G 1  and G 2 , and data lines, for example, D 1  and D 2 , which are arranged to intersect each other. For simplicity, the discussion will focus on how the ELD functions in one of the pixels. 
     First, the TFT T 1  is connected to the intersection of the gate line G 1  and data, line D 1 . In a pixel, a storage capacitor C STO  and a gate of the second TFT T 2  are connected to a drain of the first TFT T 1  in parallel. A diode EL, which is a light-emitting device, is connected to a drain of the second TFT T 2 . Note that the above-described structure is similar to that of the related art thus far. 
     However, in the present preferred embodiment of the present invention, a current mirror, which includes a second TFT T 2  and a third TFT T 3 , is connected between the data line and the diode EL. The second TFT T 2  drives the diode EL, and the third TFT T 3  has its own gate connected to its own drain as well as a source of the first TFT T 1 . 
     Relating the first TFT T 1  to the third TFT T 3 , a drain of the third TFT T 3  is connected in parallel to the data line D 1 , the drain of the third TFT T 3  is connected to a gate of the third TFT T 3 , a source of the first TFT T 1  is connected to the gate of the third TFT T 3 , and a drain of the first TFT T 1  is connected to a gate of the second TFT T 2 . 
     Because both the second and third TFTs T 2  and T 3  constitute the current mirror that operates in the saturation region, the driving current I that flows through the drive switch is I={{LEFT{W} over {L}RIGHT)}_{T 3 }} over {{LEFT({W} over {L}RIGHT)} 13  {{T 2 }}}TIMES{ 1 }_{0}. Namely, the current “I 0 ” is the input for the current mirror, which includes the third and second TFTs T 3  and T 2  as drive switches for driving the diode EL, and outputs the current “I,” such that the current I is not affected by the level of the threshold voltage V TH  of the third and second TFTs T 3  and T 2 . Because the ELD of preferred embodiments of the present invention supplies each of the pixels with uniform data current I O , all of the pixels also have an uniform current I that flows through the light-emitting diodes EL in each of the pixels. In other words, the current flowing through the diode EL is controlled by the current I 0  although there exists deviations in the threshold voltage V TH  in the switching devices in each of the pixels. Thus, uniform current flows through all of the pixels, producing uniform light-emission from the diodes EL. Therefore, preferred embodiments of the present invention provide an ELD which has uniform brightness throughout the whole ELD. 
     Note that the ELD of preferred embodiments of the present invention includes a fourth TFT T 4  including a gate that is connected to the gate lines. In a pixel, the gate of the fourth TFT T 4  is connected to the gate of the first TFT T 1 , a drain of the fourth TFT T 4  is connected to a drain of the third TFT T 3 , and a source of the fourth TFT T 4  is connected to the data line. Thus, the first and fourth TFTs T 1  and T 4  are driven simultaneously by the gate signal of the gate line. Therefore, a data signal is input to a selected pixel as the fourth TFT T 4  of the pixel is selected by the gate signal and is the only one turned on. Accordingly, preferred embodiments of the present invention make possible to operate each of the pixels independently. 
     A gate driver (not shown in the drawing) is connected to one stage of the gate lines supplying each of the gate lines with a proper scanning signal. A data driver (not shown in the drawing) is connected to one stage of the data lines, supplying each of the data lines with data voltage for driving a corresponding diode EL. 
     Generally, it is possible to carry out modeling on a data driver. However, the ELD of the first preferred embodiment of the present invention requires a current supply source which supplies current in accordance with brightness to activate the current mirror. More specifically, in the ELD of the present preferred embodiment, a data driver is the current supply source that supplies the data lines with current. 
     The operation of the ELD according to the first preferred embodiment of the present invention is explained by the following description. Gate voltage is applied to an arbitrary gate line, for example, a first gate line G 1  by a gate driver (not shown in the drawing), thereby turning on the first and fourth TFTs T 1  and T 4  simultaneously. In the mean time, a data signal that is transferred from a data driver (not shown in the drawing) through a data line is input to a selected pixel through the fourth TFT T 4 , which was already turned on by the gate signal. The data signal that is applied to the pixel via the fourth TFT T 4  is applied to a node A, turning off the gate line G 1 . Note that the first and fourth TFTs T 1  and T 4  are turned off simultaneously. 
     Until the first gate line G 1  is selected again, a storage capacitor C STO  maintains the voltage at the node A to turn on the second TFT T 2  to function as a driving switch for supplying the diode EL with a fixed current for emitting light. Note that the current that is flowing in the diode EL, which is connected to the second TFT T 2  by the current mirror including the third and second TFTs T 3  and T 2 , is controlled by the initial data current that is input to the third TFT T 3 . 
     Therefore, the data signal inputted to each of the pixel regions by the current mirror is not affected by the magnitude of the threshold voltage V TH  of the TFTs in each pixel. Instead, the data signal supplies the diode EL of each pixel with uniform current, thereby driving the diode EL. Because the pixels cover a large area, the data current that is input to each pixel also flows into and drives each diode EL despite fluctuations in the threshold voltage V TH  of the TFTs provided in each pixel. Thus, each of the pixels provide the same brightness because each of the diodes in each of the pixels have the same amount of current for driving the diodes. 
     FIG. 3 is a schematic view of an equivalent circuit of an ELD according to a second preferred embodiment of the present invention. Referring to FIG. 3, a current mirror including a third TFT T 3  and a second TFT T 2  drives a diode EL, a first TFT T 1 , which selects a pixel region by a gate signal, is connected between a drain and gate of the third TFT T 3 . Note that all other elements are arranged the same as the first preferred embodiment of the present invention. 
     Relating the first TFT T 1  to the third TFT T 3 , a drain of the third TFT T 3  is connected to the data line, a drain of the first TFT T 1  is connected to the drain of the third TFT T 3 , a gate of the third TFT T 3  is connected to a source of the first TFT T 1 , and a gate of the second TFT T 2  is connected to the gate of the third TFT T 3 . 
     In the above-described arrangement, a pixel is selected by the first TFT T 1 , which functions as a selection TFT, thereby supplying the selected pixel with current from a current driver. Then, current starts to flow in the second TFT T 2 , which functions as a driving TFT for the current mirror. Therefore, the diode EL emits light when the TFT T 2  is being driven. Note that the operation and effect of the second preferred embodiment is as good as the first preferred embodiment. 
     FIG. 4 is a schematic of an equivalent circuit of an ELD according to a third preferred embodiment of the present invention. Referring to FIG. 4, a current mirror includes a third TFT T 3  and a second TFT T 2 , where the second TFT T 2  functions as a driving TFT for driving a diode EL. A first TFT T 1  functions as a selecting TFT and is connected between the gates of the third and fourth TFTs T 3  and T 4 , and the drain of the first TFT T 1  is connected to the gate of the third TFT T 3 . Note that all other elements are preferably the same as the first preferred embodiment of the present invention. 
     Relating the first TFT T 1  to the third TFT T 3 , the drain of the first TFT T 1  is connected to the data line, the drain of the third TFT T 3  is connected to the source of the first TFT T 1 , the gate of the third TFT T 3  is connected to the drain of the first TFT T 1 , and the gate of the second TFT T 2  is connected to the gate of the third TFT T 3 . 
     In the above-described arrangement, a pixel is selected by the first TFT T 1 , which functions as a selecting TFT, thereby supplying the selected pixel with current from a current driver. Then, the current starts to flow in the second TFT T 2 , which functions as a driving TFT for the current mirror. Then, the diode EL emits light by the current supplied by the driving TFT T 2 . Note that the operation and effect of the third preferred embodiment is as good as the first preferred embodiment. 
     Note that preferred embodiments of the present invention are provided with an ELD arrangement having PMOS as the TFT, but in other embodiments, NMOS can be used as the TFTs. 
     Accordingly, preferred embodiments of the present invention provides an ELD which has uniform brightness by providing the each of the pixels that supply the respective diodes EL with a uniform drive current by providing current mirrors for each of the diodes EL in each of the pixels. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.