Abstract:
A pixel structure of a display and a driving method thereof are disclosed. The pixel structure disclosed in the invention includes a structure with less elements than that of prior art. The driving method thereof is also much easier than that of prior art. The pixel structure and driving method thereof can completely compensate the variations of the threshold voltage of a driving transistor thereof. The pixel structure includes a switching transistor, a driving transistor, a capacitor, a light emitting diode (LED) and a reset transistor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of Taiwan application serial no. 92131760, filed on Nov. 13, 2003.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a pixel structure of a display and a driving method thereof, and more particularly to a pixel structure of a display and a driving method thereof, which compensate threshold voltages of the transistors thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     Array displays include liquid crystal displays (LCD),inorganic and organic light emitting diode (LED) displays, etc. As to LCD, backlight modules, liquid crystal and thin film transistors in pixels are used to generate images. During displaying, the backlight modules should continuously generate light for the electronic devices, such as notebooks or PDA. The operation of the displays thereof will consume substantial power. Contrary, organic LED displays uses pixels on demand for displaying and consuming less power.  
         [0006]     Moreover, organic LED displays also have the other advantages, such as high luminance, low power consumption, wide viewing angles, low costs, and low weight. Therefore, organic LED displays gradually have been applied to different display applications. Referring to  FIG. 1 , a pixel structure of the active-matrix-addressed organic LED display includes two N-type thin film transistors  110  and  120 . A row selecting line  110   a  is adapted to turn on the thin film transistor  110 , in order to apply the voltage of the data signal line  110   b  to the capacitor  140  for driving the thin film transistor  120  as to generate light.  
         [0007]     Although the active-matrix-addressed organic LED displays have the aforementioned advantages, the luminance thereof is not stable, caused by several reasons. One of them is that because the luminance of the organic LED is proportional to the current, the threshold voltage of the thin film transistor  120  shifts during a long-time operation as to cause the instability of the current flowing therethrough. Another reason is the process inconsistence of the thin film transistors within each pixel resulting in different threshold voltages. Accordingly, the light generated therefrom is not stable. In addition, the material of the organic LED is another reason causing the problem. The turn on voltage of the organic LED (OLED) will be shifted because of an operational temperature change.  
         [0008]     James L. Sanford and Frank R. Libsch, of IBM inc., disclosed a pixel structure of LED display, titled “TFT AMOLED Pixel Circuits and Driving Methods,” in Society For Information Display (SID). Please refer to  FIGS. 2A and 2B . A pixel structure of a display is shown in  FIG. 2A  and the pixel structure includes three N-type transistors  210 ,  220  and  230 . A gate terminal of the transistor  210  is electrically connected to a row selecting line  210   a,  a source terminal thereof is electrically connected to a data signal line  210   b,  i.e. a data signal line and a drain terminal thereof is electrically connected to the transistors  220  and  230 , and to a light emitting diode  240  via a capacitor  250 . A gate terminal of the transistor  220  is electrically connected to an autozero line (AZ). The capacitor  250  is disposed between the gate and source terminals of the transistor  230  for storing the threshold voltage and the data voltage.  FIG. 2B  is a timing diagram of the pixel structure of the display shown in  FIG. 2A .  
         [0009]     The driving time of the organic LED display includes three time zones. The first time zone is used to store the threshold voltage in the capacitor  250 . The second time zone is used to write in data. The third time zone is used to display. The step of writing in the threshold voltage includes: maintaining the AZ signal in a high state, Vca, for storing the threshold voltage in the capacitor  250 ; raising the Vca to 10 V for turning on the thin film transistor  230 ; and lowering the Vca to 0 V for charging the capacitor  250  to the threshold voltage of the thin film transistor  230 .  
         [0010]     Then, the Vca is 0 V and the AZ signal is in a low state so that the data is written in. If the voltage drop on the light emitting diode  240  does not change, the voltage of the capacitor  250  will be Vdata+Vt, where the Vdata means the voltage for the data and the Vt means the threshold voltage. After the data is written in, the Vca is −18 V. A current flowing through the thin film transistor  230  is proportional to (Vdata+Vt−Vt) 2 , i.e. (Vdata) 2 .  
         [0011]      FIG. 2C  is a drawing showing luminance with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits. The line (A) represents the pixel structure of  FIG. 2A ; the dash line (B) represents the conventional pixel structure of  FIG. 1 . Under the same operation of Vdata, the former has a better luminance than that of the later.  FIG. 2D  a drawing showing luminance difference with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits, when the variations of the data voltage Vdata and the threshold voltage are under 2 V. The line (C) represents the pixel structure of  FIG. 2A ; the dash line (D) represents the prior art pixel structure of  FIG. 1 . When Vdata is higher than 2.5 V, the former has a worse luminance than that of the later by 20%. If Vdata is less than 2.5 V, it will be much worse. The reason of the issue is that the thin film transistor  230  induces the voltage of the light emitting diode  240  to 0 V during the writing of the data. In addition, different threshold voltages are applied to the capacitor  250  when Vca is introduced from Vt to −18 V. Therefore, the issue will affect the operation of the organic LED display.  
       SUMMARY OF INVENTION  
       [0012]     Therefore, the present invention discloses a pixel structure of a display and a driving method thereof, which are easier than those of prior art and compensate the threshold voltage of the thin film transistors.  
         [0013]     To achieve the object described above, the present invention discloses a pixel structure of a display, which comprises: a switching transistor, a driving transistor, a first capacitor, a light emitting diode and a reset transistor. A gate terminal of the switching transistor is electrically connected to a scan line, and a source terminal thereof is electrically connected to a signal line. A gate terminal of the driving transistor is electrically connected to a drain terminal of the switching transistor. The first capacitor is disposed between the gate terminal of the driving transistor and a source terminal thereof. The light emitting diode has a first terminal electrically connected to a operational voltage, and a second terminal electrically connected to a drain terminal of the driving transistor. A gate terminal of the reset transistor is electrically connected to an autozero, a drain terminal is electrically connected to the driving transistor, and a source terminal electrically connected to a ground voltage.  
         [0014]     As to the pixel structure described above, the driving method thereof comprises: turning on the switching transistor at a threshold voltage writing timing, then turning off the reset transistor and applying a start voltage to the gate terminal of the driving transistor; lowering the operational voltage to a low voltage at an data writing timing for turning off the light emitting diode, applying an data voltage to the gate terminal of the driving transistor; and  
         [0015]     turning off the switching transistor after the data writing timing, raising the operational voltage to a high voltage, turning on the reset transistor for driving the light emitting diode.  
         [0016]     As to the driving method described above, the step of turning on the switching transistor is by inputting a scan voltage via the scan line. The start voltage and the data voltage are applied to the gate terminal of the driving transistor via the signal line.  
         [0017]     In the exemplary embodiment, the reset transistor is turned off after a delay time, when the switching transistor is turned on by the scanning voltage via the scan line;  
         [0018]     and the delay time is determined by a time of tuning on the switching transistor.  
         [0019]     As to the driving method described above, the gate terminal of the reset transistor is electrically connected to an autozero line. The first terminal of the light emitting diode is an anode, and the second terminal thereof is a cathode.  
         [0020]     To achieve the object described above, the present invention discloses another pixel structure of a display, which comprises: a switching transistor, a driving transistor, a first capacitor, a light emitting diode and a reset transistor. A gate terminal of the switching transistor is electrically connected to a scan line, and a source terminal thereof is electrically connected to a signal line. A gate terminal of the driving transistor is electrically connected to a drain terminal of the switching transistor. The first capacitor is disposed between the gate terminal of the driving transistor and a source terminal thereof. The light emitting diode has a second terminal electrically connected to a ground voltage, and a first terminal electrically connected to a source terminal of the driving transistor. A gate terminal of the reset transistor is electrically connected to an autozero, a source terminal is electrically connected to the driving transistor, and a drain terminal electrically connected to an operational voltage.  
         [0021]     As to the pixel structure described above, the driving method thereof comprises: turning on the switching transistor at a threshold voltage writing timing, then raising the ground voltage to a high voltage for turning off the light emitting diode and applying a start voltage to the gate terminal of the driving transistor; turning off the reset transistor at an data writing timing for turning off the light emitting diode, and applying an data voltage to the gate terminal of the driving transistor; and turning off the switching transistor after the data writing timing, lowering the ground voltage to a low voltage for driving the light emitting diode, and turning on the reset transistor.  
         [0022]     As to the driving method described above, the step of turning on the switching transistor is by inputting a scan voltage via the scan line. The start voltage and the data voltage are applied to the gate terminal of the driving transistor via the signal line.  
         [0023]     In the exemplary embodiment, the ground voltage is raised to the high voltage after a delay time, when the switching transistor is turned on by the scanning voltage via the scan line; and the delay time is determined by a time of tuning on the switching transistor.  
         [0024]     In the exemplary embodiment, the gate terminal of the reset transistor is electrically connected to an autozero line.  
         [0025]     As to the pixel structure described above, the switching transistor, the driving transistor and the reset transistor are thin film transistors.  
         [0026]     As to the pixel structure described above, the switching transistor, the driving transistor and the reset transistor are made from poly-silicon or amorphous silicon.  
         [0027]     As to the pixel structure described above, the first terminal of the light emitting diode is an anode, and the second terminal thereof is a cathode.  
         [0028]     As to the pixel structure described above, the light emitting diode is made from an organic material.  
         [0029]     As to the driving method described above, the start voltage Vo is applied to the gate terminal of the driving transistor so that a gate voltage thereof is Vo; and a source voltage is Vo−V T , wherein the V T  is a threshold voltage of the driving transistor.  
         [0030]     As to the driving method described above, the data voltage Vdata is applied to the gate terminal of the driving transistor so that a voltage drop on the first capacitor is Vdata−(Vo−V T +ΔVdata), wherein the ΔVdata=K(Vdata−Vo). The driving current of the light emitting diode is proportional to (Vdata−Vo−ΔVdata) 2 .  
         [0031]     As to the driving method described above, K=Cs/Ctotal, Cs represents a capacitance of the first capacitor, and Ctotal is a sum of capacitances on the source terminal of the driving transistor.  
         [0032]     In the exemplary embodiment, the pixel structure further comprises a second capacitor disposed between the source terminal and the drain terminal of the reset transistor for adjusting the K. In another embodiment, the second capacitor is disposed between the first and the second terminals of the light emitting diode.  
         [0033]     In order to make the aforementioned and other objects, features and advantages of the present invention understandable, a preferred embodiment accompanied with figures is described in detail below. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0034]      FIG. 1  shows a conventional pixel structure of an organic LED display includes two N-type thin film transistors.  
         [0035]      FIG. 2A  shows another conventional pixel structure of an organic LED display.  
         [0036]      FIG.2B  shows a timing diagram of the pixel structure of the display shown in  FIG. 2A .  
         [0037]      FIG.2C  shows luminance with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits.  
         [0038]      FIG. 2D  shows luminance difference with the data voltage Vdata for the modified voltage follower (solid) and a standard voltage follower (dashed) circuits, when the variations of the data voltage Vdata and the threshold voltage are under 2 V.  
         [0039]      FIG. 3A  shows a preferred embodiment of a pixel structure of a display.  
         [0040]      FIG. 3B  shows a timing diagram related to a driving method of the pixel structure as shown in  FIG. 3A .  
         [0041]      FIG. 4A  shows another preferred embodiment of a pixel structure of a display.  
         [0042]      FIG. 4B  shows another timing diagram related to a driving method of the pixel structure as shown in  FIG. 4A . 
     
    
     DETAILED DESCRIPTION  
       [0043]     Following are the descriptions of the present to interpret the feature thereof. The scope of the present invention, however, is not limited thereto.  
         [0044]     The present invention discloses a pixel structure of a display and a driving method thereof for compensating the threshold voltage of the thin film transistors.  
         [0045]      FIG. 3A  shows a preferred embodiment of a pixel structure of a display.  FIG. 3B  shows a timing diagram related to a driving method of a preferred embodiment of the present invention. The pixel structure shown in  FIG.3A  includes three N-type transistors: a switch transistor  310 , a driving transistor  320  and a reset transistor  330 .  
         [0046]     A gate terminal of the switching transistor  310  is electrically connected to a scan line  310   a,  and a source terminal thereof is electrically connected to a signal line  310   b,  i.e. a data signal line. A drain terminal thereof is electrically connected to the driving transistor  320  and electrically connected to the reset transistor  330  via a capacitor  340 . A gate terminal of the reset transistor  330  is electrically connected to an autozero line AZ, a drain terminal thereof is electrically connected to the driving transistor  320 , and a source terminal is electrically connected to a ground voltage V SS . The anode of the light emitting diode  350  is electrically connected to an operational voltage V DD , and the cathode thereof is electrically connected to the drain terminal of the driving transistor  320 . The capacitor  340  is disposed between the gate and source terminals of the driving transistor  320  for storing the threshold voltage and the data voltage.  
         [0047]     In a preferred embodiment, the pixel structure of the present invention includes thin film transistors and made from, such as poly-silicon or amorphous silicon. In the embodiment, the light emitting diode  350  can be an organic light emitting diode. However, the present invention is not limited thereto. Any other types of transistors or light emitting diodes can also be applied in the present invention. In addition to the N-type transistors, the present invention also can use P-type transistors by simply modifying the design of the driving part.  
         [0048]      FIG. 3B  is a timing diagram related to a driving method of the preferred embodiment of the pixel structure of the display shown in  FIG. 3A . A threshold voltage (V T ) is applied to the capacitor  340  at a threshold voltage writing timing. The data signal is applied to the pixel at a data writing timing. The light emitting diode  350  then illuminates according to the data signal. At the beginning of the V T  writing timing, the scanning signal voltage Vscan on the scan line  310   a  is raised to a high voltage for turning on the switching transistor  310 . The V AZ  on the AZ line is lowered to a low voltage for turning off the reset transistor  330 . The rise of the V AZ  and the lowering of the Vscan can occur simultaneously or the rise of the V AZ  delays for a period of time as indicated by the dash line for synchronization with the switching transistor  310 . The delay time depends on a time from the raising of the Vscan to the turning on of the switching transistor  310 . Then, a start voltage Vo is applied to the signal line  310   b.  The current passes through the driving transistor  320  is zero. The voltage level V G  of the gate terminal of the driving transistor  320  is charged to Vo, and the voltage level V S  of the source terminal is charged to Vo−V T . −At the data writing timing, the operational voltage V DD  is in a low state for turning off the light emitting diode  350 , that is, no current is passed through the terminals of the operational voltage V DD  and the ground V SS . The data voltage Vdata from the signal line  310   b  is electrically connected to the source terminal of the switching transistor  310 . The voltage drop on the capacitor  340  is Vdata−(Vo−V T +ΔVdata), where ΔVdata=K(Vdata−Vo) and K=Cs/Ctotal, Cs represents the capacitance of the capacitor  340 , and Ctotal represents a sum of capacitances on the source terminal of the driving transistor  320 . Moreover, in an alternative embodiment of the present invention, another capacitor  360  can be disposed between the source and drain terminals of the reset transistor  330  for changing the Ctotal and adjusting the K in response with the design requirement.  
         [0049]     After the data writing time, the switching transistor  310  is turned off. The operational voltage V DD  is raised to a high voltage for driving the light emitting diode  350 , the V AZ  also is in a high state for turning on the reset transistor  330 . After the switching transistor  310  is turned off, the driving transistor  320  is floating. Therefore, the voltage drop on the capacitor  340  is still Vdata−(Vo−V T +ΔVdata). Because the driving transistor  320  is operated in a saturation region, the current is proportional to the [Vdata−(Vo−V T +ΔVdata)−V T]   2 , or (Vdata−Vo−ΔVdata) 2 . Accordingly, the current of the light emitting diode  350  is irrelevant to the V T  of the driving transistor  320 . Therefore, the operation of the pixel structure of the display does not depend on the V T  and is affected thereby.  
         [0050]      FIG. 4A  shows another preferred embodiment of a pixel structure of a display.  FIG. 4B  shows a timing diagram related to a driving method of another preferred embodiment of the present invention. The pixel structure shown in  FIG.4A  includes three N-type transistors: a switch transistor  410 , a driving transistor  420  and a reset transistor  430 . A gate terminal of the switching transistor  410  is electrically connected to a scan line  410   a,  and a source terminal thereof is electrically connected to a signal line  410   b,  i.e. a data signal line. A drain terminal thereof is electrically connected to the driving transistor  420  and electrically connected to the anode of the light emitting diode  450  via the capacitor  440 . A gate terminal of the reset transistor  430  is electrically connected to an autozero line AZ, a drain terminal thereof is electrically connected to the operational voltage V DD , and a source terminal is electrically connected to the driving transistor  420 . A cathode of the light emitting diode  450  is electrically connected to an ground voltage V SS . The source terminal of the driving transistor  420  is electrically connected to the anode of the light emitting diode  450 . The capacitor  440  is disposed between the gate and source terminals of the driving transistor  420  for storing the threshold voltage and the data voltage.  
         [0051]     In a preferred embodiment, the pixel structure of the present invention is composed of thin film transistors and made from, such as poly-silicon or amorphous silicon. In the embodiment, the light emitting diode  450  can be an organic light emitting diode. However, the present invention is not limited thereto. Any other types of transistors or light emitting diodes can also be applied thereto. In addition to the N-type transistors, the present invention also can use P-type transistors, by simply modifying the design of the driving part.  
         [0052]      FIG. 4B  is a timing diagram related to a driving method of the preferred embodiment of the pixel structure of the display shown in  FIG. 4A . A threshold voltage (V T ) is applied to the capacitor  440  at a threshold voltage writing timing. The data signal is applied to the pixel at a data writing timing. The light emitting diode  450  then illuminates according to the data signal.  
         [0053]     At the beginning of the V T  writing timing, the scanning signal voltage Vscan on the scan line  410   a  is raised from a low voltage level to a high voltage level for turning on t h e switching transistor  410 . The V SS  rises to a high voltage level. The rise of the V SS  and the raise of the Vscan can occur simultaneously or the rise of the V SS  delays for a period of time as indicated by the dash line for synchronization with the switching transistor  410 . The delay time depends on a time from the raising of the Vscan to the turning on of the switching transistor  410 . A start voltage Vo is then applied to the signal line  410   b.  The current passes through the driving transistor  420  is zero. In the driving transistor  420 , the voltage level V G  of the gate terminal is charged to Vo, and the voltage level V S  of the source terminal is charged to Vo−V T .  
         [0054]     At the data writing timing, the V AZ  on the AZ line is lowered to a low voltage for turning off the reset transistor  430  and avoiding any current flowing through the terminals of the V DD  and the V SS . A data voltage Vdata is applied to the signal line  410   b,  which is electrically connected to the source terminal of the switching transistor  410 . The voltage drop on the capacitor  440  is Vdata−(Vo−V T +ΔVdata), wherein ΔVdata=K(Vdata−Vo) and K=Cs/Ctotal, Cs represents the capacitance of the capacitor  440 , and Ctotal represents a sum of capacitances on the source terminal of the driving transistor  420 . Moreover, in an alternative embodiment, another capacitor  460  can be disposed between the anode and cathode of the light emitting diode  450  for changing the Ctotal and adjusting the K in response with the design requirement.  
         [0055]     After the data writing time, the switching transistor  410  is turned off. The V AZ  is raised to a high voltage for turning on the reset transistor  430 , and the V SS  is lowered to a low voltage for driving the light emitting diode  450 . After the switching transistor  410  is turned off, the gate terminal of the driving transistor  420  is floating. Therefore, the voltage drop on the capacitor  440  is still Vdata−(Vo−V T  +ΔVdata). Because the driving transistor  420  is in saturation region, the current is proportional to the [Vdata−(Vo−V T +ΔVdata)−V T ] 2 , or (Vdata−Vo−ΔVdata) 2 . Accordingly, the current of the light emitting diode  450  is irrelevant to the V T  of the driving transistor  420 . Therefore, the operation of the pixel structure of the display does not depend on the V T  and is affected thereby.  
         [0056]     Accordingly, the present invention discloses a pixel structure of a display and a driving method thereof, which are easier that those of prior art and compensate the threshold voltage of the thin film transistors.  
         [0057]     Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be constructed broadly to include other variants and embodiments of the invention which may be made by those skilled in the field of this art without departing from the scope and range of equivalents of the invention.