Patent Publication Number: US-2013249885-A1

Title: Display devices, sensing circuits and methods for sensing and compensating for threshold voltage shift of transistor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of Taiwan Patent Application No. 101110320, filed on Mar. 26, 2012, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a display device, and more particularly to a display device capable of sensing and compensating for a threshold voltage shift of a transistor. 
     2. Description of the Related Art 
     Metal Oxide Thin Film Transistor (MOTFT) has become a popular choice when manufacturing a display panel in recent years because the cost is lower than Low Temperature Poly-silicon (LTPS) TFT and the performance is better than the Amorphous Silicon (a-Si) TFT. 
     However, one drawback of the MOTFT is poor stability. When a display panel is functioning, positive and negative voltages are repeatedly applied to a gate of the MOTFT for turning on or off the MOTFT. After a long period of operation, the stress repeatedly applied to the gate of the MOTFT causes the threshold voltage of the MOTFT to gradually decrease, which may cause the MOTFT to finally have a negative threshold voltage. 
       FIG. 1  shows a current-voltage characteristic curve of a MOTFT, where the X axis represents the gate-source voltage V GS  and the Y axis represents the drain current I D . The curve  10  is an original characteristic curve of a MOTFT, and the curve  20  is a measured characteristic curve of the MOTFT after a long period of operation. According to the curve  10 , the original threshold voltage of the MOTFT is V th =V GS1 , which is a small positive voltage. Therefore, for operating the display panel, the system high voltage may be designed as a positive voltage (for example, 10V) higher than the threshold voltage V th  for turning on the MOTFT and the system low voltage may be designed as a negative voltage (for example, −3V) for turning off the MOTFT. 
     However, after a long operation period, the threshold voltage V th  is shifted from V GS1  to V GS2 , which is a negative voltage (for example, −5V). The voltage shift of the threshold voltage causes the MOTFT to be unable to be turned off normally by the system low voltage, resulting in malfunction of the display panel. 
     Therefore, a novel display device capable of sensing and compensating for a threshold voltage shift of a transistor to solve the above-mentioned problems is highly required. 
     BRIEF SUMMARY OF THE INVENTION 
     Display devices, sensing circuits and methods for sensing and compensating for threshold voltage shift of a transistor are provided. An exemplary embodiment of a display device comprises a pixel array and a sensing circuit. The pixel array comprises a plurality of active display pixels and at least one dummy display pixel. The active display pixels and the dummy display pixel respectively comprise a transistor. The sensing circuit is coupled to the transistor of the dummy display pixel for sensing a voltage at the transistor of the dummy display pixel and generating a trigger signal according to the sensed voltage. A voltage generating device generates or adjusts a first control voltage according to the trigger signal and the transistors of the active display pixels and the dummy display pixel are turned off in response to the first control voltage. 
     An exemplary embodiment of a sensing circuit comprises a constant current source, a voltage sensing device and a converting device. The constant current source is coupled to a first electrode of a transistor of a dummy display pixel. The dummy display pixel is comprised in a pixel array. The voltage sensing device is coupled to the first electrode of the transistor of the dummy display pixel for sensing a voltage at the first electrode when the transistor is turned on and generating a sensed signal according to the voltage. The converting device is coupled to the voltage sensing device for generating a trigger signal according to the sensed signal. A voltage generating device generates or adjusts a first control voltage according to the trigger signal and a plurality of transistors of a plurality of active display pixels and the transistor of the dummy display pixel in the pixel array are turned off in response to the first control voltage. 
     An exemplary embodiment of a method for sensing and compensating for threshold voltage shift of a transistor comprises: providing a constant current source to a first electrode of a transistor in a dummy display pixel of a pixel array when the transistor is turned on for sensing a voltage at the first electrode of the transistor and generating a trigger signal according to the voltage; and adjusting a level of a first control voltage according to the trigger signal. The pixel array further comprises a plurality of active display pixel, each active display pixel comprises a transistor, and the transistors of the active display pixels and the transistor of the dummy display pixel are turned off in response to the first control voltage. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a current-voltage characteristic curve of a MOTFT; 
         FIG. 2  shows one of the various types of display devices of the invention according to an embodiment of the invention; 
         FIG. 3  shows a portion of a display panel according to an embodiment of the invention; 
         FIG. 4  shows a block diagram of a sensing circuit according to an embodiment of the invention; 
         FIG. 5  is a circuit diagram of a voltage sensing device according to an embodiment of the invention; and 
         FIG. 6  is a flow chart of a method for sensing and compensating for threshold voltage shift of a transistor according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 2  shows one of the various types of display devices of the invention according to an embodiment of the invention. As shown in  FIG. 2 , the display device may comprise a display panel  201 , where the display panel  201  may comprise a gate driving circuit  210 , a data driving circuit  220 , a pixel array  230  and a controller chip  240 . The gate driving circuit  210  generates a plurality of gate driving signals to drive a plurality of display pixels in the pixel array  230 . The data driving circuit  220  generates a plurality of data driving signals to provide data to the display pixels of the pixel array  230 . The controller chip  240  generates a plurality of timing signals, comprising clock signals, a system reset signal and a start pulse, and so on, and a plurality of control voltages for controlling operations of the display panel  201 . 
     In addition, the display device of the invention may further be comprised in an electronic device  200 . The electronic device  200  may comprise the above-mentioned display panel  201  and an input device  202 . The input device  202  receives image signals and controls the display panel  201  to display images. According to an embodiment of the invention, the electronic device  200  may be implemented as various devices, comprising: a mobile phone, a digital camera, a personal digital assistant (PDA), a lap-top computer, a personal computer, a television, a vehicle displayer, a portable DVD player, or any apparatus with image display functionality. 
     According to an embodiment of the invention, the proposed display device is capable of sensing and compensating for threshold voltage shift of the transistor.  FIG. 3  shows a portion of a display panel according to an embodiment of the invention. As shown in  FIG. 3 , the pixel array  230  comprises a plurality of display pixels, such as display pixel  300 . Each pair of interlacing data electrodes (represented by D 1 , D 2 , D 3 , . . . , Dm) and gate electrodes (represented by G 1 , G 2 , G 3 , . . . , Gn+1) controls a display pixel  300 . As shown in the figure, an equivalent circuit of the display pixel  300  comprises a transistor (such as the transistors Q 11 -Q 1   m , Q 21 -Q 2   m , . . . , and Q(n+1)1-Q(n+1)m) for controlling data input and a storage capacitor (such as the capacitors C 11 -C 1   m , C 21 -C 2   m , . . . , and C(n+1)1-C(n+1)m). According to an embodiment of the invention, the transistors Q 11 -Q 1   m , Q 21 -Q 2   m , . . . , and Q(n+1)1-Q(n+1)m may be the Metal Oxide Thin Film Transistors (MOTFTs) formed by Indium Gallium Zinc Oxide (IGZO). 
     Note that in the embodiment of the invention, the display pixel  300  may be an active display pixel or a dummy display pixel. The active pixels are the display pixels disposed in the active area (AA)  320 , where each display pixel  300  in the active area  320  may correspond to a brightening point on the pixel array  230 . That is, each display pixel  300  corresponds to a single pixel for a monochromatic display or a single sub-pixel for a color display. The sub-pixel can be red (represented by “R”), blue (represented by “B”), or green (represented by “G”). In other words, a single pixel is formed by an RGB (three display pixels) combination. 
     On the other hand, the display pixels  300  disposed outside of the active area  320  are the dummy display pixels. According to an embodiment of the invention, the electronic components comprised in the dummy display pixels Q(n+1)1˜Q(n+1)m may be almost the same as those comprised in the active display pixel, except for the way in which the power source is coupled thereto (which will be discussed in more detail in the following paragraphs). In addition, in the embodiments of the invention, when the display panel  201  operates, the transistors in the dummy display pixels Q(n+1)1˜Q(n+1)m may be turned on or off according to the corresponding gate driving signals. The difference between the active display pixels and dummy display pixels is that even if the transistors of the dummy display pixels are turned on, the crystal will not be twisted. For example, the Indium Gallium Zinc Oxide (IGZO) may not be electroplated when processing the dummy display pixels Q(n+1)1˜Q(n+1)m, or the common voltage VCOM may not be supplied to the dummy display pixels Q(n+1)1˜Q(n+1)m, or others, so that the crystal on the dummy display pixels Q(n+1)1˜Q(n+1)m will not be twisted. Note that one of ordinary skilled in the art would recognize that there are still many different ways for manufacturing the dummy display pixels Q(n+1)1˜Q(n+1)m, and the invention should not be limited to those mentioned. 
     According to an embodiment of the invention, the dummy display pixels Q(n+1)1˜Q(n+1)m of the pixel array  230  may further be coupled to a sensing circuit  340 . The sensing circuit  340  may sense a voltage at the transistor of the dummy display pixels Q(n+1)1˜Q(n+1)m and generate a trigger signal S TRI  according to the sensed voltage, wherein the amount of change in the sensed voltage reflects the amount of threshold voltage shift. A voltage generating device  450  in the display device may further generate or adjust a control voltage V GL  (which will be discussed in more detail in the following paragraphs) according to the trigger signal S TRI  and the gate driving circuit  210  may receive the control voltage V GL  for turning off the transistors  401  in the active display pixels and the dummy display pixels Q(n+1)1˜Q(n+1)m. 
     According to some embodiments of the invention, the sensing circuit  340  may be integrated in the controller chip  240  and may periodically (for example, every multiple frames) or aperiodically enter a sensing mode in response to a control command received from the controller chip  240  for sensing the voltage change of the transistor in the dummy display pixels Q(n+1)1˜Q(n+1)m. Note that in some other embodiments of the invention, the sensing circuit  340  may also be an independent circuit or may be integrated with other devices or circuits in the display device. Therefore, the invention should not be limited to any specific embodiment. 
     In addition, it is noted that although a raw of dummy display pixels Q(n+1)1˜Q(n+1)m disposed under the active area  320  is shown in  FIG. 3 , the invention should not be limited thereto. According to a concept of the invention, as shown in  FIG. 4 , even if there is only one dummy display pixel  400  disposed in the pixel array  230 , the sensing circuit  440  may still sense the voltage change of the transistor through the dummy display pixel  400 . Referring back to  FIG. 3 , when there are multiple dummy display pixels Q(n+1)1˜Q(n+1)m disposed on the pixel array  230 , the sensing circuit  340  may sense the voltage change through different dummy display pixels Q(n+1)1˜Q(n+1)m in turn, or may take an average of the sensed voltage changes obtained from different dummy display pixels Q(n+1)1˜Q(n+1)m and the invention should not be limited to any specific embodiment. 
     In addition, it should be further noted that in the embodiments of the invention, the dummy display pixels are not limited to be disposed under the active area  320 . In other words, the dummy display pixels may also be disposed above the active area  320 , or on the left or right hand side of the active area  320 . Therefore, the invention should not be limited to any specific embodiment. 
       FIG. 4  shows a block diagram of a sensing circuit according to an embodiment of the invention. For brevity, in the embodiment of the invention, the sensing circuit  440  is only coupled to a dummy display pixel  400 . As shown in the figure, the sensing circuit  440  may comprise a constant current source  441 , a voltage sensing device  442  and a converting device  443 . The constant current source  441  is coupled to a first electrode of the transistor  401 . The voltage sensing device  442  is also coupled to the first electrode of the transistor  401  for sensing the voltage V O  at the first electrode when the transistor  401  is turned on and generating a sensed signal S SEN  according to the voltage V O . The converting device  443  is coupled to the voltage sensing device  442  for or generating a trigger signal S TRI  according to the sensed signal S SEN . The trigger signal S TRI  may be input to a voltage generating device  450  in the display device, wherein the voltage generating device  450  generates or adjusts the control voltage V GL  according to the trigger signal S TRI . 
     The gate driving circuit  210  receives the control voltage V GL  from the voltage generating device  450  and receives another control voltage V GH  from another voltage generating device (not shown) for controlling the voltage level on each gate line so as to turn on or off the corresponding transistor according to the control voltages V GL  and V GH . According to an embodiment of the invention, the voltage generating device may be the regulators inside of the controller chip  240  for generating and providing the control voltages V GL  and V GH . 
     According to an embodiment of the invention, a second electrode of the transistor  401  in the dummy display pixel  400  is coupled to a gate line for receiving the control voltages V GL  and V GH , and a third electrode of the transistor  401  is coupled to another control voltage. According to an embodiment of the invention, the control voltage coupled to the third electrode of the transistor  401  may be a high operation voltage of the display panel, such as the voltage V DD . In addition, because the sensing circuit  440  senses the voltage V O  at the first electrode of the transistor  401  when the transistor  401  is turned on, the second electrode of the transistor  401  shown in  FIG. 4  is directly coupled to the control voltage V GH  to represent that the transistor  401  is being turned on. 
     According to the concept of the invention, because the dummy display pixel is coupled to the active gate line as the active display pixels, the amount of time that the transistor of the dummy display pixel being turned on or off equals to that of the active display pixel. Therefore, under the same operation environment, when the threshold voltage of the transistor in the active display pixel starts to drift, the threshold voltage of the transistor in the dummy display pixel starts to drift, too. In this manner, once the sensing circuit coupled to the dummy display pixel detects the voltage change (as discussed above, the amount of change in the sensed voltage reflects the amount of threshold voltage shift), the sensed amount of voltage change may be reflected on the trigger signal so as to accordingly adjust the control voltage V GL . Because of the adjustment, even if the threshold voltage shift happens, the transistor in the display pixels (either active or dummy) may still be successfully turned off according to the adjusted control voltage V GL . 
     As shown in the figure, because the second and third electrode of the transistor  401  are respectively coupled to a constant voltage (the V GH  and V DD  as shown), and a constant current source  441  is provided by the sensing circuit  440 , the voltage V O  at the first electrode of the transistor may be determined according to the amount of current of the constant current source  441 . Referring back to  FIG. 1 , it is noted that there is only one intersection point for each characteristic curve with the constant current I. Therefore, when the constant current I is provided, a corresponding voltage V GS  may be obtained for each characteristic curve. Suppose that the first electrode of the transistor is the drain of the transistor, when the constant current is provided, the voltage at the first electrode may be V O =V GH −V GS . 
     Because the voltage V GH  is a constant voltage, from the equation shown above, when the threshold voltage of the transistor changes, the voltage V O  at the first electrode changes, accordingly. In other words, the voltage V O  at the first electrode varies with the threshold voltage of the transistor. Once the sensing circuit detects that the voltage V O  has been changed, the sensing circuit may reflect the amount of change on the trigger signal S TRI  so that the voltage generating device  450  may regenerate or adjust the control voltage V GL  according to the trigger signal S TRI . The gate driving circuit  210  may receive the adjusted control voltage V GL  from the voltage generating device  450  and therefore, the corresponding transistor may be successfully turned off in response to the adjusted control voltage V GL . 
     According to an embodiment of the invention, the voltage sensing device  442  may be an analog to digital converter for converting the sensed voltage V O  into a digital sensed signal S SEN . The converting device  443  may be a Look-Up Table (LUT) device for outputting a corresponding trigger signal S TRI  according to the input sensed signal S SEN . 
     According to another embodiment of the invention, the voltage sensing device  442  may also comprise a plurality of comparators for converting the sensed voltage V O  into a digital sensed signal S SEN .  FIG. 5  is a circuit diagram of a voltage sensing device according to an embodiment of the invention. The comparators  501 - 50   k  are arranged to compare the sensed voltage V O  with a plurality of reference voltages Vref 1 -Vrefk to generate a plurality of comparison results, such as the bits b 1 - bk shown in  FIG. 5 . The sensed signal S SEN  may be a signal composed by the bits b 1 -bk. The converting device  443  may be a Look-Up Table (LUT) device for outputting a corresponding trigger signal S TRI  according to the input sensed signal S SEN . 
       FIG. 6  is a flow chart of a method for sensing and compensating for threshold voltage shift of a transistor according to an embodiment of the invention. First of all, a constant current source is provided to a first electrode of a transistor in a dummy display pixel of a pixel array when the transistor is turned on for sensing a voltage at the first electrode of the transistor and generating a trigger signal according to the voltage (Step S 601 ). Next, a level of a control voltage V GL  is adjusted according to the trigger signal (Step S 602 ). 
     Based on the concept of the invention, the threshold voltage shift of a transistor may be sensed and compensated for without using a complicated circuit layout. In addition, the circuit introduced above may be compatible with conventional display panels. In other words, the circuit introduced above may be directly combined with the driving circuit and peripheral circuit in a conventional display panel without causing any effect thereto. In addition, because the threshold voltage shift is sensed through the dummy display pixel(s), the sensing operations will not degrade the performance of the display panel, and the extra sensing circuit will also not shrink the aspect ratio of the display panel. In addition, the introduced sensing and compensating operations may be performed any time after the display panel is powered on and a period or a time of entering the sensing mode to sense the voltage may be set via the control command of the controller chip  240 . In other words, there is no need to frequently perform the sensing and compensating operations during every frame, thus, power consumption can be greatly reduced. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.