KR20080022696A - Pixel structure for active matrix device - Google Patents

Abstract

A pixel structure for an active matrix display device is provided to suppress an image degradation such as brightness variation, stripe patterns, and crosstalk, by compensating for a leakage current in a switching TFT(Thin Film Transistor). A first switching transistor(MSW1) selects a unit pixel which is defined by data and scan lines. A driving TFT(MDRV) receives a data voltage from the first switching transistor at a gate terminal and supplies a corresponding current to an OLED(Organic Light Emitting Diode). A capacitor stores the data voltage which is applied to the gate of the driving TFT. A second switching transistor(MSW2) is controlled by a scan signal and cascoded to a drain-source region of the first switching transistor. A shield capacitor is connected between a junction between the first and second switching transistors and a voltage supply and supplies the charges leaked by a leakage current of the first and second switching transistors.

Description

Pixel Structure of Active Drive Type Display Device {PIXEL STRUCTURE FOR ACTIVE MATRIX DEVICE}

1 is a unit pixel circuit diagram of a conventional active driving type organic light emitting display device;

2 is a unit pixel circuit diagram of a conventional active driving liquid crystal display device;

3 is an exemplary diagram of a leakage current characteristic according to a voltage of a thin film transistor;

4 is a diagram illustrating a configuration of a first embodiment of an active driving type organic light emitting display device according to the present invention;

5 is an exemplary diagram of a temporal change in leakage current when the switching transistor of FIG. 4 is turned off.

6 is a view showing the configuration of a second embodiment of an active driving type organic light emitting display device according to the present invention;

7 is a view showing the configuration of a third embodiment of an active driving type organic light emitting display device according to the present invention;

8 is a diagram showing the configuration of a first embodiment of an active driving type liquid crystal display device according to the present invention;

9 is a diagram showing the configuration of a second embodiment of an active driving type liquid crystal display device according to the present invention;

FIG. 10 is a diagram showing the configuration of a third embodiment of an active driving type liquid crystal display device according to the present invention; FIG.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active driving type organic light emitting display device and a liquid crystal display device using a thin film transistor, and in particular, a pixel for minimizing a change in luminance due to leakage current of a switching transistor in a pixel, and a degradation of image quality such as streaks and crosstalk. It's about structure.

In accordance with the trend toward larger displays and higher resolution, liquid crystal displays and organic light emitting displays have adopted an active driving method.

To this end, a unit pixel of a liquid crystal display includes a liquid crystal, a storage capacitor storing a data voltage controlling the same, and a switching thin film transistor for transferring the data voltage.

In addition, the unit pixel of the organic light emitting display device transfers an organic light emitting diode and a driving thin film transistor (TFT) for driving the organic light emitting diode, a storage capacitor for storing a data voltage for controlling the driving thin film transistor, and a data voltage. A thin film transistor for switching is provided.

Thin film transistors are largely classified into polysilicon thin film transistors and amorphous silicon thin film transistors according to the type of active layer. In the case of polysilicon thin film transistors, a crystallization method using a laser is generally used, and thus the thin film transistors have high electron mobility and excellent photostability and device reliability compared to amorphous silicon transistors.

On the other hand, due to the limitations in the characteristics of laser irradiation, it is difficult to secure uniform device characteristics over a large area, and the manufacturing process is complicated.

The disadvantages are recognized as a problem that must be overcome due to low temperature process, cost reduction, and the like, and various crystallization methods have been proposed to solve this problem. However, the newly proposed method has a weak point that the leakage current of the transistor is very high in most cases, which causes a lot of difficulties in practical application.

A switching thin film transistor used in a pixel circuit of an active driving type display device needs to maintain a corresponding voltage for one frame after storing a data voltage and transferring new data. At this time, the leakage current of the switching thin film transistor is applied to the drain. Can be changed by the data signal.

The change in leakage current caused by this result causes a poor image quality of the display device such as a change in luminance and a crosstalk phenomenon due to the difference in leakage current between pixels even when the same pixel signal is applied.

1 is a circuit diagram of a unit pixel of a conventional voltage write type active driving type organic light emitting display device.

Referring to FIG. 1, a unit pixel of an active driving type organic light emitting display device includes a switching transistor MSW, a driving transistor MDRV, a capacitor C _ST , and an organic light emitting diode OLED.

In the switching transistor MSW, the drain region is connected to the data line VDATA, the source region is connected to the gate VG of the driving transistor, and the data voltage VDATA applied through the image signal line is applied to the gate signal line. The gate signal VSCAN is applied to the driving transistor MDRV and the capacitor C _ST .

The driving transistor MDRV serves as a current source element to supply a corresponding current to the organic light emitting diode OLED according to the image signal VDATA transmitted through the switching transistor MSW.

The capacitor C _ST stores the bias voltage applied to the gate terminal of the driving transistor MDRV so that the driving transistor MDRV can maintain its operation as a current source even when the switching transistor MSW is cut off.

The current ID flowing between the source and the drain of the driving transistor MDRV is as follows.

ID = 1/2 * k * (V _GS -V _TH ) ²

(Current-voltage relationship in the saturation region k = μ * C _OX * W / L, where μ is the field effect mobility, C _OX is the capacitance of the insulating layer, W is the channel width of the thin film transistor, L is a thin film transistor) Each represents a channel length)

2 is a circuit diagram of a unit pixel of a conventional active driving liquid crystal display.

Referring to FIG. 2, a unit pixel of an active driving type liquid crystal display includes a switching transistor MSW, a storage capacitor C _ST , and a common electrode VCOM which is a reference voltage for inverting driving of the liquid crystal C _LC . In addition, the final pixel circuit may additionally be modeled in such a way that the liquid crystal capacitor (CLC) is connected in parallel between both ends of the storage capacitor.

The switching transistor MSW has a drain region connected to the data line VDATA, a source region connected to one terminal of the storage capacitor C _ST , and an image according to the gate signal VSCAN applied through the gate signal line. It transmits or blocks the data voltage VDATA applied through the signal line to the storage capacitor C _ST and the liquid crystal C _LC .

The common electrode VCOM is a reference voltage for inverting driving of the liquid crystal C _LC . The image signal VDATA transferred through the switching transistor MSW has a positive value and a negative value based on the potential of the common electrode. It is periodically inverted and applied.

The storage capacitor CST stores the data voltage so that the voltage applied to the liquid crystal is maintained even when the switching transistor MSW is cut off.

3 shows leakage current characteristics according to the voltage of the thin film transistor. The leakage current of the thin film transistor MSW increases as the voltage V _DS between the drain and the source increases, and the voltage V _GS between the gate and the source. It can be seen that the higher the) is, the greater the characteristic.

Therefore, in the basic pixel structure of FIG. 2, the drain-source voltage of the switching transistor MSW not only changes in time according to the change of the data voltage VDATA within a frame, but also different drain-source depending on where the image is located. You will experience voltage. When different drain-source voltages are applied to each pixel, a deviation occurs in the average leakage current generated in each frame, which is applied to the gate because it means discharge of charge charged in the storage capacitor C _ST . The bias voltage to be changed. As a result, variations in output current and luminance between pixels may be intensified, resulting in deterioration of image quality such as streaks and crosstalk.

Accordingly, the present invention has been made to solve the above problems of the prior art, and an object of the present invention is to provide a pixel structure of an active driving type display device that can prevent image degradation due to leakage current of the thin film transistor. .

In order to achieve the above object, a pixel structure of an active driving display device according to an embodiment of the present invention includes a switching transistor for selecting a unit pixel defined by a data line and a scan line, and a pixel voltage applied from the data line. A pixel structure comprising at least a storage capacitor for storing a corresponding charge, the pixel structure further comprising a shield capacitor for supplying a charge leaked due to a leakage current of the switching transistor when the switching transistor is in an off state. do.

In addition, the present invention is an organic light emitting device; A first switching transistor for selecting a unit pixel defined by the data line and the scan line; A driving transistor receiving a data voltage transferred through the first switching transistor as a gate and supplying a corresponding current to the organic light emitting diode; A capacitor for storing a data voltage applied to a gate of the driving transistor; A second switching transistor controlled by a scan signal and connected in cascode form to a drain-source region of the first switching transistor; And a shield capacitor connected between a connection node of the first switching transistor and the second switching transistor and a power supply source, the shield capacitor supplying charges leaked due to leakage currents of the first switching transistor and the second switching transistor. do.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention; Note that the same components in the drawings are denoted by the same reference numerals and symbols as much as possible even if shown on different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

4 is a diagram illustrating a pixel circuit of an active driving type organic light emitting display device using a thin film transistor according to an exemplary embodiment of the present invention.

Referring to FIG. 4, the pixel structure of the present invention includes a first switching transistor MSW1 and a second switching transistor MSW2 for selecting each pixel defined by the data line VDATA and the scan line VSCAN; A storage capacitor C _ST for storing a corresponding charge according to the pixel voltage applied to the data line; A shield capacitor C _SHIELD for preferentially supplying charges leaked by the leakage currents of the switching transistors MSW1 and MSW2; A driving transistor (MDRV) for receiving a power supply voltage through the scan line and conducting a corresponding current according to an image signal transmitted through the switching transistors MSW1 and MSW2; An organic light emitting diode (OLED) which emits light by a current conducted to the driving transistor (MDRV) is included.

The first switching transistor MSW1 has a drain-source current path having a data line VDATA or a corresponding node and a drain-source current of one node VX of the shield capacitor C _SHIELD and the second switching transistor MSW2. It is connected between the passage and the gate is connected to the gate signal line VSCAN.

The second switching transistor MSW2 has a drain-source current path and a drain-source current path of the first switching transistor and one node VX of the shield capacitor C _SHIELD and one node VG of the storage capacitor C _ST . The gate is connected to the gate signal line VSCAN.

The operation of the unit pixel of the active driving type organic light emitting display device having the above configuration is as follows.

Referring back to FIG. 4, during the ON period of the gate signal VSCAN applied through the gate line, the first switching transistor MSW1 and the second switching transistor MSW2 are turned on and applied through the data line. In the image signal VDATA, the same voltage is stored in the shield capacitor C _SHIELD and the storage capacitor C _ST through the first switching transistor MSW1 and the second switching transistor MSW2.

When the gate signal VSCAN is turned off, the drain-source current paths of the first switching transistor MSW1 and the second switching transistor MSW2 are blocked. However, when the switching transistor has a leakage current characteristic corresponding to that of FIG. 3, the switching transistor does not completely block the flow of charge and generates a different amount of leakage current per frame according to the voltage between the drain and the source applied. do.

The leakage current characteristics of the thin film transistor are shown in FIG. 3. That is, the thin film transistors tend to increase leakage current as the gate-source voltage VGS increases, and the leakage current decreases as the drain-source voltage VDS decreases. In particular, in the latter case, no current is generated without a potential difference, and the off resistance increases as the voltage V _DS between the drain and the source is small.

The added shield capacitor C _SHIELD and the first switching transistor MSW1 serve to supply a leakage current by an external voltage, thereby minimizing a voltage across the drain-source of the second switching transistor MSW2. Therefore, leakage current by the storage capacitor is minimized.

In addition, as the voltage across the drain-source is decreased, the off resistance of the second switching transistor is additionally increased, thereby reducing the leakage current caused by the second switching transistor.

Therefore, when the capacitance of the capacitor is properly selected, the potential of the gate node VG of the driving transistor can be kept intact for one frame.

FIG. 5 illustrates, as an example, the temporal change of the leakage current when the switching transistor is off in the pixel structure of FIG. 4.

Referring to FIG. 5, the leakage current induced by the data voltage may flow in or out through only the first switching transistor MSW1 in the case of the shield capacitor C _SHIELD , whereas in the case of the storage capacitor C _ST , It appears as a leakage current of the pixel only after passing through the off resistances of the first switching transistor MSW1 and the second switching transistor MSW2.

Therefore, initially, a leakage current is generated from the shield capacitor C _{SHIELD which} is easy to supply the charge, and after the charge in the shield capacitor C _SHIELD starts to be sufficiently exhausted or accumulated, the storage capacitor is stored through the second switching transistor MSW2. (C _ST ) outflow or inflow.

One frame period is short enough for the shield capacitor to cover most of the leakage current, and the ratio of the capacity of the shield capacitor C _SHIELD to the capacity of the storage capacitor C _ST may be added or subtracted according to the characteristics of the leakage current.

6 is a circuit diagram illustrating a unit pixel structure of a second exemplary embodiment of an active driving type organic light emitting display device according to the present invention. In the pixel structure of FIG. 4, the external voltage is increased by connecting the third switching transistor MSW3 to the drain-source region of the first switching transistor MSW1 in the form of a cascode to increase the off resistance. It is configured to further suppress the variation of leakage current by

When the shield capacitor and the transistor of the data line are connected in the form of a cascode, VDATA-VX, which is the drain-source voltage V _{DS that} is applied to the first switching transistor MSW1 of FIG. 4, is converted to the third switching transistor MSW3. ), The drain-source voltage V _DS applied to the first switching transistor MSW1 can be lowered to VY-VX. Accordingly, the leakage current flowing out of the shield capacitor is further reduced as illustrated in FIG. 3. Can be obtained.

In this embodiment, since the resistance of the final leakage current path is added, the total leakage current generated during a sufficiently short period of about one frame is reduced as compared with the case of FIG. 4, which is a gate bias of the driving transistor for one frame. VG changes will also decrease.

FIG. 7 is a circuit diagram illustrating a unit pixel structure of a third exemplary embodiment of a display device using an active driving type organic light emitting diode according to the present invention. In the pixel structure of FIG. 4, the external voltage is increased by connecting the third switching transistor MSW3 to the drain-source region of the second switching transistor MSW2 in the form of a cascode to increase the off resistance. It is configured to further suppress the variation of leakage current by

When the transistors of the storage capacitor and the shield capacitor are connected in the form of a cascode, VG-VX, which is a drain-source voltage (V _DS ) applied to the second switching transistor MSW2 of FIG. 4, is converted into a second transistor ( By distributing to the MSW2 and the third switching transistor MSW3, the drain-source voltage V _DS applied to the third switching transistor MSW3 can be lowered to VY-VX, and thus, from the storage capacitance due to the drop in the VY potential. The leakage leakage current is further reduced, so that the amount of change in VG, which is the gate bias of the driving transistor, for one frame may also be reduced.

Meanwhile, the configuration of the shield capacitor of the present invention can be applied to an active driving liquid crystal display device.

FIG. 8 is a circuit diagram showing a unit pixel structure of a first embodiment of an active driving type liquid crystal display device according to the present invention, in which the pixel structure of the present embodiment is an organic light emitting element and a driving transistor in the pixel structure of the embodiment shown in FIG. Instead, it consists of liquid crystals and capacitors.

Referring to FIG. 8, the first switching transistor MSW1 has a drain-source region connected to a data line VDATA or a node corresponding thereto, and the drain-source region is connected to one node of the shield capacitor C _SHIELD . It is connected to the drain-source region of VX and the second switching transistor MSW2, and the gate region is connected to the gate signal line VSCAN.

In the second switching transistor MSW2, a drain-source region is connected to the drain-source region of the first switching transistor MSW1 and one node VX of the shield capacitor C _SHIELD , and the drain-source region is a storage capacitor. It is connected to one node VG of CST, and a gate region is connected to gate signal line VSCAN.

The operation of the unit pixel of the liquid crystal display of this embodiment having the above configuration is as follows.

During the ON period of the gate signal VSCAN applied through the gate line, the first switching transistor MSW1 and the second switching transistor MSW2 are turned on, and the image signal VDATA applied through the data line is controlled to The same voltage is stored in the shield capacitor C _SHIELD and the storage capacitor CST through the first switching transistor MSW1 and the second switching transistor MSW2.

Thereafter, when the gate signal VSCAN is turned off, the drain-source terminals of the first switching transistor MSW1 and the second switching transistor MSW2 are blocked from flowing of charge. However, when the switching transistors MSW1 and MSW2 have the leakage current characteristics corresponding to those of FIG. 3, they do not completely block the flow of charge, but have different amounts of pixels per frame depending on the voltage across the drain-source applied. Leakage current is generated.

At this time, in the pixel structure of this embodiment, as in the structure of FIG. 4 described above, most of the leakage current is supplied through the first switching transistor MSW1 by the shield capacitor C _SHIELD , and the liquid crystal capacitor C _LC and The voltage VP stored in the storage capacitor C _ST remains intact for one frame.

9 is a circuit diagram showing a unit pixel structure of a second embodiment of an active driving type liquid crystal display device according to the present invention. In the pixel structure shown in FIG. 8, the third switching transistor MSW3 is connected to the drain-source region of the first switching transistor MSW1 in the form of a cascode to increase the off resistance. It is configured to further suppress the variation of leakage current by voltage.

When the shield capacitor and the transistor of the data line are connected in the form of a cascode, VDATA-VX, which is a drain-source voltage V _DS applied to the first switching transistor MSW1 of FIG. 8, is converted into a third switching transistor MSW3. ), The drain-source voltage V _DS applied to the first switching transistor MSW1 can be lowered to VY-VX. Accordingly, the leakage current flowing out of the shield capacitor is further reduced as illustrated in FIG. 3. Can be obtained.

In this embodiment, since the resistance of the final leakage current path is added, the total leakage current generated during a sufficiently short period of about one frame is reduced as compared with the case of FIG. 8, which is a gate bias of the driving transistor for one frame. VG changes will also decrease.

10 is a circuit diagram illustrating a unit pixel structure of a third embodiment of an active driving type liquid crystal display device according to the present invention. In the pixel structure of FIG. 8, the external voltage is increased by connecting the third switching transistor MSW3 to the drain-source region of the second switching transistor MSW2 in the form of a cascode to increase the off resistance. It is configured to further suppress the variation of leakage current by

When the transistors of the storage capacitor and the shield capacitor are connected in the form of a cascode, VG-VX, which is the drain-source voltage (V _DS ) applied to the second switching transistor MSW2 of FIG. 8, is converted into a second transistor ( By distributing to the MSW2 and the third switching transistor MSW3, the drain-source voltage V _DS applied to the third switching transistor MSW3 can be lowered to VY-VX, and thus, from the storage capacitance due to the drop in the VY potential. The leakage leakage current is further reduced, so that the amount of change in VG, which is the gate bias of the driving transistor, for one frame may also be reduced.

As described above, the pixel structure of the display device and the active liquid crystal display device using the active light emitting organic light emitting diode according to the present invention is largely caused by the charge stored in the shield capacitor even when the leakage current of the switching thin film transistor is high. The pixel voltage can be maintained during the frame period by allowing the outflow or inflow. Therefore, it is possible to suppress the deterioration of image quality such as changes in luminance, streaks, crosstalks, etc. generated due to leakage currents.

In addition, the pixel structure of the display device according to the present invention can be applied to various threshold voltage compensation pixel circuits in addition to the embodiments disclosed in the present invention. In this case, no additional wiring is required in the pixel, and only a switching transistor and a shield capacitor may be added to implement a display device having excellent image quality.

In addition, according to the present invention, the sensitivity to the off characteristics of the device can be greatly alleviated, and thus, defects such as changes in luminance can be overcome without being greatly influenced by variations in process conditions or occurrence of deviation due to instability. .

Claims (7)

A pixel structure comprising at least a switching transistor for selecting a unit pixel defined by a data line and a scan line, and a storage capacitor for storing a corresponding charge according to a pixel voltage applied from the data line. And a shield capacitor for supplying charges leaked by the leakage current of the switching transistor when the switching transistor is in an off state. An organic light emitting device; A first switching transistor for selecting a unit pixel defined by the data line and the scan line; A driving transistor receiving a data voltage transferred through the first switching transistor as a gate and supplying a corresponding current to the organic light emitting diode; A capacitor for storing a data voltage applied to a gate of the driving transistor; A second switching transistor controlled by a scan signal and connected in cascode form to a drain-source region of the first switching transistor; And a shield capacitor connected between a connection node of the first switching transistor and the second switching transistor and a power supply source and supplying charges leaked due to leakage currents of the first switching transistor and the second switching transistor. A pixel structure of a display device. The method of claim 2, And a third switching transistor connected in cascade form to the drain-source region of the first switching transistor. The method of claim 2, And a fourth switching transistor connected in cascade form to a drain-source region of the second switching transistor. A first switching transistor for selecting a unit pixel defined by the data line and the scan line; A storage capacitor connected between the drain-source current path of the first switching transistor and a power supply source and storing a corresponding charge according to a pixel voltage applied from a data line; A liquid crystal whose transmittance is controlled by a voltage stored in the storage capacitor; And a second switching transistor controlled by a scan signal and connected in a cascode form to a drain-source region of the first switching transistor. The method of claim 5, wherein And a third switching transistor connected in cascade form to the drain-source region of the first switching transistor. The method of claim 5, wherein And a fourth switching transistor connected in cascade form to a drain-source region of the second switching transistor.

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