# Patent

## Title
Shift register units, display panels utilizing the same, and methods for improving current leakage thereof

## Abstract

A shift register comprising at least one shift register unit. The shift register unit comprises an input unit, at least one first TFT, and at least one second TFT. The input unit receives an input signal from the input terminal and outputs a switching control signal in accordance with a first clock signal. The gate of the first TFT is for receiving the switching control signal, the drain of the first TFT is for receiving a second clock signal, and the source of the first TFT is coupled to the output terminal. The gate and drain of the second TFT are coupled to the output terminal, and the source of the second TFT is coupled to the input unit.

## Background
This application claims the benefit of Taiwan application Serial No. 94105032 filed Feb. 21, 2005, the subject matter of which is incorporated herein by reference.
BACKGROUND
The invention relates to a shift register unit applied in a shift register, and in particular to a shift register employed in drivers of display panels.
FIG. 1 is a schematic diagram of a conventional liquid crystal display (LCD) panel. As shown in FIG. 1, LCD panel 1 comprises display array 10, data driver 11, and scan driver 12. Display array 10 comprises a plurality of pixels. Data driver 11 controls a plurality of data lines D1 to Dn, and scan driver 12 controls a plurality of scan lines S1 to Sm. Scan driver 12 sequentially outputs scan signals SD1 to SDm to scan lines S1 to Sm in response to a scan control signal to turn on the pixels corresponding to a row. When a row of pixels are turned on, data driver 11 outputs corresponding video signals with gray scale values to n pixels corresponding to the row through data lines D1 to Dn. Each of data driver 11 and scan driver 12 requires a shift register to output signals, sequentially.
Typically, a shift register comprises a plurality of identical, substantially cascaded, shift register units. For example, in a shift register of a scan driver, an output signal of each shift register unit is transmitted to a next shift register unit as its input signal and to a corresponding row of pixels through a scan line.
FIG. 2 shows a conventional shift register as disclosed in U.S. Pat. No. 4,084,106. Shift register 2 comprises two identical, substantially cascaded, shift register units 21 and 22. Clock signals CK and XCK are provided to shift register units 21 and 22, respectively. Clock signals CK and XCK have inverse phases. Each shift register unit 21 or 22 comprises input and output terminals, transistors T21 to T26, and capacitors C21 and C22. Output terminal OUT1 of shift register unit 21 is coupled to input terminal IN2 of shift register unit 22. Referring to FIGS. 2 and 3, shift register unit 21 is given as an example. In period P1, input signal IS1 and clock signal XCK are at high logic level (first state), and clock signal CK is at low logic level. Transistor T23 is thus turned on, and capacitor C21 is charged to high logic level, so that node N21 is at high logic level. Since the gate of transistor T22 is coupled to node N21, transistor T22 is turned on. Output terminal OUT1 outputs output signal OS1 at low logic level to input terminal IN2 as input signal IS2 of shift register unit 22.
In a subsequent period P2, input signal IS1 and clock signal XCK are transformed to being at low logic level, and clock signal CK is transformed to being at high logic level. Transistor T23 is thus turned off. Node N21, which is at high voltage level, has higher voltage due to the parasitic capacitance of transistor T22. Output signal OS1 is transformed to being at high logic level. However, in practice, transistor T23 may operate in the sub-threshold region or turned-off region to generate unexpected leakage current. Node N21 does not remain at high logic level, and transistor T22 is not continuously turned on. Thus, output signal OS1 does not continuously remain at high logic level, resulting in false operation of shift register unit 22 and the timing of shift register 2 is in error.
SUMMARY
Shift register are provided. An exemplary embodiment of a shift register unit applied in a shift register is described. The shift register unit comprises an input unit, at least one first thin film transistor (TFT), and at least one second TFT. The input unit receives an input signal from the input terminal and outputs a switching control signal in response to a first clock signal. The gate of the first TFT is coupled to the input unit and configured to receive the switching control signal, the drain of the first TFT is configured to receive a second clock signal, and the source of the first TFT is coupled to the output terminal. The gate and the drain of the second TFT are coupled to the output terminal, and the source of the second TFT is coupled to the input unit.
Methods for improving current leakage of a shift register are provided. An exemplary embodiment of a shift register comprises a plurality of identical, substantially cascaded, shift register units, each of which having an input terminal and an output terminal. An exemplary embodiment of the method comprises steps of: receiving an input signal; transforming the input signal to a switching control signal in response to a first clock signal; transmitting a second clock signal to the output terminal in response to the switching control signal; and maintaining the state of the switching control signal in response to the second clock signal.



DESCRIPTION OF THE DRAWINGS
The inventions will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the invention.
FIG. 1 is a schematic diagram of a conventional liquid crystal display panel;
FIG. 2 shows a conventional shift register;
FIG. 3 is an operational timing chart of the conventional shift register unit of FIG. 2;
FIG. 4 shows an embodiment of a shift register;
FIG. 5 is an operational timing chart of an embodiment of a shift register unit of FIG. 4;
FIG. 6 shows an embodiment of a shift register;
FIG. 7 is an operational timing chart of an embodiment of a shift register unit of FIG. 4;
FIG. 8 shows a display panel incorporating a shift register according to an embodiment of a display panel; and
FIG. 9 shows an embodiment of a liquid crystal display panel.



DETAILED DESCRIPTION
Shift register units are provided. In an exemplary embodiment of a shift register, as shown in FIG. 4, shift register 4 comprises a plurality of identical, substantially cascaded, shift register units and is controlled by clock signals CK and XCK opposite to each other, that is, clock signals CK and XCK have inverse phases. In this embodiment, two shift register units 41 and 42 are given as an example. Output terminal OUT1 of shift register unit 41 is coupled to input terminal IN2 of shift register unit 42. Shift register unit 41 comprises input unit U1, control unit U2, and two switching units. Referring to FIG. 4, the two switching units are implemented by transistors, for example, N-type thin film transistors T41 and T42.
The gate of transistor T41 is coupled to the input unit U1 at a node N41, and the drain thereof receives clock signal CK, and the source thereof is coupled to node N42. The gate of transistor T42 receives control signal CS1, the drain thereof is coupled to node N42, and the source thereof is coupled to ground GND at low logic level. Input unit U1 turns on the transistor T41 via node N41 due to coupling with transistor T41. Control unit U2 is coupled between node N42 and input unit U1.
FIG. 5 is an operational timing chart of the shift register unit of the present invention. Referring to FIGS. 4 and 5, shift register unit 41 is described. The entire duration of shifting an input signal IS1 by shift register unit 41 comprises periods P1 to P4. During period P1, input signal IS1 and clock signal XCK are at high logic level, that is, at first state, and clock signal CK is at low logic level, that is, at second state. Input unit U1 receives input signal IS1 in response to clock signal XCK being at high logic level and outputs switching control signal SW. In other words, input unit U1 transforms input signal IS1 into switching control signal SW in response to clock signal XCK. Node N41 is at high logic level to turn on transistor T41. Clock signal CK is transmitted to output terminal OUT1 to serve as output signal OS1 of shift register unit 41. Control signal CS1 may be at high or low logic level to turn transistor T42 on or off, respectively, during period P1. In the embodiment of FIG. 4, control signal CS1 is set at low logic level to turn off transistor T42. Thus, node N42 is at low logic level, that is, output signal OS1 is at low logic level. If control signal CS1 is set at high logic level to turn on transistor T42, the node N42 is also at low logic level.
During period P2, clock signal CK is at high logic level, and input signal IS1, the clock signal XCK, and control signal CS1 are at low logic level. Node N41 at high logic level has a higher voltage level due to the parasitic capacitance of transistor T41. In other words, the voltage level of switching control signal SW is higher, so that transistor T41 is turned on. Clock signal CK is transmitted to output terminal OUT1 to serve as output signal OS1. Control signal CS1 turns off transistor T42. Thus, node N42 is at high logic level, and output signal OS1 is also at high logic level. Control unit U2 controls input unit U1 in response to output signal OS1, which is at high logic level, thus maintaining node N41 at high logic level and further keeping transistor T41 turned on during period P2.
During periods P3 and P4, input unit U1 controls node N41 to be at low logic level, and control signal CS1 is transformed into being at high logic level to turn on transistor T42. Thus, node N42 is at low logic level, and output signal OS1 is also at low logic level.
Shift register units are provided. In an exemplary embodiment of shift register unit 41, as shown in FIG. 6, input unit U1 of shift register unit 41 comprises two transistors T61 and T62, and control unit U2 thereof comprises a diode. In this embodiment, the diode is implemented by transistor T63. Transistors T61 to T63 may be N-type thin film transistors. The gate of transistor T61 receives clock signal XCK, the drain thereof receives input signal IS1, and the source thereof is coupled to node N61. The gate of transistor T62 receives clock signal XCK, the drain thereof is coupled to node N61, and the source thereof is coupled to node N41. The gate and the drain of transistor T63 are coupled to node N42, and the source thereof is coupled to node N61, so that transistor T63 serves as a diode.
Referring to FIGS. 5 and 6, during period P1, input signal IS1 and clock signal XCK are at high logic level, that is, at first state, and clock signal CK and control signal CS1 are at low logic level, that is, second state. Transistors T61 and T62 are turned on, and node N41 is at high logic level. Transistor T41 is turned on due to node N41 being at high logic level. Control signal CS1 at low logic level turns off transistor T42. Thus, node N42 is at low logic level due to clock signal CK being at low logic level, that is, output signal OS1 is at low logic level.
During period P2, clock signal CK is at high logic level; input signal IS1, clock signal XCK, and control signal CS1 are at low logic level. Transistors T61 and T62 are turned off. Node N41 from being at high logic level to be at higher voltage level due to the parasitic capacitance of transistor T41, thus turning on transistor T41. Control signal CS1, which is at low logic level, turns off=transistor T42. Node N42 is transformed into being at high logic level, that is, =output signal OS1 is at high logic level. At the same time, =output signal OS1 at high logic level turns on=transistor T63, therefore, node N61 is at high logic level. During period P2, despite transistors. T61 and T62 are operated in the sub-threshold region, in the turned-off region or generate undesired leakage current, node N41 remains at high logic level, and transistor T41 remains turned on because of node N61 being at high logic level. Accordingly, the output terminal OUT1 outputs output signal OS1 at high logic level to shift input signal IS1.
During period P3, control signal CS1 and clock signal XCK are at high logic level; clock signal CK and input signal IS1 are at low logic level. Transistors T61 and T62 are turned on, creating a discharge path between node N41 and input terminal IN1. Node N41 is transformed into being at low logic level to turn off transistor T41. Control signal CS1 turns on transistor T42. Thus, output signal OS1 is transformed into being at low logic level.
During period P4, control signal CS1 and clock signal CK are at high logic level, and clock signal XCK and input signal IS1 are at low logic level. Transistors T61 and T62 are turned off, and transistor T42 is turned on. Node N41 is at low logic level to turn off transistor T41. Thus, the output signal OS1 remains at low logic level.
As previously described, transistors T61 and T62 may be N-type thin film transistors, as an example. If transistors T61 and T62 are P-type thin film transistors and transistor T41 is N-type thin film transistor, clock signal XCK, passing through the gates of transistors T61 and T62, and clock signal CK may be in same phases, in other words, transistors T61, T62, and T41 may receive same clock signal source.
Shift register units are provided. In an exemplary embodiment of shift register unit 41, as shown in FIG. 7, input unit U1 of shift register unit 41 comprises two transistors T71 to T74; control unit U2 thereof comprises transistor T75. Control signal CS2 is further provided to input unit U1.
In input unit U1, the gate and the drain of transistor T71 receive input signal IS1, and the source thereof is coupled to node N71. The gate of transistor T72 receives clock signal XCK, the drain thereof is coupled to node N71, and the source thereof is coupled to node N41. The gate of transistor T73 receives control signal CS2, the drain thereof is coupled to node N41, and the source thereof is coupled to node N72. The gate of transistor T74 receives control signal CS2, the drain thereof is coupled to node N72, and the source thereof is coupled to ground GND.
In control unit U2, the gate and the drain of transistor T75 are coupled to node N42, and the source thereof is coupled to node N72. In this embodiment of FIG. 7, transistors T41, T41, T71 to T75 may be N-type thin film transistors, for example.
FIG. 8 is an operational timing chart of shift register unit 41 in FIG. 7. Referring to FIGS. 7 and 8, during period P1, input signal IS1 and clock signal XCK are at high logic level, that is, at first state; clock signal CK, control signals CS1 and CS2 are at low logic level, that is, at second state. Transistors T71 and T72 are turned on, and transistors T73 and T74 are turned off, so that node N41 is at high logic level. Transistor T41 is turned on due to node N41 being at high logic level. Control signal CS1 at low logic level turns off transistor T42. Thus, node N42 is at low logic level due to clock signal CK being at low logic level, and output signal OS1 is at low logic level.
During period P2, the clock signal CK is at high logic level, and input signal IS1, clock signal XCK, control signals CS1 and CS2 are at low logic level. Transistors T71 and T72 and transistors T73 and T74 are still turned off. Node N41 at high logic level has a higher voltage level due to the parasitic capacitance of transistor T41, thus transistor T41 turns on. Control signal CS1 turns off transistor T42. Node N42 is transformed into being at high logic level, that is, output signal OS1 is at high logic level. At the same time, output signal OS1 at high logic level turns on transistor T75, and node N72 is at high logic level. During period P2, even through transistors T73 and T74 operate in the sub-threshold region or turned-off region to generate undesired leakage current, node N41 remains at high logic level, and transistor T41 remains turned on. Accordingly, output terminal OUT1 outputs output signal OS1 being at high level for shifting input signal IS1.
During period P3, control signals CS1, CS2 and clock signal XCK are at high logic level; clock signal CK and input signal IS1 are at low logic level. Transistor T71 is turned off, and transistors T72 to T74 are turned on. Since transistor T71 serves as a diode, a discharge path is created between node N41 and transistor T74, rather than between node N71 and input terminal IN1. Node N41 is transformed into being at low logic level to turn off transistor T41. Control signal CS1 turns on transistor T42. Thus, output signal OS1 is transformed into being at low logic level.
During period P4, control signal CS1 and clock signal CK are at high logic level; control signal CS2, clock signal XCK, and input signal IS1 are at low logic level. Transistors T71 to T74 are turned off, while transistor T42 is turned on. Node N41 is at low logic level to turn off transistor T41. Thus, output signal OS1 remains at low logic level.
As previously described, in some embodiments of a shift register unit, control unit U2 is used to solve or prevent the problem of undesired leakage current happened because the transistors of input unit U1 may be operated in the sub-threshold region or in the turned-off region. During period P2, input unit U1 controls node N41 being at high logic level, so that transistor T41 maintains turned on. Accordingly, output terminal OUT1 outputs output signal OS1 being at high level for shifting input signal IS1.
Some embodiments of a shift register can be applied in data or scan drivers within a display panel in FIG. 9, such as data driver 91 and scan driver 92 within display panel 9. Display panel 9 further comprises display array 90 defined by a plurality scan lines S1 to Sm and a plurality of data lines D1 to Dn. For example, an embodiment of shift register 93 is applied in scan driver 92. Shift register 93 comprises a plurality of shift register units 91 to 9m, each of which may have the same circuitry as shown in FIG. 4. An output signal of each shift register unit is provided not only to a next shift register unit to serve as an input signal thereof but also to a corresponding scan line.
In embodiments of a shift register unit, control signal CS1 may be at high or low logic level during period P1, with no effect on the logic level of output signal OS1. The embodiments of FIGS. 6 and 7 describe control signal CS1 being at low logic level as an example during period P1, without limitation.
While the invention has been described in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. A shift register unit having an input terminal and an output terminal, comprising:
an input unit, in accordance with a first clock signal, configured to receive an input signal from the input terminal and output a switching control signal;
at least one first transistor coupled to the input unit, wherein the gate of the first transistor is configured to receive the switching control signal, the drain of the first transistor is configured to receive a second clock signal, and the source of the first transistor is coupled to the output terminal; and at least one second transistor, wherein the gate and the drain of the second transistor are coupled to the output terminal, and the source of the second transistor is coupled to the input unit.

2. The shift register unit as claimed in claim 1, wherein the first and second clock signals have reverse phases.

3. The shift register unit as claimed in claim 1, further comprising a third transistor, wherein the gate of the third transistor is configured to receive a first control signal, the drain of the third transistor is coupled to the output terminal, and the source of the third transistor is coupled to ground.

4. The shift register unit as claimed in claim 1, wherein the input unit comprises:
a fourth transistor, wherein the gate of the fourth transistor is configured to receive the first clock signal and the drain of the fourth transistor is configured to receive the input signal; and
a fifth transistor, wherein the gate of the fifth transistor is configured to receive the first clock signal, the drain of the fifth transistor is coupled to the source of the fourth transistor, and the source of the fifth transistor is coupled to the gate of the first transistor.

5. The shift register unit as claimed in claim 4, wherein the fourth and fifth transistors are P-type thin film transistors, the first transistor is an N-type thin film transistor, and the first and second clock signals have same phases.

6. The shift register unit as claimed in claim 4, wherein the source of the second transistor is coupled to the source of the fourth transistor and the drain of the fifth transistor.

7. The shift register unit as claimed in claim 6, further comprising a third transistor, wherein the gate of the third transistor is configured to receive a first control signal, the drain of the third transistor is coupled to the output terminal, and the source of the third transistor is coupled to ground.

8. The shift register unit as claimed in claim 1, wherein the input unit comprises:
a fourth transistor, wherein the gate and the drain of the fourth transistor are configured to receive the input signal;
a fifth transistor, wherein the gate of the fifth transistor is configured to receive the first clock signal, the drain of the fifth transistor is coupled to the source of the fourth transistor, and the source of the fifth transistor is coupled to the gate of the first transistor;
a sixth transistor, wherein the gate of the sixth transistor is configured to receive a second control signal, the drain of the sixth transistor is coupled to the gate of the first transistor, and the source of the sixth transistor is coupled to the source of the second transistor; and
a seventh transistor, wherein the gate of the seventh transistor is configured to receive a second control signal, the drain of the seventh transistor is coupled to the source of the sixth transistor, and the source of the seventh transistor is coupled to ground.

9. A display panel comprising a shift register unit of claim 1.