CN100353394C - Pixel circuit of display - Google Patents

Abstract

A pixel circuit is used for driving a plurality of unit pixels in a display panel and at least comprises a plurality of scanning lines, a plurality of data lines and a lightly doped drain thin film transistor (LDD-TFT) which are respectively connected with the scanning lines, the data lines and a driving transistor of the unit pixels. The lightly doped drain thin film transistor at least comprises a first lightly doped drain and a second lightly doped drain with different lengths, wherein the first lightly doped drain closest to the driving transistor has the longest length.

Description

Display pixel circuit

technical field

The invention relates to an asymmetric lightly doped drain thin film transistor structure, in particular to a thin film transistor structure in a liquid crystal display pixel circuit.

Background technique

With the improvement of semiconductor manufacturing process, thin film transistor (Thin Film Transistor, TFT) components are getting smaller and smaller, and the channel length between the source and drain of the thin film transistor is shortened accordingly. When the channel length of the thin film transistor is shortened, in addition to the decrease of the threshold voltage (Threshold Voltage, Vt), leakage current occurs, and the phenomenon of hot electron effects (Hot Electron Effects) affects the operation of the transistor. In order to solve this problem, a lightly doped drain (LDD) structure has been developed in the prior art, which is used to reduce the electric field at the drain junction to alleviate the impact of the hot electron effect.

An organic light emitting display (OLED) uses a large number of thin film transistors in the design of two major functional circuits, such as its pixel circuit and peripheral driving circuit. Since the functions and operation conditions of the pixel circuit and the peripheral driving circuit are different, their respective thin film transistor characteristic requirements are also different. In terms of pixel circuits, thin-film transistors are mainly used as switching components of pixels to provide appropriate current to control the grayscale performance of organic light-emitting diodes, so it is particularly necessary to reduce the leakage current (Leakage-Current) to maintain organic light-emitting diodes. normal performance.

Please refer to Figure 1. FIG. 1 is a schematic cross-sectional view of a conventional symmetrical lightly doped drain transistor structure. The symmetrical lightly doped drain transistor 10 comprises a substrate 12, a semiconductor layer 14 is arranged on the surface of the substrate 12, a gate insulating layer 16 is arranged on the surface of the semiconductor layer 14, and a gate 18 is arranged on the upper surface of the gate insulating layer 16 . The semiconductor layer 14 includes two lightly doped drains 140, 142 and two source/drains 144, 146, which are symmetrically arranged on both sides of the gate 18, and between the lightly doped drains 140 and 142 is defined as a channel area 148 . The lightly doped drains 140 and 142 are lightly doped regions formed by N-type dopants, which are used to reduce the leakage current of the symmetrical lightly doped drain transistor 10 and avoid hot electron effects caused by excessive electric fields near the drains. .

However, affected by the low doping concentration, the resistance of the lightly doped drains 140 and 142 is relatively higher than that of the source/drains 144 and 146 on both sides, so it is easy to cause the gap between the drain and the source 144, 146. The series resistance increases, which in turn causes problems such as electron drift rate and lower operation speed of the whole assembly. In this case, in order to improve the leakage current phenomenon of thin film transistors, it is inevitable to sacrifice the operating speed of the device. Therefore, how to choose between the characteristics of the electron mobility and the leakage current has become a problem in the design of lightly doped drain devices. An important consideration when designing polar structures.

Please refer to FIG. 2 , which is a schematic diagram of a pixel circuit in a conventional display. A pixel circuit for current-driven active light-emitting display drive circuit, at least including a double-gate symmetric lightly doped drain thin film transistor N1, the source of which is connected to a data line D1, and the gate of which is connected to a scan line S1; a drive transistor P1, the gate of which is connected to the drain of the double-gate symmetrical lightly doped drain thin film transistor N1; a capacitor C1, one end of which is connected to the source of the double-gate symmetrical lightly doped drain thin film transistor N1 and a light-emitting diode L1, the anode of which is connected to the drain of the driving transistor P1.

When the scanning signal on the scanning line S1 turns on the double-gate symmetrical light-doped drain thin film transistor N1, the double-gate symmetrical light-doped drain thin film transistor N1 will pass the data signal on the data line D1 and transmit it to the capacitor C1 and The gate terminal of the driving transistor P1 is used to control the driving transistor P1 to drive the grayscale representation of the light emitting diode.

When there is no scanning signal on the scanning line S1, the double-gate symmetrical lightly doped drain thin film transistor N1 is turned off. At this time, the capacitor stores the charge so that the potential of node A is high, keeping the drive transistor P1 off, and node B The potential is a low level. Since the potential of the node A is relatively higher than that of the node B, a leakage current will flow from the node A to the node B.

Therefore, the present invention will improve the components of the pixel circuit to reduce the occurrence of leakage current.

Contents of the invention

The purpose of the present invention is to improve the components of the pixel circuit to reduce the occurrence of leakage current.

Another object of the present invention is to utilize the length asymmetry of the lightly doped drain to reduce the series resistance value between the source and the drain in the double-gate lightly doped drain thin film transistor, so as to avoid the reduction of device operation speed.

The invention discloses a pixel circuit for driving a plurality of unit pixels in a display panel. The pixel circuit at least includes: a plurality of scanning lines formed on the display panel for transmitting scanning signals of these unit pixels; a plurality of data line, formed on the display panel, and interlaced with a plurality of scanning lines, to transmit the data signals of these unit pixels; and a lightly doped drain thin film transistor (LDD-TFT), respectively connected to the scanning lines of the unit pixel, The data line and a driving transistor, the lightly doped drain thin film transistor, at least have a first lightly doped drain and a second lightly doped drain with different lengths, wherein the first lightly doped drain closest to the driving transistor The pole has the longest length.

The present invention uses the lightly doped drain to reduce the leakage current near the drain, and the lightly doped adjacent to the source region can be removed or shortened to effectively reduce the series resistance between the drain and the source. Improves the electron transfer rate and the operating speed of the entire assembly.

Description of drawings

1 is a schematic cross-sectional view of an existing symmetrical lightly doped drain transistor structure;

2 is a schematic diagram of a pixel circuit in an existing display;

3A is a schematic diagram of a pixel circuit in a preferred embodiment of the present invention;

FIG. 3B is a structural diagram of a double-gate thin film transistor with an asymmetric lightly doped drain.

Description of reference symbols

10 symmetrical lightly doped drain transistor 240 first lightly doped drain

242 second lightly doped drain 244 third lightly doped drain

246 The fourth lightly doped drain 247 The first channel

249 second channel 281 first grid

282 second grid

12, 22 substrate 14, 24 semiconductor layer

16, 26 grid insulating layer 18, 28 grid

140, 142 lightly doped drain

144, 146, 241, 243, 245 drain/source

A, B, C nodes

C1 capacitor D1 data line

L1 Light Emitting Diode P1 Transistor

S1 scan line

N1 Double Gate Symmetrical Lightly Doped Drain Thin Film Transistor

N2 Double Gate Asymmetric Lightly Doped Drain Thin Film Transistor

Detailed ways

Please refer to FIG. 3A , which is a schematic diagram of a pixel circuit in a preferred embodiment of the present invention. The pixel circuit is a driving circuit for current-driven active light-emitting displays, and at least includes a double-gate thin film transistor N2, a driving transistor P1, a capacitor C1, and a light-emitting diode L1. Wherein, the source of the double-gate TFT N2 is connected to a data line D1, and the gate is connected to a scan line S1. The gate of the driving transistor P1 is connected to the drain of the double-gate thin film transistor N2. The capacitor C1 is connected to the junction between the source of the double-gate TFT N2 and the gate of the driving transistor P1. The anode of the LED L1 is connected to the drain of the driving transistor P1. The main difference between this embodiment and the prior art is that the double-gate thin film transistor N2 used has an asymmetric lightly doped drain. Its driving principle is similar to that of the prior art and will not be repeated here.

Please refer to FIG. 3B , which is a structural diagram of a double-gate TFT with an asymmetric lightly doped drain. In a preferred embodiment of the present invention, the double-gate thin film transistor N2 is an N-type thin film transistor, but it can also be replaced by a P-type thin film transistor. The double-gate TFT N2 includes a substrate 22 , a semiconductor layer 24 disposed on the surface of the substrate 22 , a gate insulating layer 26 disposed on the surface of the semiconductor layer 24 , and a double gate 28 disposed on the upper surface of the gate insulating layer 26 .

Wherein, the double gate is the first gate 281 and the second gate 282; the semiconductor layer 24 includes: a source 241 connected to a first lightly doped drain 240, a second lightly doped drain 242 connected in sequence A drain/source 243 and a third lightly doped drain 244, a fourth lightly doped drain 246 are connected to a drain 245, and between the first and second lightly doped drains 240, 242 Then it is defined as a first channel region 247, above which is the first gate 281, and between the third and fourth lightly doped drains 244, 246 is defined as a second channel region 249, above which is the second gate. The first lightly doped drain 240 is the side closest to the driving transistor P1 and has the longest length.

In the structure of the double-gate TFT N2, the two sidewalls of the first and second gates are respectively stacked above the lightly doped drain, that is to say, the gate partially covers the lightly doped drain, but The two sidewalls of the gate do not have to cover the lightly doped drain, and the relative position between the gate and the lightly doped drain can be adjusted according to the electrical design requirements.

Generally speaking, there are two ways to calculate the length of the lightly doped drain (illustrated in FIG. 3B ), one is the length H1 defined by the distance between the two ends of the first lightly doped drain 240; the other is The length H2 is defined by the distance from one end of the first gate close to the first lightly doped drain to the other end of the lightly doped drain. In the present invention, any calculation method for the length of the lightly doped drain can be selected, which will not affect the effect of the present invention.

In this embodiment, the lengths of the first, second, third, and fourth lightly doped drains 240, 242, 244, and 246 are designed to be 3, 1, 1, and 1 unit lengths respectively. pole to reduce the leakage current near the drain, wherein the length of the first lightly doped drain 240 is increased to make it longer than the other three lightly doped drains, so that the leakage current path from node A to node B The total resistance value increases, thereby reducing the leakage current.

The lengths of the three lightly doped drains can be appropriately shortened, or even completely removed, so that the lengths of the three lightly doped drains 242, 244, and 246 are respectively less than or equal to the length of the first lightly doped drain. The length of the doped drain 240; because the three lightly doped drain lengths have little effect on the leakage current problem, shortening the length can reduce the series resistance between the drain and the source, improve the electron transfer rate and the operation of the entire assembly speed.

Therefore, it can be known that, compared with the existing symmetric thin film transistor structure, the thin film transistor structure of the present invention includes an asymmetric lightly doped drain, so it can be further improved on the drain side that is more sensitive to the problem of leakage current. Moderately increase the length of the lightly doped drain to effectively reduce the leakage current.

The present invention is merely a preferred embodiment, which is not intended to limit the scope of the present invention. In the double-gate asymmetric lightly doped thin film transistor used in the present invention, the ratio of the length of the lightly doped drain can also be changed because of its needs, for example: 1. The length of the first lightly doped drain is longer than that of the other There are three lightly doped drain lengths, and the lengths of the second, third and fourth lightly doped drains are not necessarily equal. 2. The lengths of the first and third lightly doped drains are equal, the lengths of the second and fourth lightly doped drains are equal, and the length of the first lightly doped drain is greater than the length of the second lightly doped drain. 3. The length of the first lightly doped drain is equal to that of the second lightly doped drain, the length of the third lightly doped drain is equal to that of the fourth lightly doped drain, and the length of the first lightly doped drain is greater than that of the third lightly doped drain.

The present invention is not limited to the application of double-gate asymmetric lightly doped thin film transistors, and can also be replaced by single-gate asymmetric lightly doped thin film transistors; , can also achieve the same effect.

Generally speaking, when the thin film transistor is turned off, there is still a voltage (electric field) between the drain and the substrate, so leakage current is easily generated. That is to say, the leakage current problem of thin film transistors is mainly sensitive to the region near the drain, so the present invention uses the lightly doped drain to reduce the leakage current near the drain, and the lightly doped region adjacent to the source can be adjusted. Removed to effectively reduce the series resistance between the drain and the source, improve the electron mobility and the operating speed of the entire component.

Compared with the existing symmetric thin film transistor structure, the thin film transistor structure of the present invention includes an asymmetric lightly doped drain, so the lightly doped drain can be further moderately increased on the side of the drain that is more sensitive to the leakage current problem. The length of the impurity drain can effectively reduce the leakage current. In addition, the present invention can further shorten the length of the lightly doped drain on the source side, or even completely remove the lightly doped drain structure on the source side, so as to effectively reduce the series resistance between the drain and the source and improve the electron density. Drift rate as well as overall component operating speed.

The above description is only a preferred embodiment of the present invention, and it is not intended to limit the implementation scope of the present invention. Those skilled in the art should not violate the spirit of the present invention. The modifications made should fall within the scope of the present invention. Therefore The protection scope of the present invention is based on the claims of the present invention.

Claims (15)

1. A pixel circuit for driving a plurality of unit pixels in a display panel, the circuit at least comprising: A plurality of scanning lines are formed on the display panel to transmit scanning signals of these unit pixels; A plurality of data lines formed on the display panel and intersecting with the plurality of scan lines for transmitting data signals of these unit pixels; and The lightly doped drain thin film transistor is respectively connected to the scan line, the data line and a drive transistor of the unit pixel. The lightly doped drain thin film transistor has at least a first lightly doped drain and a lightly doped drain with different lengths. a second lightly doped drain, wherein the first lightly doped drain closest to the drive transistor has the longest length, Wherein the lightly doped drain thin film transistor is a double gate thin film transistor with at least a first gate and a second gate. 2. The circuit according to claim 1, wherein the double gate TFT further comprises a third lightly doped drain, a fourth lightly doped drain, and the first gate corresponds to the first lightly doped drain impurity drain, the second lightly doped drain, the second gate corresponds to the third lightly doped drain, the fourth lightly doped drain, and the first lightly doped drain is closest to the driver transistor, and the length of the first lightly doped drain is greater than the lengths of the other three lightly doped drains. 3. The circuit of claim 2, wherein the length of the first lightly doped drain is greater than the lengths of the other three lightly doped drains, and the lengths of the second, the third and the fourth lightly doped drains are equal in length. 4. The circuit according to claim 2, wherein the lengths of the first and the third lightly doped drains are equal, the lengths of the second and the fourth lightly doped drains are equal, and the first lightly doped drains The dopant drain length is greater than the second lightly doped drain length. 5. The circuit of claim 2, wherein the first and the second lightly doped drains have equal lengths, the third and the fourth lightly doped drains have equal lengths, and the first lightly doped drains The length of the doped drain is greater than the length of the third lightly doped drain. 6. A pixel circuit for driving a current-driven active light-emitting display, each of which includes: The lightly doped drain thin film transistor is turned on in response to a scan signal to transmit a data signal, wherein the lightly doped drain thin film transistor has at least a first lightly doped drain and a second lightly doped drain with different lengths Miscellaneous drain; driving a transistor to conduct a current in response to the data signal transmitted by the lightly doped drain thin film transistor; and a light emitting diode that emits light by the current turned on by the driving transistor, Wherein the first lightly doped drain is closer to the driving transistor and has the largest length, Wherein the lightly doped drain thin film transistor is a double gate thin film transistor with a first gate and a second gate. 7. The circuit according to claim 6, wherein the double-gate TFT further comprises a third lightly doped drain, a fourth lightly doped drain, and the first gate corresponds to the first lightly doped drain impurity drain, the second lightly doped drain, the second gate corresponds to a third lightly doped drain, a fourth lightly doped drain, and the first lightly doped drain is closest to the driver The lightly doped drain of the transistor, and the length of the first lightly doped drain is greater than or equal to the length of the other three lightly doped drains. 8. The circuit of claim 7, wherein the length of the first lightly doped drain is greater than the lengths of the other three lightly doped drains, and the lengths of the second, the third and the fourth lightly doped drains are equal in length. 9. The circuit as claimed in claim 7, wherein the lengths of the first and the third lightly doped drains are equal, and the lengths of the second and the fourth lightly doped drains are equal, and the first lightly doped drains The doped drain length is greater than the second lightly doped drain length. 10. The circuit of claim 7, wherein the first and the second lightly doped drains have equal lengths, the third and the fourth lightly doped drains have equal lengths, and the first lightly doped drains The length of the doped drain is greater than the length of the third lightly doped drain. 11. An active organic light-emitting display, having a plurality of current-driven pixel circuits, and using a driving circuit to drive these pixel circuits, the pixel circuit including a switching transistor, a capacitor, a driving transistor and a display component, characterized in that: The switch transistor has at least two lightly doped drains with asymmetric lengths, and the length of the lightly doped drain close to the drive transistor of the switch transistor is greater than the length of the lightly doped drain on the other side, Wherein the switching transistor is a double-gate lightly doped drain thin film transistor. 12. The display as claimed in claim 11 , wherein the double gate transistor comprises a first gate and a second gate, wherein the first gate corresponds to a first lightly doped drain, a second lightly doped drain doped drain, the second gate corresponds to a third lightly doped drain and a fourth lightly doped drain, and the first lightly doped drain is the lightly doped drain closest to the driving transistor , and the length of the first lightly doped drain is greater than or equal to the lengths of the other three lightly doped drains. 13. The display device according to claim 12, wherein the length of the first lightly doped drain is greater than the lengths of the other three lightly doped drains, and the lengths of the second, third and fourth lightly doped drains equal. 14. The display device according to claim 12, wherein the lengths of the first and third lightly doped drains are equal, and the lengths of the second and fourth lightly doped drains are equal, and the first lightly doped drains The drain length is greater than the second lightly doped drain length. 15. The display device according to claim 12, wherein the lengths of the first lightly doped drain and the second lightly doped drain are equal, the lengths of the third lightly doped drain and the fourth lightly doped drain are equal, and the first lightly doped drain The length of the doped drain is greater than the length of the third lightly doped drain.

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