WO2004090622A1

WO2004090622A1 - A liquid crystal display having double thin film transistor pixel structure

Info

Publication number: WO2004090622A1
Application number: PCT/CN2003/000258
Authority: WO
Inventors: Chu-Hung Tsai
Original assignee: Quanta Display Inc.; Quanta Display Japan Inc.
Priority date: 2003-04-11
Filing date: 2003-04-11
Publication date: 2004-10-21
Also published as: AU2003236120A8; AU2003236120A1

Abstract

This invention provides a liquid crystal display having double thin film transistor pixel structure, comprising the first and second scanning lines, the first and second signal lines and pixels. The pixels comprise pixel electrodes, the first and the second transistors. Gate and source of the first and the second transistor are electrically connected to the first and the second scanning lines, the first and the second signal lines, respectively, drains of two of transistor are electrically connected to pixel electrodes. A ratio of channel width to its length of the first transistor is smaller than the ratios of the second transistors.

Description

Liquid crystal display with double thin film transistor pixel structure

The invention provides a liquid crystal display with a double thin film transistor (TFT) pixel structure, particularly a liquid crystal display with high resolution and high display frequency.

Background technique

Thin-film transistor liquid crystal displays (TFT-LCDs) mainly use thin-film transistors arranged in a matrix, and cooperate with electronic components such as appropriate capacitors and connection pads to drive liquid crystal pixels, thereby generating rich and beautiful images. A conventional thin film transistor liquid crystal display basically includes a transparent substrate having a plurality of thin film transistors arranged in a plurality of groups, pixel electrodes ^ orthogonal scanning lines (scan or gate) line) and signal line (data or signal line), a filter (color filter), and the liquid crystal material filled between the transparent substrate and the filter, and supplemented with appropriate electronic components to drive the liquid crystal pixels to produce rich and beautiful Graphics. TFT-LCD is widely used in portable information products such as notebook personal digital assistant (PDA) due to its thin and light appearance, low power consumption and no radiation pollution. It has even been gradually replaced. Trends in CRT monitors for traditional desktop computers.

Please refer to FIGS. 1 and 2. FIG. 1 is a schematic diagram of a 1TT-LCD, FIG. 2A is a schematic diagram of an equivalent circuit of a pixel 20 in the prior art, and FIG. 2B is a top view of a pixel 20 in the prior art. As shown in FIG. 1, a TFT-LCD 10 includes a scanning line driving circuit area 12, a signal line driving circuit area 14, and a pixel array area 16. The pixel array area 16 further includes a plurality of pixels. (Not shown).

As shown in FIG. 2A and FIG. 2B, each pixel 20 disposed in the pixel array region 16 includes a liquid crystal cell filled with liquid crystal molecules (not shown). (liquid crystal unit, LC. unit) 22, and the liquid crystal cell 22 is electrically connected to a common counter electrode (CE) and a thin film transistor (TFT) 24. A gate electrode 26 is electrically connected to the thin film transistor 24 is a scanning line G _n, a source electrode 28 is electrically connected to a signal line S _n, a drain electrode 32 is electrically connected to a pixel electrode (pixel electrode, not shown). In addition, the pixel 20 further includes a storage capacitor SC (storage capacitor) electrically connected to the liquid crystal cell 22 and a common electrode, and a gate-drain capacitor GD (gate-drain capacitor) electrically connected to the gate 26 and the drain of the thin film transistor 24 32. Among them, one of the functions of the storage capacitor SC is to reduce the influence of the leakage current on the voltage of the liquid crystal cell 22, that is, to assist the liquid crystal cell 22 to store electric charges, and the gate-drain capacitor GD is a parasitic capacitor.

Please refer to FIG. 3, which is a schematic diagram of charging the pixel 20 of FIG. 2 in the prior art. As shown in FIG. 3, a first voltage pulse is first applied to the previous column of scanning lines G _n ., According to its pulse timing, and then according to its pulse timing for the next period (next period). ) Is applied to the next column of scanning lines 0. At the same time, a second voltage pulse is also applied to the previous line of signal lines S _{n + 1} according to its pulse timing, and then according to its pulse. timing is applied on top of the next cycle line signal line S _n. when the first voltage pulse and the second voltage pulse is simultaneously applied to the scan line on the one line G _n and the signal line 8 ', the film The transistor 24 will be turned on (tum-on) to charge the pixel electrode (not shown), so as to raise the so-called pixel voltage, and then fill the liquid crystal in the liquid crystal cell (not shown) in the pixel 20 Molecules (not shown) are rotated to the desired angle to control the penetration of light.

In order to meet the specifications of high-resolution and high-display frequency liquid crystal displays, the number of scanning lines and signal lines must be greatly increased. When the number of scanning lines and signal lines is greatly increased, the charging time of each pixel is relatively large. (Ton) will be shortened. Due to the rotation of the liquid crystal molecules, a certain amount of pixel voltage is required, that is, when the charging time is not enough for the pixel voltage to be large enough, a sufficient electric field cannot be provided to charge the liquid crystal. The molecules are rotated to the expected angle, which will seriously affect the penetration of light to each pixel and even cause defective products.

Methods for solving such problems in the prior art improve channel width of thin film transistors

(channel width) to channel length (W / L value), by increasing the ratio of channel width to channel length to increase the current flowing in the thin film transistor channel (current), so that the same The time required for the pixel voltage is shortened, thereby avoiding the problem that the expected luminance voltage cannot be reached due to insufficient charging time.

However, the prior art that cited this solution has other problems. Please refer to FIG. 4. FIG. 4 is a schematic diagram of a gate-drain capacitance generated by a thin film transistor 60 in a conventional liquid crystal display. As shown in FIG. 4, since the gate 62 and the drain 64 of the thin film transistor 60 are conductive materials, and the gate 62 and the drain 64 are isolated by an insulating material (not shown), the gate of the thin film transistor 60 is The overlapped region 66 of 62 and drain 64 forms a parasitic gate-drain capacitance (GD), and when the ratio of the channel width to the channel length of the thin film transistor 60 is increased, The capacitance (Cgd) of the gate-drain capacitance is also increased.

Please refer to FIG. 2 again, it can be known that the voltage applied to the liquid crystal cell 22 is the voltage difference between the common electrode CE and the pixel electrode (not shown). When the thin film transistor 24 is turned off due to the completion of charging, the pixel The electrode (not shown) is not connected to any voltage source, so it is in a floating state. At this time, if there is any voltage change around the pixel electrode (not shown), this voltage change will be coupled to the parasitic capacitor. The pixel electrode (not shown) changes its voltage so that the voltage applied to the liquid crystal cell 22 deviates from the originally set value. This voltage variation is called the feed-through voltage (V _FD ), which can be expressed as:

V _FD = [C _GD / (C _LC + C _SC + C _GD )] * AV _G. (1) Among them, the capacitance value of the liquid crystal cell 22 in the equation (1), C _se is the capacitance value of the storage capacitor SC, C is the capacitance value of the gate-drain capacitance of the thin film transistor 24, and Δ V _Q is applied to the scan The amplitude of the pulse voltage on the line. Therefore, when the ratio of the channel width to the channel length of the thin film transistor 60 is increased, the capacitance value of the gate-drain capacitance is also increased, thereby causing a change in the V _FD value. Especially when making a large-size liquid crystal display, because the panel is too large, the current manufacturing process mostly divides the panel into several areas and exposes them in multiple times. In this case, each area is often aligned when the exposure is aligned. There will be different offsets, plus the effect of the channel width to channel length ratio being increased, it is quite easy to produce stitching defects, which will cause the final LCD to produce a shot mum phenomenon and become a manufacturing process. The last insurmountable obstacle.

Therefore, how to develop a liquid crystal display with high resolution and high display frequency can not only solve the problem of too short charging time, but also avoid line ripple caused by the increase of the capacitance value of the gate-drain capacitor ( shot mura) phenomenon has become a very important issue. Summary of the Invention

The object of the present invention is to provide a liquid crystal display (two TFT pixel structure liquid crystal display, two TFT pixel structure LCD) with a double thin film transistor pixel structure, especially a high resolution and high display frequency. display frequency).

The liquid crystal display with high display frequency of the present invention includes at least one first scan line, at least one second scan line, at least one first signal line, at least one second signal line, and at least one pixel. In a preferred embodiment of the present invention, the pixel is electrically connected to the first scan line, the second scan line, the first signal line and the second signal line, and the pixel includes a liquid crystal molecule filled with a plurality of liquid crystal molecules. Liquid crystal cell, a pixel electrode, a first switching transistor for controlling charging of the pixel electrode, and a The second switching transistor. A gate of the first switching transistor is electrically connected to the first scan line, a source is electrically connected to the first signal line, a drain is electrically connected to the pixel electrode; and a gate of the second switching transistor is Electrically connected to the second scan line, a source electrically connected to the second signal line, and a drain electrically connected to the pixel electrode. The first switching transistor has a first channel length (L,) and a first channel width, the second switching transistor has a second channel length (L ₂ ) and a second channel width (W ₂ ), and A ratio (W, / ^) of the first channel width to the first channel length is smaller than a ratio (W ₂ / L ₂ ) of the second channel width to the second channel length.

Because the liquid crystal display of the present invention uses an additional thin film transistor to charge the pixel electrode in advance when the previous line of scanning lines and the previous line of signal lines receive voltage pulses, and then receives the voltage pulses in the next line of scanning lines and the next line of signal lines. At this time, the pixel electrode continues to be charged, so that the pixel voltage rises to the expected voltage value, so not only the charging time of each pixel will change from T. _{n is} increased to 2T. _n , the picture quality of the display screen will not be affected, and no bright spots are allowed. In addition, the method of the present invention does not have to be limited to the existing method of increasing the ratio of the channel width to the channel length of the thin film transistor in response to the specifications of high resolution and high display frequency. Therefore, the capacitance value of the gate-drain capacitor is not It will be increased, so the feed-through voltage (VFD) can be greatly reduced, and when the present invention is used in an actual production line, it can also effectively produce a high-resolution, high display frequency, wireless ripple (shot rnura). Size panel. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a TFT-LCD.

FIG. 2A is a schematic diagram of an equivalent circuit of a pixel in the prior art.

FIG. 2B is a top view of a pixel in the prior art.

FIG. 3 is a schematic diagram of charging the pixel of FIG. 2 in the prior art. FIG. 4 is a schematic diagram of a gate-drain capacitance generated by a thin film transistor in a conventional liquid crystal display.

FIG. 5A is a schematic diagram of an equivalent circuit of each pixel of the present invention.

FIG. 5B is a top view of each pixel of the present invention.

FIG. 6 is a schematic diagram of charging the pixel of FIG. 5 in the present invention. detailed description

Please refer to FIG. 5, FIG. 5A is a schematic diagram of an equivalent circuit of each pixel 100 according to the present invention, and FIG. 5B is a top view of each pixel 100 according to the present invention. As shown in FIG. 5A and FIG. 5B, each pixel 100 of the present invention includes a liquid crystal unit (LC unit) 102 filled with liquid crystal molecules (not shown) ^ a pixel electrode , Not shown), a first thin film transistor (first TFT) 104 and a second thin film transistor (second TFT) 106. The liquid crystal cell 102 is electrically connected to a common counter electrode (CE), and both the first thin film transistor 104 and the second thin film transistor 106 are used as switches to control the pixel electrodes (not shown). Charging. The gate electrode 108 of the first thin film transistor 104 is electrically connected to the scan line G _n4 in the previous column, the source 112 of the first thin film transistor 104 is electrically connected to the signal line S _{n + 1} in the previous row, and the drain of the first thin film transistor 104 114 is electrically connected to the pixel electrode (not shown); and a second gate electrode 118 is electrically connected to the thin film transistor 106 after a scanning line G _n, the source of the second thin film transistor 106 is electrically connected to the line 122 signal line S _n, The drain electrode 124 of the second thin film transistor 106 is electrically connected to a pixel electrode (not shown).

It is worth noting that the first thin film transistor 104 has a first channel length (L and a first channel width (W), and the second thin film transistor 106 has a second channel length (L ₂ ) and a second channel width (W ₂ ), and the ratio of the first channel width to the first channel length (WL,) Is smaller than the ratio (W ₂ / L ₂ ) of the second channel width to the second channel length. In addition, the pixel 100 also includes at least one storage capacitor SC. A common situation shown in FIG. 5A is a case where a storage capacitor SC is electrically connected to the liquid crystal cell 102 and a common electrode. A first gate-drain capacitor (GD1) derived from an overlapping region (not shown) of the gate 108 and the drain 114 of the thin film transistor 104 is electrically connected to the gate 108 and the drain of the first thin film transistor 104 Electrode 114, and a second gate-drain capacitor GD2 (second gate-drain capacitor) derived from the overlap region (not shown) of gate 118 and drain 124 of second thin film transistor 106 is electrically connected to the second thin film The gate 118 and the drain 124 of the transistor 106. One of the functions of the storage capacitor SC is to reduce the effect of the leakage current on the voltage of the liquid crystal cell 102, that is, to assist the liquid crystal cell 102 to store the charge, and the first gate-drain capacitor GDI and the second gate-drain capacitor GD2 is a parasitic capacitor.

Please refer to FIG. 6, which is a schematic diagram of charging the pixel 100 of FIG. 5 in the present invention. As shown in FIG. 6, a first voltage pulse is first applied to the scan line of the previous column according to its pulse time, and then is applied to the scan line of the next column G _n in the next cycle according to its pulse timing. Similarly, a second voltage pulse is first applied to the signal line S _{n + 1} on the previous line according to its pulse timing, and then applied to the signal on the next line in the next period according to its pulse timing. on line S _n. When the first voltage pulse and the second voltage pulse are simultaneously applied to the scan line and the signal line Sn _{+ 1} , the first thin film transistor 104 will be turned on to charge the pixel electrode (not shown) ( charge) to raise the so-called pixel voltage to a certain level. When the first voltage pulse and the second voltage pulse are simultaneously applied to the scanning line G _n and the signal line 8 ′, the second thin film transistor 106 will be turned on to continue to the pixel electrode (not shown). Charging causes the pixel voltage to continue to rise, thereby driving liquid crystal molecules (not shown) filled in the liquid crystal cell (not shown) in the pixel 100 to rotate to a desired angle to control the degree of light transmission.

Because when the first thin film transistor 104 is turned on, the pixel 100 has already been charged. In other words, when the first voltage pulse is applied to the previous row of scanning lines and the previous row of signal lines S _{n + 1} , the pixel voltage has begun to rise to a specific value, and when the second voltage after the pulse is applied to a scanning line on G _n, and the row signal lines S _n Z, when the pixel voltage will easily and quickly elevated up to the desired voltage value. In other words, the method of the present invention is equivalent to increasing the charging time of each pixel from Ton to 2Ton, and because the charging time of each pixel is stretched to twice, so under the requirements of high resolution, The display frequency of the invented liquid crystal display can be significantly increased. In addition, since the ratio of the first channel width to the first channel length is smaller than the ratio of the second channel width to the second channel length (W ₂ / L ₂ ), the charging rate of the second thin film transistor 106 will be significantly greater than The charging rate of the first thin film transistor 104, and at the same time, because the entire charging time 2Ton is very short, the first and second voltage pulses are respectively applied to the previous row of scanning lines G _n _ and the previous one of the signal lines S _{n + 1} . During the pre pump stage, the display quality of the display screen will not be deteriorated because the first thin film transistor 104 has been turned on and the pixel electrodes are precharged. .

In addition, it is worth noting that because the first thin film transistor 104 and the second thin film transistor 106 are both used to charge the pixel electrode (not shown), when one of the transistors fails, there is still another one that can be used for charging. Since the phenomenon of light defects is not generated, the yield can be effectively improved, and even zero defect products can be produced. Moreover, since the liquid crystal display of the present invention can use the first thin film transistor 104 for pre-charging, the charging time will be relatively lengthened. Therefore, the present invention does not need to be as general as the aforementioned prior art. The ratio of the channel width to the channel length (W ₂ / L ₂ ) of the transistor 106 is used to solve the problem that the luminance voltage cannot be reached. In this way, not only the second gate formed by the overlap of the gate 118 and the drain 124 -The capacitance value (Cgd2) of the drain capacitor GD2 will not be increased to reduce the feed-through voltage (VFD), and relatively speaking, the process of the present invention is also less prone to produce a shot mura phenomenon. . In short, since the liquid crystal display of the present invention utilizes the addition of a thin film transistor, the pixel electrodes are charged in advance when receiving voltage pulses in the previous line of scanning lines and the signal lines in the previous line, and then in the next line of scanning lines and the next line of signal lines. When the voltage pulse is received, the pixel electrode is continuously charged to increase the pixel voltage to the expected voltage value, which not only can increase the charging time, but also can keep the capacitance value of the gate-drain capacitor from being increased.

Compared with the prior art, the present invention uses the addition of a thin film transistor to charge the pixel electrode in advance when the previous line of scanning lines and the previous line of signal lines receive voltage pulses, and then the next line of scanning lines and the next line of signals When the line receives the voltage pulse, it continues to charge the pixel electrode, so that the pixel voltage rises to the expected voltage value, so not only the charging time of each pixel will change from T. _{n is} increased to 2T. _Π , the picture quality of the display screen will not be affected, and no bright spots are allowed. In addition, the method of the present invention does not have to be limited to the existing method of increasing the ratio of the channel width to the channel length of the thin film transistor in response to the specifications of high resolution and high display frequency. Therefore, the capacitance value of the gate-drain capacitor is not It will be increased, so the feed-through voltage (VFD) can be greatly reduced, and when the present invention is used in an actual production line, it can also effectively produce a large-scale, high-resolution, high-frequency display, wireless shot mura (shot mura). Size panel.

The above description is only the preferred embodiments of the present invention, and any equivalent changes and modifications made in accordance with the scope of the patent application of the present invention shall fall within the scope of the patent of the present invention. Reference Signs

10TFT-LCD 12 scan line driver circuit area

14 signal line driver circuit area 16 pixel array area

20 pixels 22 LCD

24 thin film transistor 26 gate

28 source 32 drain Thin film transistor 62 gate

Drain 66 overlap area

0 pixel 102 LCD cell

First thin film transistor 106 Second thin film crystal 8 Gate 112 Source

Drain 118 Gate

Source 124 Drain

Claims

1. A liquid crystal display with a high display frequency, comprising: at least one first scanning line;

At least one second scan line;

At least one first signal line;

Right

At least one second signal line; and

At least one pixel, the pixel is electrically connected to the first scan line, the second scan line, the first signal line, and the second signal line, and the pixel includes:

A liquid crystal cell filled with a plurality of liquid crystal molecules;

A pixel electrode

A first switching transistor for controlling charging of the pixel electrode, and a gate of the first switching transistor is electrically connected to the first scanning line, a source is electrically connected to the first signal line, and a drain is electrically connected. Connected to the pixel electrode; and

A second switching transistor for controlling charging of the pixel electrode, and a gate of the second switching transistor is electrically connected to the second scanning line, a source is electrically connected to the second signal line, and a drain is electrically connected. Connected to the pixel electrode;

The first switching transistor has a first channel length (L and a first channel width), the second switching transistor has a second channel length (L ₂ ) and a second channel width (W ₂ ), and the first The ratio of the channel width to the first channel length (Wi / LJ is smaller than the ratio of the second channel width to the second channel length (W ₂ / L ₂ ).

2. The liquid crystal display as claimed in claim 1, wherein when a first voltage pulse is applied to the first scan line and a second voltage pulse is applied to the first signal line, the first switch The transistor will be turned on to charge the pixel electrode.

3. The liquid crystal display as claimed in claim 2, wherein, during the first scanning After the line receives the first voltage pulse, the second scan line receives the first voltage pulse in the next cycle in accordance with the timing of the first voltage pulse, and after the first signal line receives the second voltage pulse, the first The two signal lines receive the second voltage pulse in the next cycle according to the timing of the second voltage pulse.

4. The liquid crystal display of claim 3, wherein when the first voltage pulse is applied to the second scan line and the second voltage pulse is applied to the second signal line, the second switch The transistor will be turned on to charge the pixel electrode.

The liquid crystal display of claim 1, wherein the liquid crystal display is a liquid crystal display with a double thin film transistor pixel structure.

6. The liquid crystal display as claimed in claim 1, wherein a first overlapping region is provided between the gate and the drain of the first switching transistor, and the gate and the second switching transistor of the first switching transistor have a first overlapping region. There is a second overlapping region between the drains.

The liquid crystal display of claim 6, wherein the first overlapping region causes a parasitic capacitance between the gate and the drain of a first switching transistor, and the second overlapping region causes a second switch Parasitic capacitance between the gate and the drain of the transistor.

The liquid crystal display of claim 1, wherein the first switching transistor and the second switching transistor are both used to charge the pixel electrode to promote the rotation of the liquid crystal molecules in the liquid crystal cell. 9.

The liquid crystal display of claim 1, wherein a ratio of the first channel width to the first channel length is smaller than a ratio of the second channel width to the second channel length, so that the second switch The charging rate of the transistor is greater than the charging rate of the first switching transistor.

10. The liquid crystal display of claim 1, wherein the pixel further comprises at least one storage capacitor to assist the liquid crystal cell to store electric charges.