JPH0827464B2 - Active matrix LCD display - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix liquid crystal display device in which two transistors are formed in each pixel and a display defect does not occur even if one transistor has a defect. It is a thing.

[Prior Art] Recently, there has been a strong demand for development of thin displays such as OA equipment terminals and flat-screen TVs. One of them is a liquid crystal display device in which electrodes are arranged in a matrix, and transistors such as transistors are provided near intersections of the electrodes. Active matrix liquid crystal display devices have been actively developed in which active elements are arranged to drive liquid crystals.

FIG. 2 is an equivalent circuit diagram of a typical example of this active matrix liquid crystal display device. In the figure, 21 is a liquid crystal layer, 22 is a capacitor for holding a voltage applied to the liquid crystal layer, and 23 is a transistor for controlling a voltage for driving the liquid crystal layer. X _2,1 , X _2,2 ,
X _2,3 ... are row select lines that control the gate of the transistor 23, and Y _2,1 , Y _2,2 , Y _2,3 ... are columns for applying the voltage necessary to drive the liquid crystal. It is a signal line and is driven line-sequentially.

When putting such an active matrix liquid crystal display device into practical use, the biggest obstacle is defects such as disconnection and short circuit. Since it is necessary to integrate tens of thousands to hundreds of thousands of transistors on one substrate, it is extremely difficult to obtain a defect-free display.
In order to remove such a defect, a structure in which two transistors are arranged in advance for each pixel is known.

3 and 4 show equivalent circuit diagrams of an example in which two transistors are provided for each pixel.

FIG. 3 is an example of a structure in which transistors are arranged above and below a row selection line. 33 is a first transistor, 34 is a second transistor, and 31 is a liquid crystal of a pixel provided with the two transistors. The layers are shown. Also, X ₃ , i _-1 , X ₃ ,
i, X ₃ , i ₊₁ , X ₃ , i _{+2 are} row selection lines, Y ₃ , j _-1 , Y ₃ , j, Y ₃ , j ₊₁
Indicates a column signal line.

In this example, the gate electrodes of the two transistors are connected to different row selection lines. In the example of FIG. 3,
In the 31 pixel, the gate electrode of the first transistor 33 is
The gate electrode of the second transistor 34 is connected to the X ₃ , i _-1 row selection line, and is connected to the X ₃ , i row selection line.

Therefore, if it is found that the second transistor 34 has a defect, the second transistor 34 is separated from the row selection line by a laser or the like, and only the first transistor 33 drives the liquid crystal layer. . Therefore, the liquid crystal layer 31 of the pixel displays an image on one line.

FIG. 4 shows an example of a structure in which two transistors are arranged in parallel on one row selection line, and 43 is the first transistor,
Reference numeral 44 denotes a second transistor, and 41 denotes a liquid crystal layer of a pixel provided with the two transistors. Also,
X ₄ , i _-1 , X ₄ , i, X ₄ , i _{+1 are} row selection lines, Y ₄ , j _-1 , Y ₄ , j, Y ₄ , j
₊₁ indicates a column signal line.

In this example, the gate electrodes of the two transistors are connected to the same row selection line. In the example of FIG. 4, 41
In the pixel of, the gate electrode of the first transistor 43 is also the second
The gate electrode of the transistor 44 is also connected to the row selection line of X ₄ , i ₊₁ .

Therefore, if it is found that the first transistor 43 has a defect, the first transistor 43 is cut and separated from the row selection line by a laser or the like, and only the second transistor 44 drives the liquid crystal layer. . Therefore, the liquid crystal layer 41 of the pixel displays the original image of the pixel.

[Problems to be Solved by the Invention] With the structure shown in FIG. 3, when laser trimming is performed, the pixel is displayed with a line offset by one line. Therefore, a moving image is displayed like a television image. However, this is an effective defect remedy method when the display of adjacent pixels is not so different, but there is a drawback that incorrect information is displayed in the case of character / graphic display. Therefore,
The structure shown in FIG. 3 cannot be adopted in a display device intended for displaying characters and figures.

Therefore, it is necessary to adopt the structure shown in FIG. 4 for the device for displaying the character and graphics, but this also has the following problems.

That is, the transistor has a parasitic capacitance between the gate electrode and the pixel electrode, and the potential of the pixel electrode is changed by the parasitic capacitance, the capacitance of the liquid crystal layer between the substrates, and the storage capacitance of the pixel capacitor. Known to shift.

Therefore, in the case of a liquid crystal display device having a structure of one transistor per pixel, by driving by shifting the potential of the counter electrode by the shifted size, there is no problem in terms of driving in AC driving. Is possible. However, in the case of a liquid crystal display device in which two transistors are provided in one pixel in order to reduce display defects, when one of the transistors is trimmed, the drive potential is shifted, and there is a risk of erroneous display. there were.

[Means for Solving Problems] The present invention has been made to solve such problems, and electrodes are arranged in a matrix on an insulating substrate, and the row electrodes are provided in the vicinity of intersections of the electrodes. Active matrix liquid crystal display in which liquid crystal is sandwiched between an active matrix substrate provided with a pixel electrode in which two transistors each having a gate electrode connected to each pixel are arranged in parallel per pixel, and a counter substrate provided with a counter electrode. In the device, the parasitic capacitance of the transistor is C _P , the capacitance of the liquid crystal between the substrates is C _L ,
When the storage capacity of the pixel capacitor is C _S , the selection signal voltage of the gate electrode is V _G , and the lighting threshold voltage of the liquid crystal is V _L , The present invention provides an active matrix liquid crystal display device characterized by the following.

With this structure, one pixel is
Even if it is driven by one transistor, one of the
Even when driven by only individual transistors, the original display is accurately displayed, and the risk of misidentifying the display can be almost eliminated.

As a result, an active matrix substrate in which a large number of transistors are integrated on the substrate can be produced with a high yield,
The productivity can be improved, and the active matrix liquid crystal display device can be used for a display device which does not have a defect of one pixel such as a character graphic display.

FIG. 1 is an equivalent circuit diagram of a typical example of the active matrix liquid crystal display device of the present invention.

In FIG. 1, a pixel in which two transistors are arranged in parallel on one row selection line, 3 is a first transistor, 4 is a second transistor, and 1 is the two transistors The liquid crystal layers 2 and 2 are storage capacitors.

Further, Xi _-1 , Xi, and Xi ₊₁ indicate row selection lines, and Yj _-1 , Yj, and Yj ₊₁ indicate column signal lines.

In the present invention, the gate electrodes of the two transistors are connected to the same row selection line. In the pixel 1 shown in FIG. 1, both the gate electrode of the first transistor 3 and the gate electrode of the second transistor 4 are connected to the row selection line of Xi ₊₁ .

Therefore, if it is found that the first transistor 3 has a defect, the first transistor 3 is cut off from the row selection line by a laser or the like, and the liquid crystal layer is driven only by the second transistor 4. Good. On the contrary, the second
If it is found that the second transistor 4 has a defect, the second transistor 4 may be cut and separated from the row selection line by a laser or the like, and the liquid crystal layer may be driven only by the first transistor 3. In any of these cases, since the liquid crystal layer 1 of the pixel is driven by the same row selection line in any case, the image of the original pixel is displayed.

The insulating substrate of the present invention is a substrate having insulating properties such as glass, plastic, and ceramics, and may be a substrate in which an insulating film is formed on a conductive or insulating substrate. Electrodes made of a metal film or a transparent conductive film are arranged in a matrix on the insulating substrate to form row electrodes (row selection lines) and column electrodes (column signal lines), respectively. A pixel electrode in which two transistors each having a gate electrode connected to the row electrode are arranged in parallel per pixel is provided near the intersection of the row electrode and the column electrode. As these two transistors, transistors of various known shapes can be used, and normally transistors having the same structure may be used. For the semiconductor layers of these transistors, amorphous, polycrystalline, or single crystal semiconductors such as Si, CdSe, and GaAs can be used.

If necessary, a color filter and an alignment film are formed on the pixel electrode to form an active matrix substrate. A counter electrode is formed on the other substrate to serve as a counter substrate.

A liquid crystal is sandwiched between the active matrix substrate and the counter substrate to form an active matrix liquid crystal display device.

Since this only shows the basic configuration, within a range that does not impair the effects of the present invention, for example, one of the electrodes may be a reflective electrode, or a portion other than the pixel electrode may be covered with a light-shielding layer, or a substrate. A conventionally known structure may be added by disposing a spacer for adjusting the gap.

[Operation] The operation of the present invention will be described.

In FIG. 1, the parasitic capacitance between the gate electrode and the pixel electrode of each transistor is C _P , the liquid crystal capacitance between the substrates when no electric field is applied is C _L , and the pixel capacitor The storage capacitance is C _S and the selection signal voltage of the gate electrode is
When V _G is set, the potential of the pixel electrode shifts by ΔV as shown in the following expression (1) when the signal potential of the selection line drops.

Therefore, in the case of driving with two transistors, it is sufficient to shift the potential of the counter electrode by this size and drive.

However, when one transistor has a defect, one transistor is trimmed, and the potential shift ΔV ′ shifts as shown in the following expression (2).

Therefore, the difference between the pixel driven by two transistors and the pixel driven by one transistor is Δ
The driving potential will shift by Vc = ΔV−ΔV ′.

Therefore, when the lighting threshold voltage of the liquid crystal is _VL ,
If this potential shift difference ΔVc exceeds the lighting threshold voltage V _L , an erroneous display will occur.

The lighting threshold voltage V _L of the liquid crystal is the case where the transmittance of the light of the pixel when viewed from the front of the liquid crystal display device is 0 when the transmittance is 0 and the maximum is 1 when the transmittance is 1. In addition, it means the effective applied voltage at which the transmittance becomes 0.9 in the positive type display, and the effective applied voltage at which the transmittance becomes 0.1 in the negative type display.

Therefore, by designing the parasitic capacitance of the transistor so as to satisfy the condition of the following expression (3), such an erroneous display can be prevented.

In this formula, the parasitic capacitance C _P is considerably smaller than the capacitance C _{L of} the liquid crystal and the storage capacitance C _S of the pixel capacitor, so this formula can be approximated by the following formula (4). .

In particular, in this case, it is understood that the parasitic capacitance C _P of the transistor can be set as shown in the following expression (5) by eliminating the capacitor of each pixel.

Further, when a capacitor is provided and two transistors are used as they are, the capacitor is used, and when one of the transistors is trimmed, the capacitor is also trimmed at the same time. The potential shift ΔV 'is as follows.

Therefore, in this case as well, from equations (1) and (6),
The calculation of Vc is as follows.

That is, by making the capacitance C _L of the liquid crystal and the storage capacitance C _S of the pixel capacitor substantially equal to each other here, ΔVc can be almost zero regardless of the size of the parasitic capacitance of the transistor. The potential difference between the non-trimmed pixel and the trimmed pixel can be almost eliminated, and erroneous display does not occur.

By doing so, even if one of the transistors in one pixel has a defect, by trimming this with a laser trimmer and cutting and separating, a high yield, high productivity, and accurate display can be performed. It is possible to obtain an active matrix liquid crystal display device which can be manufactured.

[Examples] Example 1 Amorphous silicon was used as a semiconductor, and an inverted stagger type transistor as shown in FIG. 5 was used as a transistor structure. In FIG. 5, 50 is a glass substrate, 51 is a gate electrode, 52 is a gate insulating film, 53 is a semiconductor layer, 54 is a protective insulating layer, 55A and 55B are source and drain electrodes, respectively, and 56 is a display pixel electrode. . The size of the parasitic capacitance of the transistor is determined by the width of the overlap between the gate electrode and the display pixel electrode, the channel width of the transistor, and the contribution from the channel capacitance.

Using such an inverted stagger type thin film transistor, two transistors were formed in one pixel as shown in FIG. No auxiliary storage capacitor was provided.

In this embodiment, based on the distance between the row selection line and the column signal line,
The cell gap was 400 μm, the cell gap was 9.5 μm, and the aperture ratio was 70%. The liquid crystal display system was a twisted nematic (TN) type, and a liquid crystal having a dielectric constant of 5 when no electric field was applied was used.

The liquid crystal capacitance C _L between the substrates when no electric field is applied in this liquid crystal display device is C _L = 0.52 pF, and since no capacitor is provided, the storage capacitance C _S of the pixel capacitor is C _S =
It was 0. Further, the selection signal voltage V _G of the gate electrode is V _G =
It was set to 25V.

The size of this transistor is such that the thickness of the gate insulating film 52 is 260 nm, the dielectric constant is 5, the overlap between the gate electrode 51 and the drain electrode 55B is 2 μm, and the channel width and the channel length of the transistor are 10 μm and 9 μm, respectively. And

In this case, the parasitic capacitance C _P was C _P = 0.012 pF due to the contribution of the drain side half of the channel capacitance and the overlapping portion of the gate electrode 51 and the drain electrode 55A. This allows
The potential shift was ΔV _C = 0.54V. This is sufficiently lower than the lighting threshold voltage V _L of the liquid crystal, which is 2.1 V. For pixels driven by two transistors, the defective transistor is trimmed and removed by trimming with one transistor. The driven pixels were also able to be driven without any problems, and erroneous display did not occur.

Example 2 The spacing between row selection lines is 220 μm, and the spacing between column signal lines is 190 μm
A twisted nematic (TN) type liquid crystal display device was manufactured in the same manner as in Example 1 except that the aperture ratio was 60% and the storage capacitor was provided.

The capacitance C _L of the liquid crystal between the substrates when the electric field of this liquid crystal display device was not applied was C _L = 0.19 pF, and the storage capacitance C _S of the pixel capacitor was C _S = 0.35 pF. Further, the selection signal voltage V _G of the gate electrode is set to V _G = 25V.

The potential shift of this liquid crystal display device was ΔVc = 0.52V. This was sufficiently lower than the lighting threshold voltage _VL of the liquid crystal, 2.1 V, and no erroneous display occurred.

Example 3 A twisted nematic (TN) type liquid crystal display device was manufactured in the same manner as in Example 1 except that the aperture ratio was 60% and the storage capacitor was provided.

The liquid crystal capacitance C _L between the substrates when no electric field was applied to this liquid crystal display device was C _L = 0.45 pF, and the storage capacitance C _S of the pixel capacitor was C _S = 0.45 pF. Further, the selection signal voltage V _G of the gate electrode is set to V _G = 25V.

In this liquid crystal display device, when the pixel transistor is defective, the defective transistor is removed and the capacitor is also removed.

In this liquid crystal display device, since the capacitance C _L of the liquid crystal between the substrates and the storage capacitance C _S of the pixel capacitor are made the same, (C _P · C _L −C _P · C _S ) becomes 0, the potential shift ΔV _C is almost 0 regardless of the values of the selection signal voltage V _G of the gate electrode and the parasitic capacitance C _P of the transistor.
Becomes

As a result, the potential shift ΔV _C is almost 0, which is, of course, sufficiently lower than the lighting threshold voltage V _L of 2.1 V of the liquid crystal, and the pixel driven by the two transistors is also defective. A pixel driven by one transistor by cutting and removing the transistor and the capacitor by trimming could also be driven without any problem, and no erroneous display occurred.

In these embodiments, two transistors are used and the structure of the transistor is an inverted stagger type. However, three or more transistors may be used, or a stagger type or a coplanar type may be used.
The two transistors may have different shapes.

For example, if the two transistors have different shapes, and the parasitic capacitances of the _two transistors are shown as C ₁ and C _{2, respectively,} and the transistor of the parasitic capacitance C ₂ is removed, the following equation is obtained. Will be satisfied.

That is, as the C _P of the equation (3) molecules, substitutes the parasitic capacitance of the excised transistors, by substituting the synthesized parasitic capacitance of the two transistors as the denominator of 2C _P, transistor remaining in C _P of the denominator It means that the parasitic capacitance of is substituted.

Even when three or more transistors are formed in one pixel, this formula is similarly established, and the combined parasitic capacitance of all the transistors is represented by (C ₁ + C ₂ ), and one or two or more cut-off transistors are combined. The parasitic capacitance may be represented by C ₂ , and the combined parasitic capacitance of the remaining one or more transistors may be represented by C ₁ .

The liquid crystal is not limited to TNLCD, and ferroelectric liquid crystal such as SmC ^* can be used.

[Effects of the Invention] As described above, according to the present invention, by providing a plurality of transistors in one pixel, even if a defective transistor occurs, only the defective transistor is cut off and only one transistor is used. The productivity of the active matrix liquid crystal display device which can be driven and has a low manufacturing yield can be improved.

Further, even if the pixel is driven by only one transistor, no erroneous display occurs with a pixel driven by two transistors, and all pixels are accurately displayed as in the case of character and graphic display. No erroneous display occurs even on a required display.

In addition to the above, the present invention can be applied in various ways within a range that does not impair the effects of the present invention.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of the active matrix liquid crystal display device of the present invention. FIG. 2 is an equivalent circuit diagram of an active matrix liquid crystal display device in which one transistor is provided for each pixel in the conventional example. FIG. 3 is an equivalent circuit diagram of an active matrix liquid crystal display device in which two transistors are provided in one pixel. FIG. 4 is an equivalent circuit diagram of an active matrix liquid crystal display device in which two transistors are provided in parallel in one pixel. FIG. 5 is a sectional view of an inverted staggered transistor. Liquid crystal layer: 1,21,31,41 Capacitor: 2,22 Transistor: 3,4,23,33,34,43,44

Claims (5)

[Claims] 1. A pixel electrode in which electrodes are arranged in a matrix on an insulating substrate and two transistors each having a gate electrode connected to the row electrode are arranged in parallel in the vicinity of an intersection of the electrodes. In an active matrix liquid crystal display device in which a liquid crystal is sandwiched between the provided active matrix substrate and a counter substrate provided with a counter electrode, the parasitic capacitance of the transistor is C _P and the liquid crystal capacitance between the substrates is C _{P. L} , the storage capacity of the pixel capacitor is C _S , the selection signal voltage of the gate electrode is V _G , and the lighting threshold voltage of the liquid crystal is V _L , An active matrix liquid crystal display device characterized by the following. 2. A capacitor is not provided in each pixel, The active matrix liquid crystal display device according to claim 1. 3. The active matrix liquid crystal display device according to claim 1, wherein when a transistor is defective, the defective transistor is separated and used. 4. The active matrix liquid crystal display device according to claim 3, wherein each pixel is provided with a capacitor, and when a transistor is defective, the defective transistor and capacitor are separated and used. 5. The active matrix liquid crystal display device according to claim 4, wherein the capacitance C _L of the liquid crystal between the substrates and the storage capacitance C _S of the pixel capacitor are made substantially equal to each other.