JPH04225330A - Active matrix liquid crystal display device - Google Patents

Abstract

PURPOSE:To display good images free from 'flickering', etc., by impressing sufficient electric fields to the liquid crystals between picture element electrodes and counter electrodes even at the time of non-selection when active circuits attain a shut-off state. CONSTITUTION:Electrodes 27 for capacitors facing the picture element electrodes 13 via insulating films 26a are provided in correspondence to a part of the picture element electrodes 13. Capacity compensation capacitors Cs are formed of such electrodes 27 for the capacitors, the picture element electrodes 13 and the insulating films 26a therebetween so that the capacity of the liquid crystal capacitors formed of the picture element electrodes 13, the counter electrodes 11 and the liquid crystals therebetween is compensated by the capacity compensation capacitors Cs.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix liquid crystal display device using thin film diodes as active elements for selecting pixel electrodes.

0002

2. Description of the Related Art There is an active matrix liquid crystal display device that uses a thin film diode as an active element for selecting a pixel electrode.・
There is something called a to-back (BTB) structure.

5 and 6 are partial circuit diagrams of a conventional active matrix liquid crystal display device, with FIG. 5 showing a diode ring structure and FIG. 6 showing a back-to-back structure.

This active matrix liquid crystal display device has a plurality of striped counter electrodes (transparent electrodes) 1 formed parallel to each other at close intervals on one side of a pair of transparent substrates facing each other with a liquid crystal layer in between. On the other substrate, a large number of signal lines 2 perpendicular to the length direction of the counter electrode 1 are wired in parallel with each other at intervals slightly wider than the area of the display pixels, and are arranged along each of the signal lines 2. A large number of pixel electrodes (transparent electrodes) 3 facing each of the above-mentioned counter electrodes 1 are formed in an array, and each of the above-mentioned signal lines 2 and each pixel electrode 3 along this signal line 2 is connected to a thin film diode (hereinafter simply referred to as a thin-film diode). They are connected via an active circuit consisting of a diode (also called a diode).

As shown in FIG. 5, the liquid crystal display device with the diode ring structure has a pair of active circuits A1 each having a plurality of (two in the figure) diodes connected in series at both ends of the pixel electrode 3. , A2, and the diode of one active circuit A1 connected to one end of the pixel electrode 3, and the diode of the active circuit A2 connected to the other end of the pixel electrode 3. The directions of the diodes are opposite to each other.

In this liquid crystal display device with a diode ring structure, each of the active circuits A1 and A2 connecting the signal line 2 and both ends of the pixel electrode 3 is constructed by connecting a plurality of diodes in series. This is to increase the ON voltage for displaying pixels.

That is, each of the above active circuits A1 and A2
If the active circuits A1 and A2 were constructed with only one thin film diode, the threshold voltage at which the active circuits A1 and A2 become conductive with respect to forward currents would be too low, and voltage due to noise etc. would be applied to the signal line 2. This voltage will still be applied to the pixel electrode 3 even if the active circuits A1 and A2 are connected in series, but if each active circuit A1 and A2 is configured with a plurality of thin film diodes connected in series, the threshold voltage of the active circuits A1 and A2 will be becomes high, and when a signal with a voltage higher than this threshold value is applied to the signal line 2, the display drive voltage is applied to the pixel electrode 3. Therefore, by increasing the difference between the ON voltage and the OFF voltage, noise is reduced. Display driving that is not affected by voltage or the like can be performed.

This liquid crystal display device is driven for display by sequentially applying a scanning signal to each opposing electrode 1 and applying an image data signal to each signal line 2 in synchronization with this. Note that the scanning signal and the image data signal are both signals whose polarities are inverted at the same period, and the polarities of the scanning signal and the image data signal are opposite to each other.

To explain the display operation of this liquid crystal display device, suppose that a data signal with a positive voltage (+Vc) higher than the thresholds of the active circuits A1 and A2 is applied to the signal line 2. The liquid crystal capacitor consisting of the electrode 1, the pixel electrode 3, and the liquid crystal between them can be considered to be transiently conductive, whereas the active circuits A1 and A2 consisting of thin film diodes have high resistance.
At the moment the data signal is applied, the voltage +Vc of the data signal is applied across the active circuits A1 and A2.

Since the voltage +Vc of this data signal is higher than the threshold of the active circuits A1 and A2, an active circuit (connected to the upper end of the pixel electrode 3 in the figure) consisting of a diode in the forward direction from the signal line 2 A current flows to the pixel electrode 3 through the active circuit A1.

[0011] Also, at this time, since a scanning signal of negative voltage, which is opposite in polarity to the data signal applied to the pixel electrode 3, is applied to the counter electrode 1, there is a gap between the pixel electrode 3 and the counter electrode 1. A charge corresponding to the voltage is accumulated in the liquid crystal capacitor, and the pixels in this area enter a writing (display) state. In addition,
The voltage across the active circuit A1 becomes lower as charge is accumulated in the liquid crystal capacitor, and when this voltage becomes lower than a threshold value, the active circuit A1 is cut off.

Active circuits A1 and A2 are also connected to the signal line 2.
When a data signal with a negative voltage (-Vc) higher than the threshold value of , the active circuits A1 and A2
A voltage with a polarity opposite to that when the positive voltage (+Vc) data signal is applied is applied to both ends of the data signal. In this case as well, the voltage -Vc of the data signal is applied to the active circuit A1,
Since the current is higher than the threshold value of A2, current flows from the pixel electrode 3 to the signal line 2 through the active circuit (active circuit connected to the lower end of the pixel electrode 3 in the figure) consisting of a diode in the opposite direction. .

Furthermore, at this time, since a scanning signal with a positive voltage is applied to the counter electrode 1, charges of opposite polarity are accumulated in the liquid crystal capacitor, and the pixels in this portion perform writing (display) of opposite polarity. ) state. At this time as well, the voltage across the active circuit A2 decreases as charge is accumulated in the liquid crystal capacitor, and when this voltage becomes lower than the threshold value, the active circuit A2 is cut off. become a state.

Even after the active circuits A1 and A2 are cut off, charge is accumulated in the liquid crystal capacitor, so that the liquid crystal between the pixel electrode 3 and the counter electrode 1 is voltage is continuously applied,
This maintains the display state of the pixel.

On the other hand, a back-to-back liquid crystal display device, as shown in FIG. 6, connects the signal line 2 and the pixel electrode 3 with an active circuit B in which two thin film diodes are connected in series in opposite directions. The configuration is connected with .

The display operation of this back-to-back liquid crystal display device is basically the same as that of the diode ring structure. However, in this back-to-back liquid crystal display device, the active circuit B is
Since the configuration is such that thin film diodes are connected in series in opposite directions, the threshold voltage of active circuit B is
This is the voltage at which the diode in the opposite direction to the direction of current among the two thin film diodes loses its diode function and a reverse current begins to flow through this diode.

[0017]

However, in the conventional active matrix liquid crystal display device, when the active circuit A1, A2 or B is cut off, the voltage of the liquid crystal capacitor, that is, the voltage across the pixel electrode 3 is The voltage between the selected pixel and the electrode 1 drops significantly, and as a result, it is no longer possible to apply a sufficient electric field to the liquid crystal continuously during the non-selection period until the selected pixel is selected next. There was a problem that "flickering" etc. occurred.

This is because the thin film diode constituting the active circuit A1, A2 or B has a capacitance. That is, a thin film diode has a semiconductor layer with a P-I-N (or N-I-P) junction structure or a semiconductor layer with a P-N (or N-P) junction structure interposed between a pair of opposing electrodes. However, in a thin film diode with a P-I-N junction structure, the I-type semiconductor film between the P-type semiconductor film and the N-type semiconductor film of the semiconductor layer corresponds to the insulating layer of the capacitor. In a thin film diode with a P-N junction structure, when a reverse voltage is applied to it, the P-N
A depletion layer is formed at the junction, and this depletion layer acts as an insulating film for the capacitor, so thin film diodes with any junction structure have capacitance.

[0019] In this way, the active circuits A1 and A2
Alternatively, if the thin film diode constituting B has a capacitance, the voltage of the liquid crystal capacitor that has accumulated the charge of the data signal when selected, that is, when the above active circuit is in a conductive state, is applied to the active circuit A1, A2 or B, which is cut off. state (non-selected state), the voltage is divided between the liquid crystal capacitor and the active circuit according to the ratio of the capacitance of the liquid crystal capacitor to the capacitance of the active circuit (total capacitance of each thin film diode constituting the active circuit). As a result, the liquid crystal capacitor voltage decreases significantly, and as a result, it becomes impossible to apply a sufficient electric field to the liquid crystal between the pixel electrode 3 and the counter electrode 1, and "flickering" or the like occurs in the display.

An object of the present invention is to maintain a sufficient electric field in the liquid crystal between the pixel electrode and the counter electrode even in the non-selected state when the active circuit is cut off, even though the active circuit is configured with thin film diodes. An object of the present invention is to provide an active matrix liquid crystal display device that can display good images without "flickering" or the like by applying .

[0021]

[Means for Solving the Problems] The present invention has a plurality of striped counter electrodes formed in parallel to each other on one side of a pair of transparent substrates facing each other with a liquid crystal layer in between, and the opposite electrodes are formed on the other substrate. A large number of signal lines perpendicular to the length direction of the electrodes are wired in parallel to each other, and a large number of pixel electrodes are arranged and formed along each of the signal lines to face each of the opposing electrodes, In an active matrix liquid crystal display device in which each signal line and each pixel electrode along this signal line are connected via an active circuit consisting of a thin film diode, a part of the pixel electrode is insulated from the pixel electrode. A capacitor electrode is provided that faces each other through a film, and the capacitor electrode, the pixel electrode, and an insulating film therebetween accumulate charges of a signal applied to the pixel electrode when the active circuit becomes conductive. The present invention is characterized in that a capacitance compensation capacitor is formed.

[0022]

[Operation] That is, in the active matrix liquid crystal display device of the present invention, by forming a capacitance compensation capacitor corresponding to a part of the pixel electrode, the liquid crystal capacitor formed by the pixel electrode, the counter electrode, and the liquid crystal therebetween is The capacitance is supplemented by the capacitance compensation capacitor described above, and since one electrode of this capacitance compensation capacitor is part of the pixel electrode, similar to the liquid crystal capacitor described above, the signal applied to the pixel electrode at the time of selection is accumulates a charge of

Also in this liquid crystal display device, when the active circuit is in the cutoff state (non-selected state), the voltage of the liquid crystal capacitor is equal to the capacitance of the liquid crystal capacitor and the capacitance of the active circuit (which forms the active circuit) The voltage is divided between the liquid crystal capacitor and the active circuit according to the ratio (total capacitance of multiple thin film diodes), but since the capacitance compensation capacitor is connected in parallel to the liquid crystal capacitor, the actual voltage of the liquid crystal capacitor is The capacitance is the total capacitance of the liquid crystal capacitor itself and the capacitance of the capacitance compensation capacitor,
Therefore, since the actual capacitance of the liquid crystal capacitor is sufficiently larger than the capacitance of the active circuit, it is possible to reduce the voltage divided into the active circuit and hold a sufficient voltage in the liquid crystal capacitor.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 4 regarding an active matrix liquid crystal display device having a diode ring structure. Figure 1 shows one of the liquid crystal display devices.
2 and 3 are plan views of two pixel display sections, II of FIG.
FIG. 4 is an enlarged cross-sectional view taken along lines -II and III-III, and FIG. 4 is an equivalent circuit diagram of the liquid crystal display device.

The liquid crystal display device of this embodiment has a striped counter electrode 1 on one substrate (not shown) of a pair of transparent substrates (glass substrates, etc.) facing each other with a liquid crystal layer in between.
A large number of signal lines 12 are formed in parallel to each other on the other substrate 10, and a large number of signal lines 12 are arranged in parallel to each other and are perpendicular to the length direction of the counter electrode 11. In addition, each of the above-mentioned counter electrodes 11
A large number of pixel electrodes 13 facing each other are formed in an array,
Each of the signal lines 12 and both ends of each pixel electrode 13 along the signal line 12 are connected via active circuits A1 and A2 made of thin film diodes, and each corresponds to a part of each pixel electrode 13. In this case, a capacitance compensation capacitor Cs is formed.

Each of the active circuits A1 and A2 includes a plurality of (two in this embodiment) thin film diodes D1.
a, D1b and D2a, D2b are connected in series, and the direction of the thin film diodes D1a, D1b of one active circuit A1 connected to one end of the pixel electrode 13, and the other end of the pixel electrode 3. the other active circuit A connected to
The directions of the two thin film diodes D2a and D2b are opposite to each other.

The structure of the first active circuit A1 connected to one end of the pixel electrode 13 will be explained. As shown in FIG. Two thin film diodes D1a and D formed side by side
1b.

Both of the thin film diodes D1a and D1b have a P-I-N junction structure.
1a and D1b each have a lower conductive film 22 and a P-type semiconductor film 2 on a base electrode 20 formed on a substrate 10.
3p, an I-type semiconductor film 23i, an N-type semiconductor film 23n, and an upper conductive film 24 are formed in this order to form a semiconductor layer 21, and an upper electrode 25 is formed on this semiconductor layer 21. There is. Note that the upper electrode 25 is the same as the base electrode 20.
and an insulating film (silicon nitride film, etc.) covering the semiconductor layer 21
26 , and is connected to the upper conductive film 24 of the semiconductor layer 21 through a contact hole provided in the insulating film 26 .

Note that each semiconductor film 23 of the semiconductor layer 21
p, 23i, and 23n are made of amorphous silicon or polysilicon, and the lower conductive film 22 and the upper conductive film 24 are made of a metal such as chromium that has good contact with the semiconductor film.

Then, the diode D on the signal line 12 side
1a and the diode D1b on the pixel electrode 13 side, the upper electrode 25 of the pixel electrode side diode D1b is connected to the insulating film 2.
They are connected in series by being connected to the base electrode 20 of the signal line side diode D1a through a contact hole provided at 6, thereby forming a first active circuit A1.

One end of the first active circuit A1, that is, the upper electrode of the signal line side diode D1a is connected to the signal line 1.
2, and the other end, that is, the pixel electrode side diode D1b
The base electrode 20 of is connected to the pixel electrode 13. Note that the base electrodes 20 of both diodes D1a and D1b are
It is formed of the same transparent conductive film as the pixel electrode 13, the base electrode 20 of the signal line side diode D1a is an independent island-shaped electrode, and the base electrode 20 of the pixel electrode side diode D1b is a part of the pixel electrode 13. It is also used.

The signal line 12 is made of a metal film such as chromium on a lower film 12a made of the same transparent conductive film as the pixel electrode 13, or a metal film made by laminating a low resistance film such as aluminum on a chromium film. The upper electrode 25 of both diodes D1a and D1b is formed of the same metal film as the upper layer 12b of the signal line 12.

The second active circuit A2 connected to the other end of the pixel electrode 13 includes two thin film diodes D formed side by side on the substrate 10, as shown in FIG.
2a and D2b.

These thin film diodes D2a and D2b are also P-
It has an I-N junction structure, and is constructed by forming a semiconductor layer 21 on a base electrode 20 formed on a substrate 10, and forming an upper electrode 25 on this semiconductor layer 21. The electrode 25 includes the base electrode 20 and the semiconductor layer 21
The semiconductor layer 21 is connected to the upper conductive film 24 of the semiconductor layer 21 through a contact hole provided in the insulating film 26 . Note that the semiconductor layer 21 of the diodes D2a and D2b is similar to that of the diode D1 constituting the first active circuit A1 shown in FIG.
Since it has the same laminated structure as the semiconductor layer 21 of a and D1b, the explanation thereof will be omitted by giving the same reference numeral in the figure.

Then, the diode D on the signal line 12 side
2a and the diode D2b on the pixel electrode 13 side are connected in series by connecting the upper electrode 25 of the signal line side diode D2a to the base electrode 20 of the pixel electrode side diode D2b through a contact hole provided in the insulating film 26. and constitutes the second active circuit A2.

One end of this second active circuit A2, that is, the base electrode 20 of the signal line side diode D2a, is connected to the signal line 12, and the other end, that is, the upper electrode 25 of the pixel electrode side diode D2b is connected to the pixel electrode 13. Therefore, the diodes D2a and D2b of the second active circuit A2 are in the opposite direction to the diodes D1a and D1b of the first active circuit A1.

Note that the base electrodes 20 of both diodes D1a and D1b of this second active circuit A2 are also connected to the pixel electrode 1.
The base electrode 20 of the pixel electrode side diode D2b is an independent island-shaped electrode, and the base electrode 20 of the signal line side diode D2a is formed of the same transparent conductive film as in No. 3.
is connected to the lower layer film 12a of the signal line 12.

On the other hand, the capacitance compensation capacitor Cs is
As shown in FIGS. 1 and 2, by providing a capacitor electrode 27 corresponding to one side edge of the pixel electrode 13 and facing the pixel electrode 13 with an insulating film 26a in between,
It is formed by the capacitor electrode 27, the pixel electrode 13, and the insulating film 26a therebetween.

The capacitor electrode 27 of the capacitance compensation capacitor Cs is connected to a capacitor line 28 which is wired between the rows of the pixel electrodes 13 and perpendicular to the signal line 12, and both ends of each capacitor line 28 are connected to the side edge of the substrate 10. A ground line 29 wired parallel to the signal line 12 in the
connected to. Note that the intersection between the signal line 12 and the capacitor line 28 is insulated by an insulating film (not shown) provided between the lines 12 and 28.

This capacitance compensation capacitor Cs stores the charge of the image data signal applied to the pixel electrode 13 when either of the active circuits A1 and A2 becomes conductive. Since one electrode is a part of the pixel electrode 13, as in the equivalent circuit shown in FIG.
and the liquid crystal capacitor CL between them.
Connected in parallel with C.

Further, the capacitor electrode 27 and the insulating film 26a are formed in a horizontally long manner along one side edge of the pixel electrode 13 in order to increase the capacitance of the capacitance compensation capacitor Cs. That is, the capacitance of the capacitance compensation capacitor Cs is the same as the capacitance of the two electrodes 13, which face each other with the insulating film 26a in between.
The larger the opposing area of Cs 27 is, the larger the area is.
capacity can be increased.

Furthermore, in this embodiment, the insulating film 26a of the capacitance compensation capacitor Cs and the insulating film insulating the intersection of the signal line 12 and the capacitor line 28 are
Thin film diode D1 forming active circuits A1 and A2
The capacitor electrode 27, the capacitor line 28, and the ground line 29 are formed of the same insulating film (silicon nitride, etc.) as the insulating film 26 of a, D1b, D2a, and D2b.
, the above thin film diodes D1a, D1b, D2a, D2
b upper electrode 25 and upper layer film 12b of signal line 12
The capacitance compensation capacitor Cs can be manufactured using the manufacturing process of the active circuits A1 and A2.

That is, the liquid crystal display device of this example is as follows:
By forming the capacitance compensation capacitor Cs corresponding to a part of the pixel electrode 13, the capacitance of the liquid crystal capacitor CLC formed by the pixel electrode 13, the counter electrode 11, and the liquid crystal therebetween is compensated by the capacitance compensation capacitor Cs. This capacitance compensation capacitor Ds is
Since one of the electrodes is a part of the pixel electrode 13, the active circuits A1 and A2 are similar to the liquid crystal capacitor CLC described above.
The charge of the image data signal applied to the pixel electrode 13 at the time of selection when the pixel electrode 13 becomes conductive is accumulated.

Also in this liquid crystal display device, when the active circuits A1 and A2 are in the cutoff state (non-selected state), the voltage of the liquid crystal capacitor CLC is equal to the capacitance of the liquid crystal capacitor CLC and the active circuits A1 and A2. The liquid crystal capacitor CLC and the active circuit A1 are adjusted according to the ratio of the capacitance of A2 (the two thin film diodes D1a, D1b and the total capacitance of D1a, D1b constituting the active circuits A1, A2).
, A2, but since the capacitance compensation capacitor Cs is connected in parallel to the liquid crystal capacitor CLC, the actual capacitance of the liquid crystal capacitor CLC is the capacitance of the liquid crystal capacitor CLC itself and the capacitance compensation capacitance described above. is the total capacitance with the capacitance of capacitor Cs, and therefore,
The actual capacitance of the liquid crystal capacitor CLC is the active circuit A1, A
2, the active circuits A1 and A2
By reducing the voltage divided into liquid crystal capacitor CL
C can hold sufficient voltage.

Note that the actual capacitance of the liquid crystal capacitor CLC (
The total capacitance of the liquid crystal capacitor CLC itself and the capacitance of the capacitance compensation capacitor Cs is the active circuit A1, A2.
If the capacitance of the active circuits A1 and A2 is 1/10 or less of the actual capacitance of the liquid crystal capacitor CLC, the voltage divided into the active circuits A1 and A2 can be almost ignored.

Therefore, according to this liquid crystal display device, the voltage of the image data signal applied between the pixel electrode 13 and the counter electrode 11 at the time of selection is applied to the non-selection state in which the active circuits A1 and A2 are cut off. Therefore, a sufficient electric field can be applied to the liquid crystal between the pixel electrode 13 and the counter electrode 11 even during the non-selection period, and a good image without "flickering" etc. can be displayed. can do.

Note that in the above embodiment, the active circuits A1,
The thin film diodes D1a, D1b and D1a, D1b constituting A2 each have a P-I-N junction structure, but this thin film diode has a N-I-N junction structure in which the semiconductor layer 21 has a stacked structure opposite to that of the above embodiment. It may be a P-junction structure, or it may be a P-N junction structure or an N-P junction structure in which the I-type semiconductor film 23i is removed from the semiconductor layer 21 and the P-type semiconductor film 23p and the N-type semiconductor film 23n are directly laminated. Good too.

Further, the liquid crystal display device of the above embodiment has a diode ring structure, but in the present invention, the signal line and the pixel electrode are connected to one active active layer in which two thin film diodes are connected in series in opposite directions. Of course, the present invention can also be applied to active matrix liquid crystal display devices having a back-to-back structure connected by circuits.

[0049]

Effects of the Invention In the active matrix liquid crystal display device of the present invention, a capacitor electrode is provided corresponding to a part of the pixel electrode and facing the pixel electrode via an insulating film, and the capacitor electrode, the pixel electrode and The insulating film between them forms a capacitance compensation capacitor that stores the charge of the signal applied to the pixel electrode when the active circuit becomes conductive, so the active circuit is composed of thin film diodes. However, even in the non-selection state when the active circuit is cut off, a sufficient electric field can be applied to the liquid crystal between the pixel electrode and the counter electrode, and a good image without "flickering" etc. can be displayed. .

[Brief explanation of the drawing]

FIG. 1 is a plan view of one pixel display section of a liquid crystal display device showing one embodiment of the present invention.

FIG. 2 is an enlarged sectional view taken along line II-II in FIG. 1;

FIG. 3 is an enlarged sectional view taken along line III-III in FIG. 1;

FIG. 4 is an equivalent circuit of a liquid crystal display device.

FIG. 5 is a circuit diagram of a conventional liquid crystal display device having a die ring structure.

FIG. 6 is a circuit diagram of a conventional back-to-back liquid crystal display device.

[Explanation of symbols]

11... Counter electrode, 12... Signal line, 13... Pixel electrode,
A1, A2...active circuit, D1a, D1b, D2a,
D2b... Thin film diode, Cs... Capacity compensation capacitor, 26a... Insulating film, 27... Capacitor electrode.

Claims (1)

[Claims] 1. A plurality of striped counter electrodes are formed parallel to each other on one of a pair of transparent substrates facing each other with a liquid crystal layer in between, and the other substrate has stripe-shaped counter electrodes formed parallel to each other in the length direction of the counter electrodes. A large number of orthogonal signal lines are wired parallel to each other, and along each signal line,
An active matrix liquid crystal display device in which a large number of pixel electrodes facing each of the counter electrodes are formed in an array, and each of the signal lines and each pixel electrode along the signal line are connected via an active circuit made of a thin film diode. A capacitor electrode is provided corresponding to a part of the pixel electrode and facing the pixel electrode via an insulating film, and the active circuit is electrically connected by the capacitor electrode, the pixel electrode, and the insulating film therebetween. 1. An active matrix liquid crystal display device, characterized in that a capacitance compensation capacitor is formed to accumulate charge of a signal applied to the pixel electrode when the pixel electrode is in the active matrix liquid crystal display state.