JPH0210955B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a substrate provided with a thin film transistor array used as a driving switching element.

This type of display device includes, for example, a display electrode substrate having a driving transistor array formed using thin film technology, a counter electrode substrate disposed with a small gap from the display electrode substrate, and a counter electrode substrate placed between these electrode substrates. It is composed of a display means that converts an electrical change into an optical change in an interposed liquid crystal, etc., and the devices described in the specifications of U.S. Patent Nos. 3,824,003 and 3,840,695 are conventionally known. . Hereinafter, a specific structure of a conventional device of this type will be described using a display device described in US Pat. No. 3,824,003 as an example.

Figure 1 is a diagram showing the structure of the display electrode substrate used in the above device.The display electrode substrate includes a glass substrate S and a matrix formed on the glass substrate S at a density of about 2 to 10 electrodes/mm. It consists of a thin film transistor (TFT) array. That is, the thin film transistor array includes a plurality of gate lines 1a, 1a', . . . formed in parallel on a substrate S at predetermined intervals, and
A thin film semiconductor SC formed on these gate lines 1a, 1a', . . . with an insulating film I interposed therebetween, and this semiconductor SC.
A plurality of source lines 3, 3', ... and a plurality of rectangular drain electrodes 4, 4', formed thereon.
It is composed of 4″, 4, ….
The source lines 3, 3', . . . are the gate lines 1a,
1a',... are arranged in parallel in the direction orthogonal to
As shown in the figure, the drain electrodes 4, 4', 4'', 4, . . .
They are arranged so as to face the gate lines 1a, 1a', . That is, the semiconductor SC has the source line 3
A field effect transistor _T11 is connected to the drain electrode 4 and the gate line 1a, and a field effect transistor T11 is connected to the source line 3', the drain electrode 4' and the gate line 1a.
A field effect transistor T ₂₁ is formed by the source line 3, the drain electrode 4'', and the gate line _1a ', and a field effect transistor _T22 is formed by the source line 3', the drain electrode 4'', and the gate line 1a'. Note that since thin film transistors are well known, their explanation will be omitted.

Each drain electrode of the thin film transistor array thus formed is used as a display electrode. That is, the display electrode substrate has a second
As shown in the figure, a display device is constructed by sandwiching a liquid crystal between a counter electrode substrate and a counter electrode substrate.

FIG. 2 shows a cross section of a display device constructed using the display electrode substrate shown in FIG. 1, taken along line AA' in FIG. In the diagram,
S is the above glass substrate, 1a is a gate line, I is an insulating film, SC is a semiconductor, 3, 3' are source lines, 4, 4'
is the drain electrode. The display electrode substrate is made of In ₂ O ₃ ,
Glass substrate 7 on which a transparent conductive layer 8 such as SnO ₂ is formed
They are placed opposite each other with a small gap between them, and the small gap is filled with liquid crystal 11. That is, the glass substrate 7 and the transparent conductive layer 8 constitute a counter electrode substrate, and the liquid crystal 11 is sandwiched between the display electrode substrate and the common electrode substrate. Note that an insulating layer 9 is formed between the transparent conductive layer 8 and the liquid crystal 11 as necessary. Further, a protective layer P made of a mixture of calcium fluoride and silicon oxide, quartz, or the like is formed on the surface of the semiconductor SC.

The above-mentioned display device uses liquid crystal as a display means and constitutes a reflective display device. For this reason, the drain electrodes 4, 4', 4'', 4,
... is a metal with good reflectivity, such as Al, so that it acts as a reflector. however,
It is also possible to construct a transmissive display device using a display electrode substrate on which a thin film transistor array is formed, and such a device is described in the above-mentioned US Pat. No. 3,840,695. Depending on whether the display device is configured as a reflective or transmissive type, or whether the liquid crystal is used in a display mode such as dynamic scattering mode (DSM) or twisted nematics (TN), there are various The alignment state of liquid crystal molecules and optical detection means such as a polarizing plate, a λ/4 plate, and a reflecting plate are appropriately set.

In order to perform display using the above display device, for example, image signals are scanned and applied to the gate lines 1a, 1a', . . . and driving voltages are scanned and applied to the source lines 3, 3', . be able to. That is,
When a signal is input to the gate line 1a, the source 3, 3'-drain 4,
4' becomes conductive, and an electric field is generated between the drain electrodes 4, 4', which are display electrodes, and the transparent conductive layer 8, which is a counter electrode, and the arrangement state of liquid crystal molecules of the liquid crystal 11 changes. Therefore, electrical changes are displayed as optical changes.

As described above, the display device can be driven using a line sequential method in which driving voltages are scanned and sequentially supplied. In such a driving method, if the time for displaying one screen (frame time) is constant, the addressing time for each picture element becomes shorter as the number of gate lines increases. Therefore, if you try to improve the screen resolution by increasing the number of gate lines,
A situation arises in which the reaction time of a display means such as a liquid crystal cannot follow the scanning time, and normal display cannot be performed. To prevent this situation, a capacitor is connected in parallel to each pixel, and drive signal charge is accumulated in this capacitor during the addressing period. A display method is being considered in which a driving voltage is continuously applied to the display. In this sense, the device shown in FIGS. 1 and 2 above has a gate line 1 as shown in FIG.
a, 1a', . . . themselves act as counter electrodes of the capacitors of the drain electrodes 4, 4', 4'', 4, . . . which are display picture elements. In other words, FIG. This figure shows an equivalent circuit of the shown device, and in the figure, T ₁₁ , T ₁₂ , T ₂₁ , T ₂₂ , ... indicate each transistor configured by the above-mentioned thin film structure. LC ₁₁ , LC ₁₂ , LC ₂₁ , LC ₂₂ , ... are held between the drain electrodes 4, 4', 4'', 4, ... of the transistors T ₁₁ , T ₁₂ , T ₂₁ , T ₂₂ , ... and the counter electrode 8, respectively. The display medium is a liquid crystal 11 in this case. C ₁₁ , C ₁₂ , C ₂₁ , C ₂₂ ,... are gate lines 1a, 1a',... and drain electrodes 4, 4',
4'', 4, . . . . In this way, the gate lines 1a, 1a', . . .
By configuring the capacitors to act as counter electrodes for the capacitors 4', 4'', 4, . When 1a',... itself is used as the counter electrode of a capacitor, a bias equal to the gate signal voltage for addressing is applied to the opposite electrode of the capacitor.Therefore, as a write signal, this gate voltage is applied to the opposite electrode of the capacitor. Moreover, when the gate voltage is removed after this and the transistor enters a high resistance state, it will show a value different from the drain voltage applied at the time of writing, resulting in complicated operation. This is inconvenient when trying to drive a display device with accurate voltage control, and it becomes difficult to control, especially when displaying gradations corresponding to voltage.

On the other hand, a display device similar to the one described above
IEEE Trans.on Electron Devices ED―20,
P.995 (1973), which shows a structure in which the counter electrode for a storage capacitor is connected to an adjacent gate line.

FIG. 4 is a plan view showing the structure of the display electrode substrate used in the display device, and FIG. 5 shows an equivalent circuit of the display device constructed using the display electrode substrate shown in FIG. There is. As shown in FIG. 4, this device forms gate lines 1a and 1a' on a substrate (not shown), and also forms gate lines 1a and 1a' on a substrate (not shown).
A conductive film E is formed between a and 1a'. Now, focusing on the transistor operating on the gate line 1a', the conductive film E is formed so that most of it is located below the drain electrode surface of this transistor, and the conductive film E is formed so that most of it is located below the drain electrode surface of this transistor. It is electrically connected to a part of the line 1a. Then, an insulating layer (not shown) is provided on the gate lines 1a, 1a' and the conductive film E, and semiconductors are placed on the insulating layer at predetermined positions above the gate lines 1a, 1a'.
Form SC. And as shown in the figure,
A source line 3 is formed at one end of this semiconductor SC, a drain electrode 4 is formed at the other end, and a thin film transistor is formed.
Configure T ₁ . In addition, for convenience of explanation, FIG.
Although the figure shows a case in which one transistor is formed, it goes without saying that in practical use, a plurality of transistors are formed in an array like the display electrode substrate shown in FIG. Furthermore, since the device shown in FIG. 4 is intended to constitute a transmissive display device, the semiconductor SC is formed separately according to each transistor, and the semiconductor SC is located below the drain electrode 4. However, there is no essential difference in the basic operation of the transistor array from the device shown in FIG.

When a display device is constructed using a display electrode substrate constructed in this manner, a capacitor _C1 formed by a conductive film E facing the drain electrode 4 with an insulating layer interposed therebetween is formed as shown in FIG. The gate line 1a' of the transistor _T1 of interest is connected to the adjacent gate line 1a. Therefore, when driving the transistor _T1 , if a driving method is adopted in which a gate voltage is applied only to the gate line 1a' and the other gate lines 1a are grounded, the signal on the source line 3 is now applied to ground potential. Therefore, it is easier to apply a clear operating voltage to the display medium than in the devices shown in FIGS. 1 and 2 described above. However, even in this case, when a signal is supplied to the gate line 1a,
The potential of the drain electrode 4 changes depending on the voltage state of the gate line 1a, which may affect the display.

This invention was made in view of the above-mentioned circumstances, and its purpose is to easily control the operating voltage accurately and to provide stable display without being affected by the voltage state of the gate line. An object of the present invention is to provide a display device configured to

This invention provides a display device using a thin film transistor array as a driving switching element, in which a conductive surface that is insulated and separated from the gate line is formed on a substrate forming the gate line of the thin film transistor array, and this conductive surface is used for charge storage. It is characterized by being used as a counter electrode for a capacitor for use.

Embodiments of the present invention will be described in detail below with reference to the drawings. 6 and 7 are drawings showing the structure of a display electrode substrate used in a display device according to an embodiment of the present invention, and the components having the same functions as those in FIGS. 1 to 5 described above include , using the same symbols. That is, S is an insulating substrate made of a transparent material such as glass, and on this substrate S, a plurality of gate lines 1a, 1a', 1a'', . . . are formed in parallel at predetermined intervals. On the substrate S, conductive films 1b, 1b', . . . are formed in stripes with predetermined insulation intervals 1C, 1C', 1C", 1C, . . . between the gate lines 1a, 1a', 1a", . The gate lines 1a and 1a are formed parallel to each other.
An insulating layer I is formed on the conductive films 1a', 1a'', . . . and conductive films 1b, 1b', .
SC is formed. This semiconductor SC forms a thin film transistor array constituting a drive switching element, and is connected to the gate lines 1a, 1a', 1
Source lines 3, 3', . ... are formed in ohmic contact with each other, and these source lines 3, 3'... are connected to the semiconductor SCs arranged in the column direction.
It has become common. That is, source line 3,
3',... are formed parallel to the direction perpendicular to the gate lines 1a, 1a', 1a'',..., and the source lines 3, 3',... and the gate lines 1a, 1a', 1a'',...
The semiconductor SC is arranged at the intersection with the SC. Rectangular drain electrodes 4, 4', 4'', . . . are formed in the areas surrounded by the source lines 3, 3', . . . and the gate lines 1a, 1a', 1a'', . The drain electrodes 4, 4', 4'',...
A part of it is extended and is in ohmic contact with the other end (the right end in FIG. 6) of the semiconductor SC. The drain electrodes 4, 4', 4'', . . . are the drain parts of the thin film transistor constituted by the semiconductor SC, and are also used as display electrodes. When configuring the display device, it is configured as a transparent conductive film such as In ₂ O ₂ or SnO ₂ , and when configuring a reflective display device, it is configured as a metal thin film such as Au, Al, Pd, etc.

In the display electrode substrate configured as described above, the conductive films 1b, 1b', . , 4', 4'', . . . to form a capacitor for accumulating drive charges. Therefore, an equivalent circuit of a display device constructed by using this display electrode substrate and sandwiching a liquid crystal between opposing electrode substrates is shown in FIG. In the figure, T ₁ is a field effect transistor formed at the intersection of the gate line 1 a and the source line 3 . A display medium is connected between the drain electrode 4 of this transistor _T1 and the grounded counter electrode 8.
LC ₁ (liquid crystal in this case) is held in place. C ₁ is a capacitor formed by the drain electrode 4 and the conductive film 1b. As is clear from the figure, this capacitor _C1 has a configuration independent of the gate lines 1a and 1a'.

That is, in the above display device, the conductive films 1b, 1b', .
a, 1a', and 1a can be operated completely electrically independently. Therefore, unlike conventional devices, the potential of the capacitor counter electrode can be set without considering the gate line signal. The simplest example of setting the potential of the capacitor counter electrode is when the capacitor counter electrode is set to the ground potential. In this case, the terminal of the conductive film 1b shown in FIG.
V _B can be connected to the ground potential at the bottom. In this way, the charge accumulated in the capacitor _C1 will not be adversely affected by the voltage state of the gate lines 1a and 1a', making it easy to accurately control the operating voltage. As a result, stable display can always be performed regardless of the voltage state of the gate lines 1a and 1a'.

Another example of setting the potential of the capacitor counter electrode is when a desired bias voltage is supplied to the terminal V _B. In this case as well, the charge accumulated in the capacitor _C1 is not adversely affected by the voltage state of the gate lines 1a and 1a'.
Needless to say, the same effect as in the case where the ground potential is set as described above can be obtained. Furthermore, in this case, if an electro-optical element with a constant threshold characteristic, such as a liquid crystal, is used, the voltage of the write input signal can be adjusted by applying a bias voltage slightly lower than the threshold voltage. It becomes possible to set the bias voltage lower by this bias voltage. Further, if the bias voltage is configured to be arbitrarily changed by external operation, the voltage operating point of the electro-optical characteristics of the liquid crystal can be changed for the same write input signal. This means that brightness can be adjusted in areas where the operating point changes linearly, and contrast characteristics can be adjusted in areas where the operating point changes non-linearly. moreover,
When using a display means such as a liquid crystal whose electro-optical characteristics change according to temperature changes, if a temperature compensation voltage is supplied as the bias voltage, it will always be able to withstand temperature changes. Stable display becomes possible.

As explained above, the display device according to the present invention can obtain extremely useful effects in terms of circuit operation, but on the other hand, due to its structure, it is more difficult to manufacture than conventional display devices of this type. It also has the advantage of not adding any complexity. This is because the striped capacitor counter electrodes 1b, 1
Since b',... are formed on the same insulating substrate S as the gate lines 1a, 1a', 1a'',..., the manufacturing process of the gate lines 1a, 1a', 1a'',... This is because they can be formed simultaneously. For example, to explain one example of the manufacturing process, first, a conductive film is formed over the entire display area on an insulating substrate S such as glass. For this conductive film, a metal film such as Al is used when configuring a reflective display device, and a transparent conductive film such as In ₂ O ₂ is used when configuring a transmissive display device. . Thereafter, using techniques such as photolithography, the conductive film is separated into predetermined gaps 1C, 1C', 1C'', which are necessary for insulation separation.
1C, . . . should be removed. The subsequent steps are the same as the conventional manufacturing steps. That is, an insulating layer I of SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ etc. is formed on the entire surface of these surfaces.
A semiconductor SC is formed at a predetermined position on this insulating layer. Source lines 3, 3', . . . are provided at one end of the semiconductor SC, and portions of drain electrodes 4, 4', 4'', . ,... and the drain electrodes 4, 4', 4'',... are made of Al, like the above-mentioned conductive film, when configuring a reflective display device.
A metal film such as the following is used. Further, as the semiconductor SC, CdS, CdSe, Te, amorphous silicon, etc. are used, and an inert film is appropriately formed on the semiconductor SC as necessary. Then, a display device is constructed by sandwiching the liquid crystal between the display electrode substrate constructed in this way and a counter electrode substrate via a spacer, and shielding the periphery.

In the above-described embodiment device, the striped conductive film 1 used as the counter electrode of the capacitor
b, 1b' were in common contact with each drain electrode 4 or 4', 4'' of the transistor array arranged in the row direction, but each conductive film 1b, 1
b' were not electrically connected to each other. Therefore, when setting the voltage of the opposite electrode of the capacitor,
It was necessary to electrically connect the conductive films 1b, 1b' to each other outside the display device. However, the present invention is not limited to the device of the above embodiment, and as shown in FIG.
A configuration in which a common connection part b is formed in a direction orthogonal to b' and the conductive films 1b and 1b' are commonly connected is also considered. With this configuration, there is no need to connect the conductive films 1b, 1b' in common outside the display device, and it is possible to reduce the number of external circuit connection terminals formed in the display device.

Further, although the device of the above embodiment is a case in which the semiconductor SC is provided separately for each transistor, it is also possible to construct the device using an integrated semiconductor as in the device shown in FIG. However, in this case, since a semiconductor layer is interposed between the drain electrode and the capacitor counter electrode, it is necessary to fully consider the influence on the capacitor.

Furthermore, in the above-mentioned embodiment device, a case has been described in which liquid crystal is used as the electro-optical changing means. Currently, liquid crystal is the most common display medium, but it is not limited to this. For example, a similar display device can be constructed using EC (electrochromic). At present, usable display media are limited by the characteristics of thin film transistors, but it is expected that the number of usable display media will increase as the performance of thin film transistors improves. In short,
Any display medium may be used as long as it converts electrical changes occurring between electrodes into optical changes.

As explained above, the display device according to the present invention uses a thin film transistor array as a driving switching element, and has a conductive surface on a substrate forming the gate line of the thin film transistor array, which is insulated and separated from the gate line. form,
This conductive surface is used as a counter electrode of a capacitor for charge storage. Therefore, according to the display device of the present invention, since the capacitor counter electrode is formed separately from the gate line, the write drive voltage can be set without considering the influence of potential changes on the gate line. Therefore, the operating voltage can be easily controlled accurately. Further, stable display can be performed without changing the display state due to changes in the potential of the gate line. Furthermore, since the capacitor counter electrode can be set to any potential, by appropriately setting and adjusting the potential of the capacitor counter electrode, it is possible to adjust the write drive voltage value, brightness adjustment, contrast adjustment, etc. as a display device. be. Furthermore, although the display device of the present invention provides extremely beneficial effects in terms of circuit operation as described above, its structure is extremely simple and can be constructed without increasing the manufacturing process of conventional devices of this type. can do.

[Brief explanation of drawings]

1 to 5 are diagrams for explaining a conventional display device. FIG. 1 is a partially vertical perspective view of a display electrode substrate in a conventional display device, and FIG. 2 is a partial vertical perspective view of a display electrode substrate in FIG. 3 is a cross-sectional view of the display device used, FIG. 3 is its equivalent circuit diagram, FIG. 4 is a plan view showing another example of a conventional display electrode substrate, and FIG. Equivalent circuit diagrams of the display device, FIGS. 6 to 8 are diagrams for explaining a display device according to an embodiment of the present invention, and FIG. 6 shows a diagram of a display electrode substrate used in an embodiment of the device of the present invention. Partial vertical perspective view, No. 7
The figure is a plan view thereof, and FIG. 8 is an equivalent circuit diagram of a display device using the display electrode substrate of FIGS. 6 and 7.
FIG. 9 is a plan view showing another example of the invention. In the figure, S is a substrate, 1a, 1a', 1a'' are gate lines, 1b, 1b' are conductive films (capacitor counter electrodes), 3,
3' is a source line, 4, 4', 4'', 4 is a drain electrode, I is an insulating layer, SC is a semiconductor, T ₁ , T ₁₁ ,
T ₁₂ , T ₂₁ , and T ₂₂ indicate transistors.

Claims (1)

[Scope of Claims] 1 a. A transistor portion formed of a gate, a source, a drain, and a semiconductor layer; a charge storage device formed of the drain, a transparent conductive film, and an insulating film disposed between the drain and the transparent conductive film; a thin film transistor substrate having a capacitor portion, and the gate and the transparent conductive film are insulated from each other; b. a counter substrate disposed to face the thin film transistor substrate and having a counter electrode provided to face the drain of the thin film transistor substrate; , c) A liquid crystal device having means for applying a bias voltage lower than a threshold voltage of the liquid crystal to a liquid crystal disposed between a thin film transistor substrate and a counter substrate, and d) to a transparent conductive film of a charge storage capacitor section.