JPH04223446A - Active matrix liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide the active matrix liquid crystal display device of a back-to- back structure which is reduced in cost by reducing the production cost of the thin-film diodes constituting the active circuit thereof. CONSTITUTION:Two pieces of the thin-film diodes D11, D12 constituting the active circuit B are made into the structure interposed with a semiconductor layer 23 formed by laminating a P type semiconductor film 23p and an N type semiconductor film 23n by holding an I type semiconductor 23i between a base electrode 21 and an upper electrode 22 in place of the conventional Schottky effect type, by which the formation of the electrodes 21, 22 of the thin-film diodes D11, D12 of the inexpensive material having good workability is enabled.

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.

FIGS. 3 and 4 are circuit diagrams of a portion of the active matrix liquid crystal display device, with FIG. 3 showing a diode ring structure and FIG. 4 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. 3, a liquid crystal display device with a diode ring structure has active circuits A1 and A2 each having a plurality of (two in the figure) diodes connected in series at both ends of the pixel electrode 3. The diode of one active circuit A1 connected to one end of the pixel electrode 3 and the pixel electrode 3
The diodes of the active circuit A2 connected to the other end are in opposite directions.

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 threshold voltage 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, so the data signal At the moment when is applied, the voltage of the data signal +
Vc is applied across 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 decreases as charge is accumulated in the liquid crystal capacitor, and when this voltage falls below a threshold, the active circuit A1 is cut off.

[0012] Also, active circuits A1 and A are connected to the signal line 2.
When a data signal with a negative voltage (-Vc) higher than the threshold value of 2 is applied, at this time, the active circuits A1 and A
2 is applied with a voltage of opposite polarity to that when the positive voltage (+Vc) data signal is applied. In this case as well, the voltage -Vc of the data signal is the active circuit A1.
, A2, the current flows from the pixel electrode 3 to the signal line 2 through the active circuit (the active circuit connected to the lower end of the pixel electrode 3 in the figure) consisting of a diode in the opposite direction. flows.

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, the active circuit A2 is cut off. Turn off.

On the other hand, in a back-to-back liquid crystal display device, as shown in FIG. 4, the signal line 2 and the pixel electrode 3 are connected to 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.

By the way, when comparing the above-mentioned diode ring structure and back-to-back structure, in the diode ring structure shown in FIG. Moreover, in order to increase the threshold voltage of each active circuit A1, A2, each active circuit A1, A2 is constructed by connecting a plurality of thin film diodes in series, so that the active circuit The area occupied by the pixel electrode 3 is large, and the area of the pixel electrode 3 is correspondingly small, resulting in a poor aperture ratio of the liquid crystal display device.

On the other hand, in the back-to-back structure shown in FIG. 4, there is only one active circuit B that connects the pixel electrode 3 to the signal line 2, and the number of diodes is only two. Therefore, since the area occupied by the active circuit is small, the area of the pixel electrode 3 can be increased to increase the aperture ratio. Furthermore, since the active circuit B is made up of two thin film diodes connected in series in opposite directions, the threshold voltage is high and the I (current)-V (voltage) characteristics are also sharp. Therefore, the display operation characteristics are also good.

Therefore, as an active matrix liquid crystal display device using thin film diodes as active elements, a back-to-back structure is better than a diode ring structure.

Conventionally, active matrix liquid crystal display devices with this back-to-back structure are known in which the active circuit is constructed of Schottky effect type thin film diodes.

FIG. 5 is a sectional view of an active circuit portion of a conventional back-to-back liquid crystal display device. In FIG. 5, 10 is a transparent substrate (glass substrate) of a liquid crystal display device;
2 and 3 are a signal line and a pixel electrode formed on this substrate 10, and an active circuit B that connects this signal line 2 and pixel electrode 3 includes two Schottky effect thin film diodes D1 and D2. It is constructed by lining up and forming.

The Schottky effect type thin film diode D
1 and D2, a semiconductor layer 13 consisting of an I-type semiconductor film 13i and an N-type semiconductor film 13n laminated thereon is formed on the lower electrodes 11 and 12, and an upper electrode is formed on this semiconductor layer 13. 14 and the semiconductor layer 13
A Schottky effect is created between the N-type semiconductor film 13n and the upper electrode 14, and the upper electrode 14 is made of platinum in order to obtain a good Schottky effect.

Further, the upper electrode 14 has both diodes D1
, D2, and both diodes D1
, D2 are connected in series with opposite polarities by the upper electrode 14 to form an active circuit B. One end of this active circuit B, that is, the lower electrode 11 of one diode D1 is connected to the signal line 2,
The other end of the active circuit B, that is, the lower electrode 12 of the other diode D2 is connected to the pixel electrode 3.

Note that the signal line 2 and pixel electrode 3 are
Both are made of a transparent conductive film such as ITO, and the lower electrode 11 of one diode D1 is formed from a part of the signal line 2, and the lower electrode 12 of the other diode D2 is formed of a part of the signal line 2.
is formed of a part of the pixel electrode 3. Although FIG. 5 shows that the semiconductor layers 13 of both diodes D1 and D2 are continuous, conventional liquid crystal display devices include
In some cases, the semiconductor layer 13 is separated between both diodes D1 and D2.

[0024]

[Problems to be Solved by the Invention] However, in the above-mentioned conventional liquid crystal display device in which the active circuit B is constituted by the Schottky effect type thin film diodes D1 and D2, the upper electrodes 14 of the above-mentioned diodes D1 and D2 cannot be properly connected. It must be made of platinum, which produces a Schottky effect.
This platinum is very expensive and has poor processability (etchability), so the manufacturing cost of the diodes D1 and D2 increases, resulting in an increase in the price of the liquid crystal display device.

An object of the present invention is to provide an active matrix liquid crystal display device with a back-to-back structure, which can be manufactured at a low cost by reducing the manufacturing cost of thin film diodes constituting its active circuit. It is in.

[0026]

[Means for Solving the Problems] The present invention provides an active matrix liquid crystal with a back-to-back structure in which a signal line and a pixel electrode are connected through an active circuit in which two thin film diodes are connected in series in opposite directions. In the display device, two thin film diodes constituting the active circuit,
The structure is characterized in that a semiconductor layer is interposed between a pair of upper and lower electrodes, in which a P-type semiconductor film and an N-type semiconductor film are laminated either directly or with an I-type semiconductor film sandwiched therebetween.

[0027]

[Operation] That is, the present invention replaces the conventional Schottky effect type thin film diode constituting the active circuit with P
A P-N (or N-P) junction or a P-I-N (or N-I-P) junction structure in which a type semiconductor film and an N-type semiconductor film are stacked directly or with an I-type semiconductor film sandwiched therebetween. The thin film diode with this junction structure does not utilize the Schottky effect between the N-type semiconductor film and the electrode like the Schottky effect type thin film diode, so the upper and lower electrodes of this thin film diode can be formed from a material that is inexpensive and has good workability.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 2 is a plan view of a portion of the liquid crystal display device of this embodiment, and FIG. 1 is an enlarged sectional view taken along line II in FIG. 2.

In FIGS. 1 and 2, 1 is a striped counter electrode (transparent electrode) formed on one of a pair of transparent substrates facing each other with a liquid crystal layer in between, and 2 is the other substrate 10.
3 is a pixel electrode arranged along the signal line 2, B is an active circuit connecting the signal line 2 and the pixel electrode 3, and this active circuit B consists of two It is constructed by forming thin film diodes D11 and D12 side by side.

The thin film diodes D11 and D12 each include a P-type semiconductor film 23p and an N-type semiconductor film 23n between a base electrode 21 formed on a substrate 10 and an upper electrode 22, and an I-type semiconductor film 23i. In this embodiment, the thin film diodes D11 and D12 are P-I-N, with the lower layer of the semiconductor layer 23 being P-type and the upper layer being N-type. It has a bonded structure.

Reference numeral 24 denotes an insulating film made of silicon nitride or the like that covers the base electrode 20 and the semiconductor layer 23 thereon. It is in contact with the upper surface of the semiconductor layer 23 (the upper surface of the N-type semiconductor film 23n).

The base electrode 20 has both diodes D1
This base electrode 20 is formed independently in the form of an island, and is shared by both diodes D1 and D12.
Semiconductor layers 23 11 and D12 are formed on both ends of this base electrode 20 . Also, both diodes D1
The upper electrodes 22 of 1 and D12 are connected to both diodes D11 and D12.
It is separated between 12.

[0033] Both the diodes D11 and D12
are connected via the base electrode 20 to a state where the polarities are opposite to each other (the P-type semiconductor films 2 of both diodes D11 and D12
3p and 23p are connected in series (with diodes D11 and D12 facing each other), and one end of the active circuit B composed of both diodes D11 and D12, that is, the upper electrode 22 of one diode D11, is connected to the signal line 2, and the active circuit B is connected to the signal line 2. The other end of the circuit B, that is, the upper electrode 22 of the other diode D12 is connected to the pixel electrode 3.

The base electrode 20 is formed of a transparent conductive film such as ITO, which is the same as the pixel electrode 3, and both diodes D
Each semiconductor film 23p, 23 of the semiconductor layer 23 of 11, D12
i and 23n are made of amorphous silicon. Moreover, the upper electrodes 21 and 2 of both diodes D11 and D12
2 are each made of chromium, which has good contact with the semiconductor layer 23, or a metal film in which aluminum is laminated on chromium.

Further, the signal line 2 is formed by forming the upper electrode 21 on the same transparent conductive film 2a made of ITO or the like as the pixel electrode 3.
, 22 has a structure in which the line resistance is lowered by laminating the same metal films as the signal line 2, and the upper metal film of the signal line 2 and the upper electrode 21 of the thin film diode D11 on the signal line 2 side are integrated. There is.

That is, the liquid crystal display device of this example has the following characteristics:
The thin film diodes D11 and D12 constituting the active circuit B are replaced with the conventional Schottky effect type, and are replaced with P-type semiconductor films 2.
3p and an N-type semiconductor film 23n are laminated with an I-type semiconductor film 23i sandwiched therebetween, which has a P-I-N junction structure.
Thin film diodes D11 and D1 with this P-I-N junction structure
2 is N like a Schottky effect type thin film diode.
The thin film diodes D11 and D12 do not utilize the Schottky effect between the type semiconductor film and the electrode.
The upper and lower electrodes, that is, the base electrode 21 and the upper electrode 22, are made of an inexpensive and easily processable material (ITO in this embodiment).
It can be formed using a transparent conductive film such as chromium or a metal film in which aluminum is laminated on chromium.

Therefore, according to this liquid crystal display device,
The manufacturing cost of the thin film diodes D11 and D12 constituting the active circuit B can be reduced, and the price of the display device can be reduced.

In the above embodiment, the base electrode 21 of the two thin film diodes D11 and D12 constituting the active circuit B is used as a common electrode, and the upper electrode 22 is used as the common electrode of the two thin film diodes D11 and D12.
1 and D12, but on the contrary,
The upper electrode is used as a common electrode that also serves as the connection wiring for both diodes, the base electrode is separated between both diodes, the base electrode of one diode is connected to the signal line, and the base electrode of the other diode is connected to the pixel electrode. It's okay.

Furthermore, in the above embodiment, both diodes D1
1, each semiconductor film 23p, 23i of the semiconductor layer 23 of D12
, 23n are made of amorphous silicon,
All or at least one of these semiconductor films 23p, 23i, and 23n may be formed of polysilicon.If each semiconductor film 23p, 23i, and 23n is formed of polysilicon, the diodes D11 and D12 It is possible to lower the resistance value and allow more current to flow.

Furthermore, the diodes D11 and D12 are not limited to the P-I-N junction structure, but also have an N-I-P structure in which the semiconductor films 23p, 23i, and 23n of the semiconductor layer 23 are laminated in the opposite manner to the above embodiment. It may be a junction structure or a P-N (or N-P) junction structure in which the I-type semiconductor film 23i is eliminated and the P-type semiconductor film 23p and the N-type semiconductor film 23n are directly joined. Note that the P-N (or N-P) junction structure makes it difficult to form a junction compared to the P-I-N (or N-I-P) junction structure, but it eliminates the need for the process of forming an I-type semiconductor film. Therefore, the number of manufacturing steps for thin film diodes can be reduced.

Furthermore, in the above embodiment, both diodes D1
Although the semiconductor layers 23 of 1 and D12 have the same junction structure, the semiconductor layers of both diodes may have junction structures that are opposite to each other. In that case, the base electrode and upper electrode of both diodes are separated, and one The base electrode of one diode may be connected to the upper electrode of the other diode, and both diodes may be connected in series in opposite directions.

Furthermore, in the above embodiment, both diodes D
The semiconductor layer 23 of 11 and D12 is connected to each diode D11 and D12.
Both diodes D11,
When the semiconductor layer 23 of D12 has the same junction structure, the semiconductor layer 23 of both diodes D11 and D12 may be a continuous layer.

[0043]

Effects of the Invention The active matrix liquid crystal display device of the present invention replaces the conventional Schottky effect type thin film diode constituting the active circuit with a P-type semiconductor film and an N-type semiconductor film directly or with an I-type semiconductor film. It has a P-N (or N-P) junction or a P-I-N (or N-I-P) junction structure in which films are stacked on both sides, and a thin film diode with this junction structure has a Schottky effect. Unlike conventional thin film diodes, this does not utilize the Schottky effect between the N-type semiconductor film and the electrodes, so the upper and lower electrodes of this thin film diode can be formed from materials that are inexpensive and easy to process. Therefore, the manufacturing cost of the thin film diode constituting the active circuit can be reduced, and the price of the display device can be lowered.

[Brief explanation of the drawing]

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

FIG. 2 is a plan view of a portion of a liquid crystal display device showing an embodiment of the present invention.

FIG. 3 is a circuit diagram of a portion of a liquid crystal display device with a diode ring structure.

FIG. 4 is a circuit diagram of a portion of a liquid crystal display device with a back-to-back structure.

FIG. 5 is a sectional view of an active circuit portion of a conventional back-to-back liquid crystal display device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... Counter electrode, 2... Signal line, 3... Pixel electrode, B... Active circuit, D11, D12... Thin film diode, 21... Base electrode, 22... Upper electrode, 23... Semiconductor layer, 23p... P
type semiconductor film, 23i... I type semiconductor film, 23n... N type semiconductor film, 24... Insulating film.

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,
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 is connected to an active circuit in which two thin film diodes are connected in series in opposite directions. In the active matrix liquid crystal display device, the two thin film diodes constituting the active circuit are connected to each other by directly connecting a P-type semiconductor film and an N-type semiconductor film or by interposing an I-type semiconductor film between a pair of upper and lower electrodes. 1. An active matrix liquid crystal display device characterized in that it has a structure in which semiconductor layers are sandwiched between layers.