JP2000164873A - Liquid-crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent degradation in image quality caused by variation in current capacity of a thin-film transistor provided at each pixel. SOLUTION: A pixel 13 is formed using a thin-film transistor 14 at an intersection between gate wires 11-1, 11-2, 11-3, and 11-4 and signal wires 12-1, 12-2, 12-3, and 12-4, etc., wired into a matrix form, with a light-shielding layer and a control part 20 connected to it provide. The light-shielding layer is provided on the side counter to a gate electrode with a silicon layer (semiconductor layer), where a source and drain are formed of the thin-film transistor 14 in between. The control part 20 controls a biasing electric potential of the light- shielding layer, so that the current voltage of the thin-film transistor 14 becomes a prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device in which pixels are formed using thin film transistors at intersections between gate lines and signal lines arranged in a matrix.

[0002]

2. Description of the Related Art Generally, liquid crystal display devices used for OA equipment such as personal computers and word processors, televisions, and the like include a simple matrix system and an active matrix system. In particular, in recent years, an active matrix method for realizing high-definition display has been frequently used in order to cope with multi-color display.

FIG. 5 is a circuit diagram showing a conventional example of an active matrix type liquid crystal display device. In FIG. 5, gate lines 51-1, 51-2, 51-3, 51-4,...
And signal lines 52-1, 52-2, 52-3, 52-4 for a plurality of columns,
Are wired in a matrix, and the intersection of the
3 form a liquid crystal panel.

The pixel 53 includes a thin film transistor (TFT) 54 serving as a pixel transistor,
It comprises a liquid crystal cell 55 and a hold capacitor 56. In this pixel 53, a thin film transistor 54
Means that the gate electrodes of the gate lines 51-1, 51-2, 51-
3, 51-4,..., The source electrodes of which are signal lines 52-1,
52-2, 52-3, 52-4,...

[0005] Gate lines 51-1, 51-2, 51-3, 51-
Each end of 4,... Is connected to the output end of each row of the gate drive circuit 57 serving as a vertical drive circuit. The gate drive circuit 57 is for performing vertical scanning by selecting each pixel 53 of the liquid crystal panel on a row basis.

On the other hand, signal lines 52-1, 52-2, 52-3, 5
One end of each of 2-4,... Is connected to the output end of each column of the source drive circuit 58 which is a horizontal drive circuit. The source drive circuit 58 is for sequentially supplying a signal voltage corresponding to a gradation to each pixel 53 of the liquid crystal panel.

[0007]

The writing of a signal voltage based on a certain image signal is performed by the gate driving circuit 57.
Is performed only on the pixel 53 selected by the source driving circuit 58 and supplied with the signal voltage by the source driving circuit 58,
The writing time at that time is determined by the time constant of the current capability of the thin film transistor 54 and the capacitance of the hold capacitor 56.

Among these, as for the capacitance of the hold capacitor 56, the thickness of the insulating film that determines the capacitance can be accurately and uniformly controlled, but the current capability of the thin film transistor 54 has a very large variation. I have. Therefore, when the current capability of the thin film transistor 54 becomes low (when the threshold voltage Vth is high), the pixel 5
When a signal voltage is written to No. 3, the pixel voltage does not reach a desired voltage and becomes a bright spot. Conversely, when the current capability of the thin film transistor 54 is increased (the threshold voltage V
(th is low), the leakage current increases accordingly, and the holding voltage leaks to become a bright spot. As described above, in the related art, there is a problem that the image quality is deteriorated due to the variation in the current capability of the thin film transistor 54.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a liquid crystal device capable of preventing a deterioration in image quality due to a variation in current capability of a thin film transistor provided in each pixel. A display device is provided.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above-mentioned object, and a pixel is formed by using a thin film transistor at an intersection between a gate line and a signal line arranged in a matrix. The liquid crystal display device employs a configuration including a light-shielding layer and a control unit that controls a bias potential of the light-shielding layer. The light-shielding layer is provided on a side facing the gate electrode with the semiconductor layer on which the source and the drain of the thin film transistor are formed interposed. Further, the control unit controls the bias potential of the light shielding layer so that the current voltage of the thin film transistor becomes a predetermined value.

In the liquid crystal display device having the above-described structure, a light-shielding layer is provided on the side opposite to the gate electrode with the semiconductor layer on which the source and drain of the thin-film transistor are formed interposed therebetween. Functions as a gate. The light-shielding layer is biased by the control unit such that the current capability of the thin film transistor becomes a predetermined value. Thus, the current capability of the thin film transistor is maintained at a predetermined value without being affected by the variation in the current capability of the thin film transistor itself.

[0012]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a liquid crystal display device according to the present invention, and FIG. 2 is a schematic diagram showing a cross-sectional structure of a thin film transistor (TFT) in the present embodiment.

First, in FIG. 2, a transparent substrate 1 on which a liquid crystal display device is formed is made of, for example, a quartz substrate or a non-alkali glass substrate, and a conductive light shielding layer 2 is formed on the transparent substrate 1. I have. The light-shielding layer 2 is made of, for example, a metal film of aluminum, titanium, tungsten, molybdenum, or molybdenum silicide, titanium silicide, tungsten silicide, or the like.

A first insulating film 3 is formed on the transparent substrate 1 so as to cover the light shielding layer 2. The first insulating film 3 is made of, for example, a silicon oxide film, a silicon nitride film, or the like. Further, a first insulating film 3 is provided above the light shielding layer 2.
, A silicon layer (ie, a semiconductor layer) 4 of the thin film transistor is formed. The silicon layer 4 is made of, for example, polysilicon or amorphous silicon.

On the first insulating film 3, a second insulating film 5 is formed so as to cover the silicon layer 4. Second
The insulating film 5 functions as a gate oxide film of the thin film transistor, and is composed of, for example, a silicon oxide film, a silicon nitride film, or the like. Further, a gate electrode 6 is formed on the second insulating film 5. The gate electrode 6 is made of, for example, conductive polysilicon having impurities introduced therein.

Further, a third insulating film 7 is formed on the second insulating film 5 so as to cover the gate electrode 6. Third
The insulating film 7 is made of, for example, phosphorus silicate glass (PSG).
It is composed of a membrane. On the third insulating film 7, a pair of extraction electrodes 8 and 9 are formed on both sides of the gate electrode 6. The pair of extraction electrodes 8 and 9 are made of, for example, aluminum. One extraction electrode 8 is connected to the source region S of the thin film transistor to form a source electrode, and the other extraction electrode 9 is connected to the drain region D of the thin film transistor. Connected to form a drain electrode.

Here, when a pixel is formed using a thin film transistor as described above, the leakage current when the gate is turned off cannot be ignored, which causes the generation of a bright spot and a decrease in contrast. As a countermeasure, in many pixel structures, a hold capacitor (accumulation capacitance) is provided in order to increase a voltage holding ability affected by a leak current. Further, since the incidence of light from the transparent substrate 1 also leads to an increase in leakage current, a light-shielding layer 2 is formed below the thin film transistor (transparent substrate 1 side) for the purpose of preventing the leakage current. Light incident from the substrate 1 side is blocked, and generation of a leak current due to the light is suppressed.

FIGS. 3A and 3B are symbol diagrams of a thin film transistor. FIG. 3A shows a case where a light shielding layer is provided, and FIG. 3B shows a case where no light shielding layer is provided. As shown, when a light-shielding layer is present, it functions as a back gate, so that the terminal structure has a four-terminal structure of a source S, a drain D, a gate G, and a back gate B. On the other hand, the terminal structure without the light-shielding layer has a three-terminal structure of a source S, a drain D, and a gate G.

By the way, the voltage of the light-shielding layer 2 has been conventionally connected because the conductive light-shielding layer 2 such as a metal film is connected to the power supply electrode or the counter electrode (common electrode) in the liquid crystal display device. The voltage is set to a fixed voltage such as the power supply voltage (VDD, VSS) or the voltage of the common electrode (VCOM), and the thin film transistors provided in each pixel have large variations in current capability.

FIG. 4 is a voltage-current characteristic diagram when the bias voltage of the light-shielding layer serving as the back gate is changed in the n-channel type thin film transistor having the light-shielding layer 2.
Incidentally, in FIG. 4, the drain voltage VD = 10 V
Constant, the gate voltage VG = -5 to 15 V, the voltage of the light-shielding layer (back gate voltage) Vb = -10 to 10 V, variable in 2 V steps, the results measured under the conditions of source voltage VS = 0 V are shown. ing.

As is apparent from FIG. 4, when the voltage of the light-shielding layer (voltage applied to the back gate) is changed to positive or negative (+/-), the current capability of the thin film transistor increases / decreases accordingly. I understand. For example, VG
= 2.5 V, the current capability of the thin film transistor changes about 10 times between the condition of Vb = −10 V and the condition of Vb = 10 V as shown by a circle in the figure. Such a change in the current capability of the thin film transistor is caused by the fact that the light-shielding layer provided on the side opposite to the gate electrode with the silicon layer interposed therebetween serves as a back gate, and by changing the back gate voltage, the threshold voltage V
This occurs when th is modulated, and this is what is called the so-called back gate effect. Although illustration is omitted here, the current capability can be varied by the back gate effect even in a p-channel type thin film transistor. However, the back gate voltage of the n-channel thin film transistor is inverted.

In the present invention, attention is paid to the back gate effect, and the circuit configuration shown in FIG. 1 is employed in order to effectively utilize the back gate effect. In FIG. 1, a plurality of rows of gate lines 11-1, 11-2, 11-3, 11-4,... And a plurality of columns of signal lines are formed on a transparent substrate made of, for example, a quartz substrate (not shown). , 12-1, 12-2, 12-3, 12-4,... Are arranged in a matrix, and pixels 13 are formed at the intersections to form a liquid crystal panel.

The pixel 13 includes a thin film transistor 14 serving as a pixel transistor, a liquid crystal cell 15, and a hold capacitor 16. In the pixel 13, the thin film transistor 14 has a gate electrode whose gate electrode 11
-1, 11-2, 11-3, 11-4,..., And their source electrodes are connected to signal lines 12-1, 12-2, 12-3, 12-4,. One electrode of each of the liquid crystal cell 15 and the hold capacitor 16 is connected to the drain electrode of the thin film transistor 14. Then, the liquid crystal cell 15
A common voltage (reference voltage) VCOM is applied to each other electrode of the hold capacitor 16 and the other electrode.

Gate lines 11-1, 11-2, 11-3, 11-
Each end of 4,... Is connected to the output end of each row of the gate drive circuit 17 serving as a vertical drive circuit. The gate drive circuit 17 is for performing vertical scanning by selecting each pixel 13 of the liquid crystal panel on a row basis.

On the other hand, the signal lines 12-1, 12-2, 12-3, 1
One end of each of 2-4,... Is connected to the output end of each column of the source drive circuit 18 serving as a horizontal drive circuit. The source drive circuit 18 is for sequentially supplying a signal voltage corresponding to a gradation to each pixel 13 of the liquid crystal panel.

Further, the gate lines 11-1, 11-2, 11-
Back gate lines 19-1, 19-
, Are provided on the back gate lines 19-1, 19-2, 19-3, 19-4,... The light-shielding layer formed immediately below the silicon layer 4 is electrically connected. Back gate lines 19-1, 19-2, 19-3, 1
Each end of 9-4,... Is connected to the control unit 20.

The control unit 20 includes the thin film transistor 14
The bias potential of the light shielding layer is controlled so that the current capability of the light shielding layer becomes a predetermined value. For example, in the case of the n-channel type thin film transistor 14, when the current capability is reduced, the light blocking layer is biased in the positive direction by the control unit 20, and the threshold voltage Vt
h is modulated to decrease the current capacity to a predetermined value. On the other hand, when the current capability of the thin film transistor 14 increases, the light blocking layer is biased in the negative direction by the
Of the threshold voltage Vth is modulated to increase the current capability to a predetermined value.

In the liquid crystal display device having the above configuration, when the current capability of the thin film transistor 14 is reduced (the threshold voltage Vth
Is high), the light blocking layer is biased in the positive direction by the control unit 20, and the current capability increases. Thus, the signal voltage supplied from the source drive circuit 18 via the signal lines 12-1, 12-2, 12-3, 12-4,... Within the 1H period is selected by the gate drive circuit 17. It is possible to sufficiently write in the pixel 13.

On the other hand, when the current capability of the thin film transistor 14 increases (when the threshold voltage Vth is low), the light shielding layer is biased in the negative direction by the control unit 20, and the current capability of the thin film transistor 14 decreases. . As a result, the leak current of the thin film transistor 14 is reduced, and not only the leak current due to light but also the pixel voltage held in the hold capacitor 16 does not leak.

As a result, when a signal voltage is written to the pixel 13, the pixel voltage can reach a desired voltage, and the leakage of the holding voltage held in the pixel 13 by writing the signal voltage by the light shielding layer 2 can be achieved. Can be prevented, so that it is possible to reliably avoid the deterioration of the image quality due to the occurrence of bright spots and the decrease in contrast.

[0031]

As described above, according to the liquid crystal display device of the present invention, the control section for controlling the bias potential of the light-shielding layer is connected to the light-shielding layer functioning as the back gate of the thin-film transistor disposed in each pixel. By doing so, the current capability of the thin film transistor can be kept constant without being affected by variations in the current capability of the thin film transistor itself. As a result, even when the current capability of the thin film transistor becomes low, the signal voltage can be sufficiently written with a desired voltage, and even when the current capability of the thin film transistor becomes high, the written pixel voltage can be obtained. Can be held without leaking. As a result, it is possible to reliably avoid the deterioration of the image quality due to the occurrence of bright spots and the decrease in contrast.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a liquid crystal display device according to the present invention.

FIG. 2 is a schematic diagram illustrating a cross-sectional structure of a thin film transistor according to an embodiment.

FIG. 3 is a symbol diagram of a thin film transistor.

FIG. 4 is a voltage-current characteristic diagram of the thin-film transistor according to the embodiment.

FIG. 5 is a circuit diagram showing a conventional example.

[Explanation of symbols]

2 ... light shielding layer, 4 ... silicon layer (semiconductor layer), 6 ... gate electrode, 11-1, 11-2, 11-3, 11-4 ... gate line, 1
2-1, 12-2, 12-3, 12-4 ... signal lines, 13 ... pixels,
14 ... Thin film transistor, 20 ... Control unit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme Court ゛ (Reference) H01L 29/78 617N (72) Inventor Yuji Hayashi 6-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Stock Company F-term (reference) 2H092 JA24 JB51 JB69 KA04 KA05 NA01 NA24 PA06 2H093 NA45 NB29 ND01 ND19 ND46 ND58 NE01 5C006 BB16 BC06 BC08 BF31 FA21 FA36 5F110 CC02 DD02 DD03 DD06 DD13 DD14 NN30 GG02 NN02 GG02 NN02 GG02 NN02 GG02 NN NN47 NN72

Claims (2)

[Claims] 1. A liquid crystal display device in which a pixel is formed using a thin film transistor at an intersection of a gate line and a signal line arranged in a matrix, with a semiconductor layer in which a source and a drain of the thin film transistor are formed interposed therebetween. A liquid crystal display device comprising: a light-shielding layer provided on a side facing a gate electrode; and a control unit for controlling a bias potential of the light-shielding layer. 2. The liquid crystal display device according to claim 1, wherein the control unit controls a bias potential of the light shielding layer so that a current voltage of the thin film transistor has a predetermined value. Liquid crystal display.