JPH11354803A - Thin film transistor and display using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the leakage current from occurring by superposing a part of a second electrode on the channel of a thin film transistor connected to a first electrode, in the part of the second electrode adjacent to the first electrode a signal different in polarity from that of a signal applied to the first electrode is applied thereto. SOLUTION: When the polarity of an image signal applied to display electrodes of display pixels b, c, d, g, h, i is positive and that of other display pixels a, e, f is negative, the display electrode of any of the display pixels b, c, d, g, h, i is superposed above the channel of a TFT connected to the display electrode of the display pixel (a), image signals different in polarity are, as a result, applied to display electrodes 16a, 16b above TFT channels 7a, 7b connected to the display electrode 16a, and hence the leak current generation can be suppressed by applying signals of a positive polarity different from that of the display electrodes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor (hereinafter, referred to as "TFT") and a display device using the TFT as a switching element.

[0002]

2. Description of the Related Art In recent years, a TFT using a polycrystalline silicon film as an active layer as a driving driver element or a pixel driving element has been developed.
An FT and a display device using the TFT as a switching element have been developed.

As a display device, for example, an active matrix type liquid crystal display device (Liquid Cry) including a pair of substrates composed of a TFT substrate and a substrate arranged opposite to the TFT substrate.
stalDisplay: Hereinafter, referred to as “LCD”. ) Is under development. However, in the conventional LCD, impurities or impurity ions from the sealing adhesive generated when the two substrates are bonded and cured are attached to the surface of the flattening film of the TFT, and the surface of the flattening film is charged. As a result, polarization occurs above and below a film such as a flattening film, so that a back channel is formed in the TFT and the threshold voltage of the TFT changes.

For this reason, when this TFT is used for an LCD, when the threshold voltage of the TFT changes in the increasing direction, TF
When a predetermined potential is not applied to the display electrode due to a decrease in the ON current of T, a point defect occurs. On the contrary, when the threshold voltage changes in a decreasing direction, the OFF current increases and a similar point defect occurs. Good display cannot be obtained and each TFT
However, when the threshold voltage varies, there is a drawback that a display with uniform brightness cannot be obtained in the plane.

Therefore, as shown in FIG. 9, the generation of polarization is suppressed by providing the display electrode so as to overlap the channel of the TFT connected to the display electrode.

A conventional LCD in which a display electrode is provided so as to overlap a channel of a TFT connected to the display electrode is described below.
Will be described.

FIG. 9 is a plan view of a conventional display pixel portion.
FIG. 10 is a sectional view of the LCD taken along the line BB in FIG.

As shown in FIG. 9, a TFT in a display pixel portion
Is provided near an intersection of a gate signal line G for supplying a gate signal and a drain signal line D for supplying a video signal, and the source 5 is connected to the display electrode 16.

The structure of the TFT will be described with reference to FIG.

On an insulating substrate 1 made of quartz glass, non-alkali glass or the like, chromium (Cr), molybdenum (M
a) a gate electrode 2 made of a refractory metal such as o), a gate insulating film 3, and an active layer 4 made of a polycrystalline silicon film.

The active layer 4 is provided with a channel 7 on the gate electrode 2 and on both sides of the channel 7 a source 5 and a drain 6 formed by ion implantation using the stopper 8 on the channel 7 as a mask. ing.

Then, the gate insulating film 3, the active layer 4, and the S
The entire surface of the stopper 8 made of an insulating film such as an SiO ₂ film is
An interlayer film 12 is formed by stacking an iO ₂ film 9, a SiN film 10 and a SiO ₂ film 11 in this order, and a contact hole provided corresponding to the drain 6 is filled with a metal such as Al to form a drain electrode 13. . Flattening film 1 for further flattening the surface
5 is formed. Then, a contact hole is formed at a position corresponding to the source 5 of the flattening film 15, and the ITO contacting the source 5 through the contact hole is formed.
A display electrode 16 made of (Indium Thin Oxide) and also serving as the source electrode 14 is formed on the flattening film 15. Then, an alignment film 19 for aligning the liquid crystal 20 is provided on the display electrode 16.
To form At this time, the display electrode 16 is provided so as to overlap the channel 7.

The insulating substrate 1 provided with the TFT thus prepared and the counter substrate 17 provided with the counter electrode 18 and the alignment film 19 facing the substrate 1 are surrounded by a sealing adhesive 2.
1 and the formed gap is filled with the liquid crystal 20 to complete the LCD.

[0014]

However, video signals having positive and negative polarities are alternately applied to the display electrode connected to the TFT with reference to the potential of the counter electrode. Therefore, when a video signal of positive polarity is applied for a certain period, the threshold voltage changes in a direction of increasing, and when a video signal of negative polarity is applied, the threshold voltage changes in a direction of decreasing. . Therefore, in the case of an LCD having this TFT,
When the threshold voltage of the TFT changes in the direction of increasing, the ON current of the TFT decreases and a point defect occurs. On the contrary, when the threshold voltage changes in the direction of decreasing, the off current increases and the point defect similarly occurs. In addition, there is a problem that a good display cannot be obtained, and a display of uniform brightness cannot be obtained in a plane when a threshold voltage varies in each TFT.

In view of the above, the present invention has been made in view of the above-mentioned drawbacks, and uses a TFT having a stable threshold voltage to reduce display defects such as white spots. It is an object of the present invention to provide an LCD capable of obtaining a display with uniform brightness.

[0016]

According to the present invention, there is provided a thin film transistor which is adjacent to a first electrode and to which a signal having a polarity different from the polarity of a signal applied to the first electrode is applied.
And a third electrode connected to the second electrode is provided so as to overlap a channel of the thin film transistor connected to the first electrode.

Further, the second electrode is an electrode adjacent to the first electrode in a direction in which a scanning signal line extends.

Further, the second electrode is an electrode adjacent to the first electrode in a direction in which a video signal line extends.

A display device according to the present invention is a display device including any one of the above-described thin film transistors.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An LCD according to the present invention will be described below.

FIG. 1 is a plan view of an LCD display pixel portion of the present invention, and FIG. 2 is a cross-sectional view of the LCD taken along line AA in FIG.

As shown in FIG. 1, the display electrodes 16a and 16a are surrounded by a scanning signal line G and a video signal D, and near an intersection thereof.
A TFT connected to the TFT 16b is provided.

A display pixel having the display electrode 16a shown in FIG. 1 will be described with reference to FIG.

In FIG. 2, the steps from the formation of the gate electrodes 2a and 2b made of a high melting point metal such as Cr and Mo on the insulating substrate 1 made of quartz glass, non-alkali glass or the like to the formation of the flattening film 15 are the same as in the prior art. Therefore, the description is omitted.

A flattening film 1 made of, for example, an organic resin over the entire surface
After the formation of the source electrode 14, a contact hole is formed at a position corresponding to the source 5 of the flattening film 15, and the display electrode 16a is a transparent electrode made of a transparent conductive material such as ITO and also serving as the source electrode 14 in contact with the source 5. To form

At this time, as shown in FIGS. 1 and 2, the display electrode 16a is formed so as to cover at least the channel 7a. In addition, the channel 7b is covered by a display electrode of an adjacent display pixel portion in a direction in which the scanning signal line extends, that is, a display electrode 16b connected to the source of the TFT connected to the preceding scanning signal line. . On the display electrodes 16a and 16b, an alignment film 19 for aligning the liquid crystal 20 is formed.

A counter electrode substrate 17 provided with a counter electrode 18 and an alignment film 19 and a TFT substrate 1 on which a TFT is formed are bonded with a sealing adhesive 21, and a liquid crystal 20 is provided between the substrates 1 and 17.
To complete the LCD.

Here, the relationship between the polarity of the video signal applied to a certain display electrode and the polarity of the video signal of the display electrode superimposed above the channel of the TFT connected to that display electrode will be described.

FIG. 3 shows display pixels a, b,
Indicates the polarities of the video signals applied to the display electrodes of c, d, e, f, g, h, and i.

As shown in FIG. 3A, for example, the polarity of the video signal applied to the display electrodes of the display pixels b, c, d, g, h, and i is positive (+), and the other display pixels a, e,
When the polarity of the video signal of the display electrode of f is negative (-),
That is, in the case where the polarity of the video signal is different for each horizontal period, in the present embodiment, considering the display pixel a,
The display electrode of any of the display pixels b, c, d, g, h, and i is superimposed above the channel of the TFT connected to the display electrode of the display pixel a.

In the case where the polarity of the video signal is inverted for each field as shown in FIG. 3 (b), the TFT connected to the display electrode of the display pixel a is considered on the basis of the display pixel a. The display electrode of any one of the display pixels c, e, f, and h, which has a negative polarity in the odd field, is superposed above the channel.

By doing so, when the polarities of the display electrodes of the adjacent display pixels are different, the display electrodes 16 above the channels 7a and 7b of the TFT connected to the display electrode 16a.
Since video signals having different polarities are applied to the a and 16b, when the polarity of the video signal applied to one conventional display electrode is negative, the leakage current of the TFT connected to that display electrode is negative. Was generated, but by applying a positive polarity signal having a polarity different from that of the display electrode,
It is possible to suppress the generation of the leak current.

Therefore, since the threshold voltage of the TFT can be stabilized, light transmission due to a point defect generated, for example, in the periphery of the liquid crystal display panel, that is, white spots can be reduced. LC with the indication of
D can be obtained.

In the present invention, the second display electrode adjacent to the first display electrode means that the display electrode of the display pixel a shown in FIGS. 3A and 3B is the first display electrode. It refers to the display electrode of any of the display pixels b, c, d, e, f, g, h, and i. Further, in the present invention, an electrode made of a metal such as Al may be formed at the same time as the drain electrode and connected to the second display electrode, and the electrode may be provided so as to overlap above the channel.

Here, a characteristic diagram of the LCD of the present invention is shown.

FIG. 4 shows the characteristics of the LCD of the present invention (solid line) and the conventional LCD (dashed line). The operation is performed when a normally white black display signal is given.

In the figure, the horizontal axis shows the gate-off margin voltage, and the vertical axis shows the light transmittance of the panel at that voltage. Note that the gate-off margin voltage is the off-level voltage of the gate signal. If this voltage is increased, the potential difference between the gate and the drain becomes smaller, so that a leak current flows between the source and the drain, and the display electrode originally The voltage to be supplied is not applied, and a white spot through which light is transmitted occurs as an LCD display. Therefore, it can be said that the device has higher reliability as the gate-off margin voltage is higher or as the luminance of the white spot is lower.

As shown in the figure, in the conventional LCD, when the gate-off margin voltage was increased, the light transmittance was increased up to around 27%. That is, in the conventional LCD, white spots through which a large amount of light was transmitted occurred, but in the LCD of the present invention, the white spots could be suppressed to several percent. That is, it can be seen that white spots through which light passes are reduced. FIG. 5A shows the amount of change over time of the gate-off margin voltage of the LCD of the present invention, and FIG.

In the figure, the abscissa indicates the current supply time to the TFT, and the ordinate indicates the gate-off margin shift voltage which is the amount of change of the gate-off margin voltage during the current supply time. The symbols ◆ (black diamond) and ◆ (black square) in FIGS. 5A and 5B represent the center of the LCD panel, respectively.
The values at the periphery are shown.

As shown in FIG. 5 (b), in the conventional LCD, the gate-off margin shift voltage gradually increases in the central portion and the peripheral portion as the power-on time elapses. In particular, the value is larger in the peripheral portion. This is because the threshold voltage of the TFT decreases over time due to the influence of impurities from the seal portion.

On the other hand, as shown in FIG.
It can be seen that the LCD of the present invention does not change significantly with the passage of the power-on time as in the prior art. Further, it can be seen that the central portion and the peripheral portion do not vary as in the related art.

As described above, according to the LCD of the present invention, the display electrodes of the adjacent display pixels having different polarities of the video signal overlap the channel of the TFT of the display pixel to enter the panel. It is possible to suppress the occurrence of white spots due to an increase in the off-state current of the TFT of the display electrode with time due to the influence of impurities, and to make the luminance in the liquid crystal display panel surface uniform. <Second Embodiment> Hereinafter, a second embodiment of the present invention will be described.

FIG. 6 is a plan view of a display pixel portion according to a second embodiment of the present invention.

As shown in the figure, the TFT of the second embodiment
The structure is the same as the structure up to the planarization film 15 shown in the first embodiment.

The difference from the first embodiment is that, in the first embodiment, one channel 7a is covered by the display electrode 16a and the other channel 7b is covered by the preceding display electrode 16b. Channel 7b is adjacent to the display electrode 16a to the left, that is, the display electrode 16 in the direction in which the scanning signal extends.
c.

As described above, even when the other channel 7b is covered with the display electrode 16c, the generation of the leak current can be suppressed as in the first embodiment.

Accordingly, since the threshold voltage of the TFT can be stabilized, it is possible to reduce the variation in the luminance in the plane of the liquid crystal display panel. <Third embodiment>
FIG. 7 is a plan view of a display pixel portion according to the third embodiment of the present invention.

As shown in the figure, the present embodiment is different from the first embodiment in that it is an LCD having a so-called single gate structure TFT having one gate.

As shown in the figure, above the channel of the TFT to which the display electrode 16a is connected, above the display electrode 16a and the display electrode 16 of the display pixel adjacent to the display electrode 16a.
b is superimposed. Both display electrodes 16 covering the channel
What is necessary is just to provide a and 16b so that the area which overlaps with a channel may become the same so that both may not contact.

As described above, even with the so-called single gate structure, the same effect as that of the so-called double gate structure can be obtained.

In this embodiment, the display electrodes of adjacent display pixels are connected to the display electrodes adjacent to each other in the direction in which the video signal lines extend, that is, the TFTs connected to the preceding scanning signal lines.
Although the display electrodes are connected via the same, the adjacent display electrodes may be overlapped in the direction in which the scanning signal lines extend as in the second embodiment.

In the present invention, as shown in FIG. 8, an electrode 23 made of a metal such as Al is connected to the second display electrode 16b, and an electrode 23 made of metal such as Al is connected to the first display electrode 16a.
It may be provided on the channel 7b of the FT.

At this time, the electrode 23 is connected to the drain electrode 1
3 may be formed at the same time as the formation, and a mask pattern for forming the electrode 23 may be added to the mask pattern for forming the drain electrode, so that the formation can be performed without increasing the number of manufacturing steps. In addition, since the display channel is formed on the interlayer insulating film 12 above the channel, the generation of the back channel can be suppressed in the same manner as when the display electrode is formed on the planarization insulating film above the channel.

In each of the above embodiments, the case where the TFT of the present invention is used for an LCD has been described.
The present invention is not limited thereto.
The present invention can be applied to an L (Electro Luminescence) display device, and the same effects as those described above can be obtained. Note that, when employed in an organic EL display device, the first or second electrode corresponds to an anode or a cathode of the organic EL element.

[0055]

According to the present invention, it is possible to obtain a TFT in which back channel generation due to polarization above and below an interlayer film and a flattening film can be suppressed, and a change in threshold voltage with time is suppressed and stabilized. Therefore, by using the TFT, it is possible to obtain a display device in which display defects such as white spots are reduced and display with uniform brightness is obtained in a plane.

[Brief description of the drawings]

FIG. 1 is a plan view of an LCD showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of the LCD showing the first embodiment of the present invention.

FIG. 3 is a diagram illustrating the polarity of a signal applied to a display pixel of the present invention.

FIG. 4 is a characteristic diagram showing characteristics of the LCD of the present invention.

FIG. 5 is a characteristic diagram showing characteristics of the LCD of the present invention.

FIG. 6 is a plan view of an LCD showing a second embodiment of the present invention.

FIG. 7 is a plan view of an LCD showing a third embodiment of the present invention.

FIG. 8 is a plan view of an LCD showing a fourth embodiment of the present invention.

FIG. 9 is a sectional view of a conventional LCD.

FIG. 10 is a plan view of a conventional LCD.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Insulating substrate 2 Gate electrode 4 Active layer 5 Source 6 Drain 7a, 7b Channel 8 Stopper 12 Interlayer film 15 Flattening film 16 Display electrode 16a Display pixel 16b, 16c Display pixel adjacent to display pixel 16a

Claims (4)

[Claims] 1. A portion of a second electrode adjacent to a first electrode to which a signal having a polarity different from the polarity of a signal applied to the first electrode is connected or connected to the second electrode. A third electrode provided so as to overlap a channel of the thin film transistor connected to the first electrode. 2. The thin film transistor according to claim 1, wherein the second electrode is an electrode adjacent to the first electrode in a direction in which a scanning signal line extends. 3. The thin film transistor according to claim 1, wherein the second electrode is an electrode adjacent to the first electrode in a direction in which a video signal line extends. 4. A display device comprising the thin film transistor according to claim 1. Description: