Liquid crystal display device

A liquid crystal display device according to an aspect of the invention includes a display region having a plurality of sub pixels. The sub pixels each include a pixel electrode and a thin film transistor electrically coupled to the pixel electrode. A transient leak current of each thin film transistor included in the sub pixels at both ends of the display region among the sub pixels provided on a line passing through the center of the display region in plan view is smaller than a transient leak current of each thin film transistor included in the sub pixels in a central portion including the center.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Application No. 2015-116572, filed on Jun. 9, 2015, the contents of which are incorporated by reference herein in its entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a liquid crystal display device.

2. Description of the Related Art

As a technique to reduce power consumption of liquid crystal display devices, low frequency driving and intermittent driving have been proposed. The low frequency driving and the intermittent driving are driving methods that reduce the number of rewriting times of a video signal per unit time (e.g., refer to International Patent Application Laid-open Publication No. 2013-190912).

One of the problems to be solved in such liquid crystal display devices is to reduce a flicker. For example, when a liquid crystal panel is left in a high humidity environment for a long time, external moisture enters a liquid crystal layer from the end portion of the liquid crystal panel, thereby reducing the voltage holding ratio of the liquid crystal layer. The reduction of the voltage holding ratio is large at the end portion of a display region (particularly, at a corner section of the display region). The flicker, thus, easily occurs at the end portion of the display region. The technique disclosed in International Patent Application Laid-open Publication No. 2013-190912 cannot sufficiently reduce the flicker.

SUMMARY

A liquid crystal display device according to an aspect of the invention includes a display region having a plurality of sub pixels. The sub pixels each include a pixel electrode and a thin film transistor electrically coupled to the pixel electrode. A transient leak current of each thin film transistor included in the sub pixels at both ends of the display region among the sub pixels provided on a line passing through the center of the display region in plan view is smaller than a transient leak current of each thin film transistor included in the sub pixels in a central portion including the center.

DETAILED DESCRIPTION

The following describes embodiments of the present invention in detail with reference to the accompanying drawings. The description of the following embodiments does not limit the invention. The constituent elements of the following embodiments include elements easily envisaged by those skilled in the art and identical elements. The constituent elements described below can also be combined as appropriate. The disclosure is made by way of examples. All modifications and changes that may be easily achieved by those skilled in the art within the spirit of the invention are included in the scope of the invention. To explain the drawings clearer, the widths, thicknesses, and shapes of respective components may be more schematically illustrated than the actual ones. Those are illustrated by way of examples and do not limit the interpretation of the invention. In the present specification and the respective drawings, the same elements already described in the previous drawings are labeled with the same symbols and the detailed description thereof may be omitted as appropriate.

First Embodiment

FIG. 1is a perspective view illustrating a schematic structure of a liquid crystal display device100according to a first embodiment.

The liquid crystal display device100includes a liquid crystal panel110and a backlight120. The liquid crystal panel110includes a first substrate10and a second substrate30. The second substrate30is provided to be opposed to the first substrate10. A seal material39having a frame shape is provided in a periphery of an opposed region where the first substrate10faces the second substrate30. A liquid crystal layer40(refer toFIG. 4) is sealed in a space surrounded by the first substrate10, the second substrate30, and the seal material39. A display region110A is provided in the region surrounded by the seal material39.

The backlight120emits illumination light toward the display region110A. The backlight120includes a light guide plate121and a plurality of light sources122, for example. The light sources122are arranged side by side along an end face (light incident surface) of the light guide plate121. Illumination light emitted from the light sources122propagates through the light guide plate121while being totally reflected and is emitted from the principal surface of the light guide plate121that faces the liquid crystal panel110. InFIG. 1, a side light structure is employed for the backlight120. The structure of the backlight120is, however, not limited to the example. For example, a backlight may be employed that has a structure where a plurality of point-shaped light sources is arranged directly under the liquid crystal panel110.

FIG. 2is a schematic diagram illustrating an electrical structure of the liquid crystal display device100.

In the display region110A, a plurality of scan lines16and a plurality of signal lines18are provided so as to intersect with each other. A thin film transistor SW is provided at each intersection between the scan line16and the signal line18. A gate electrode of the thin film transistor SW is electrically coupled to the scan line16. A source electrode of the thin film transistor SW is electrically coupled to the signal line18. A drain electrode of the thin film transistor SW is electrically coupled to a pixel electrode23.

A common electrode21shared by the pixel electrodes is provided in the display region110A. An orientation of the liquid crystal layer40(refer toFIG. 4) is controlled by an electric field generated between the pixel electrode23and the common electrode21. A region where the orientation of the liquid crystal layer40is controlled by the single pixel electrode23and the common electrode21is a single sub pixel PX. The display region110A is formed by a plurality of the sub pixels PX arranged in a matrix along the extending directions of the scan lines16and the signal lines18.

As a drive unit that drives the sub pixels PX, a gate driver101and a source driver102are provided. The scan lines16are electrically coupled to the gate driver101. The signal lines18are electrically coupled to the source driver102. The gate driver101includes a first gate driver101A and a second gate driver101B. The scan lines16are coupled to two gate drivers101(the first gate driver101A and the second gate driver101B) in such a manner that they are alternately coupled to the first gate driver101A and the second gate driver101B one by one. The gate driver101and the source driver102are arranged in a region surrounding the display region110A, for example.

The gate driver101sequentially selects the scan lines16in the order of GL1, GL2, GL3, GL4, . . . , GLm−1, and GLm in a single vertical scan period. The gate driver101supplies a gate signal to the selected scan line16so as to supply an on voltage to the gate electrodes of the thin film transistors SW coupled to the scan line16. It is preferable that the on voltages supplied to the respective scan lines are the same. The source driver102simultaneously or sequentially selects the signal lines18of SL1, SL2, SL3, . . . , SLn in a single horizontal scan period. The source driver102supplies a video signal to the selected signal line18. The video signal is supplied to the source electrode of the thin film transistor SW coupled to the signal line18and supplied to the pixel electrode23via a channel section and the drain electrode of the thin film transistor SW.

The operation of the gate driver101and the source driver102is controlled by a control circuit103provided outside the liquid crystal panel110. The control circuit103supplies a common voltage VCOM to the common electrode21via common signal lines28. The control circuit103controls the operation of the backlight120.

The control circuit103changes the number of rewriting times of the video signal per unit time in accordance with the displayed image. When a still image in which importance is not attached to moving image visibility is displayed, for example, the control circuit103performs low frequency driving or intermittent driving. The low frequency driving is a drive system that reduces power consumption by reducing a drive frequency of the liquid crystal display device100to ½ or ¼ of a standard condition, for example. The intermittent driving is a drive system that reduces power consumption by setting a pause period of several display periods after the liquid crystal display device100performs writing in a display period. Both drive systems may cause the occurrence of adverse reaction such as a moving image blur since a video signal rewriting cycle becomes longer. Each of the drive systems is, however, effective for power consumption reduction in displaying still images in which importance is not attached to the moving image visibility.

In the specification, a time interval in which the video signal is rewritten is called a “frame period” or “single frame” and a reciprocal of the frame period is called a “frame frequency” in relation to the low frequency driving and the intermittent driving.

For example, it is assumed that a standard frame frequency is 60 Hz (i.e., the video signal is rewritten every sixteenth of a second). When the moving image is displayed, the control circuit103performs rewriting of the video signal at a standard 60 Hz. When a still image in which importance is not attached to moving image visibility is displayed, the control circuit103performs writing in 1/60 sec, and thereafter sets a pause period of 1/60, 3/60, 7/60, or 59/60 sec, for example. The operation of the control circuit103stops during the pause period. As a result, power consumption, which is a temporal average in a total period including the writing period and the pause period, is reduced.

When a DC voltage is applied to a liquid crystal material for a long time, a temporal change occurs in display characteristics by charge-up. The driving is, thus, generally performed by inversing the polarity of the video signal for every frame such that an average of a DC voltage is substantially zero. If a response characteristic (luminance-voltage characteristic) differs between the positive polarity frame and the negative polarity frame, the positive polarity frame and the negative polarity frame have different luminance, thereby causing brightness and darkness to vary in each frame. As a result, a flicker occurs. The flicker can be minimized by adding a minute offset voltage to an average of the positive polarity and the negative polarity video signals (average of the DC voltages) or adjusting common potential. It is, however, difficult to completely eliminate the flicker by completely absorbing a temporal shift of the luminance-voltage characteristic and a difference between gradations in optimum condition, for example.

For reducing the flicker, the control circuit103performs inversion such as line inversion, column inversion, or dot inversion. For example, the line inversion can cause the flicker not to be apparent by inverting phases of temporal polarity inversion row by row so as to macroscopically cancel a difference in luminance response between polarities. The column inversion and the dot inversion can cause the flicker not to be apparent in the similar manner as the line conversion. The column inversion inverses the phases of the polarity inversion column by column. The dot inversion inverses the phases of the polarity inversion in a checkerboard pattern (i.e., phases of the polarity inversion are inversed row by row and column by column).

The line inversion and the dot inversion perform writing to the pixels while performing polarity inversion line by line in screen scan. Charging and discharging of the signal lines in the panel need to be performed every H period (horizontal period). As a result, power consumption is increased. In contrast, the column inversion performs no polarity inversion in rows. The column inversion is, thus, advantageous in view of reduction of power consumption. Various inversion methods are employed in a mobile liquid crystal display device in accordance with a product specification. The column inversion method is the most desirable in view of reduction of power consumption.

FIG. 3is a plan view of the first substrate for explaining the structure of the sub pixel PX.FIG. 4is a sectional view along line IV-IV′ ofFIG. 3.FIG. 5is a schematic diagram illustrating coupling capacitance between the pixel electrode23and the common electrode21.

As illustrated inFIG. 3, the pixel electrode23and the common electrode21are provided so as to partially overlap with each other in the sub pixel PX. The longitudinal direction of the pixel electrode23is the extending direction of the signal lines18. The common electrode21is provided in a belt-like shape along the extending direction of the scan lines16so as to cross over a plurality of the pixel electrodes23arranged in the extending direction of the scan lines16.

The pixel electrode23has a plurality of belt-like electrodes23a, a first joining section23b1, a second joining section23b2, and a contact section23c. Each of the belt-like electrodes23aextend in the extending direction of the signal lines18. The belt-like electrodes23aare provided so as to be arranged in the extending direction of the scan lines18. The first joining section23b1joins one ends of the belt-like electrodes23a. The second joining section23b2joins the other ends of the belt-like electrodes23a. The contact section23cbranches from the first joining section23b1toward the scan line16. The contact section23cis electrically coupled to a drain electrode19of the thin film transistor SW via a contact hole H3at a position beyond the scan line16.

The pixel electrode23may be formed in a platy shape without being patterned in a belt-like shape. In this case, the common electrode21includes a single or a plurality of belt-shaped electrodes, for example. In the example illustrated inFIG. 3, the pixel electrode23is disposed on the upper layer side (a side near the liquid crystal layer) than the common electrode21. The common electrode21may be disposed on the upper layer side than the pixel electrode23. The common electrode21and the pixel electrode23may be adjacently arranged side by side on the same layer. The common electrode21and the pixel electrode23each may be formed in a belt-like shape and provided such that they are apart from each other with a distance therebetween and do not overlap with each other in plan view, for example.

The scan lines16and the signal lines18are provided along gaps between the pixel electrodes23. The scan line16has a main line section16athat extends in a direction intersecting the signal line18and a branched section16bthat branches from the main line section16ain a direction parallel to the signal line18. The thin film transistor SW is provided in the vicinity of the intersection of the scan line16and the signal line18.

The thin film transistor SW includes a semiconductor layer14. One end of the semiconductor layer14is provided at a position overlapping with the signal line18. The one end of the semiconductor layer14is electrically coupled to the signal line18via a contact hole H1. The section of the signal line18electrically coupled to the semiconductor layer14serves as a source electrode18a(refer toFIG. 4) of the thin film transistor SW.

The semiconductor layer14bends in an L-shape from a position overlapping with the signal line18, and extends along the signal line18toward the scan line16. The semiconductor layer14bends in a direction parallel to the scan line16at a position beyond the scan line16, and extends to a position beyond the branched section16b. The other end of the semiconductor layer14is electrically coupled to the drain electrode19via a contact hole H2at the position beyond the branched section16b.

The semiconductor layer14intersects with the main line section16aand the branched section16b. The section of the main line section16ainteresting with the semiconductor layer14serves as a first gate electrode16c(refer toFIG. 4) of the thin film transistor SW. The section of the branched section16bintersecting with the semiconductor layer14serves as a second gate electrode16d(refer toFIG. 4) of the thin film transistor SW.

The semiconductor layer14includes a first channel section14C1that faces the first gate electrode16c, a second channel section14C2that faces the second gate electrode16d, a source section14S provided between the first channel section14C1and the source electrode18a(refer toFIG. 4), a drain section14D provided between the second channel section14C2and the drain electrode19, and an intermediate section14M provided between the first channel section14C1and the second channel section14C2.

In each of the source section14S, the drain section14D, and the intermediate section14M of the thin film transistor SW, a lightly doped impurity region is provided on a side near the channel section and a highly doped impurity region is provided on a side far from the channel section. The impurity concentration of the lightly doped impurity region is lower than that of the highly doped impurity region.

A light blocking layer12is provided on the backlight120side (refer toFIG. 4) of the semiconductor layer14. The light blocking layer12blocks light entering the semiconductor layer14from the backlight120. The light blocking layer12is provided on a lower layer (a layer on the backlight120side) of the thin film transistor SW, for example. The light blocking layer12includes a first light blocking layer12athat overlaps with the first channel section14C1and a second light blocking layer12bthat overlaps with the second channel section14C2. An area of the first light blocking layer12ais larger than that of the first channel section14C1. The area of the second light blocking layer12bis larger than that of the second channel section14C2. As a result, illumination light toward a channel section14C from the backlight120at various angles is blocked.

As illustrated inFIG. 4, the first substrate10includes a platy first base11. The light blocking layer (the first light blocking layer12aand the second light blocking layer12b) is formed on the inner side (on the liquid crystal layer40side) of the first base11. A first interlayer insulating layer13is formed on the first base11so as to cover the light blocking layer12. The semiconductor layer14is formed on the first interlayer insulating layer13.

For the material of the semiconductor layer14, a known material such as polysilicon or an oxide semiconductor can be used. An oxide semiconductor composed of indium (In), gallium (Ga), zinc (Zn), and oxygen (O) can reduce a transient leak current and, thus, has a high ability (holding ability) of holding a voltage for video display for a long time. The oxide semiconductor thus described is effective for reducing the flicker in the intermittent driving. The use of polysilicon has advantages of high mobility and a high aperture ratio.

The semiconductor layer14may be formed of amorphous silicon. When the semiconductor layer14is formed of amorphous silicon, no lightly doped impurity region and no highly doped impurity region are required.

A gate insulating layer15is formed on the first interlayer insulating layer13so as to cover the semiconductor layer14. The scan line16, the first gate electrode16c, and the second gate electrode16dare formed on the gate insulating layer15. A second interlayer insulating layer17is formed on the gate insulating layer15so as to cover the scan line16, the first gate electrode16c, and the second gate electrode16d.

The signal line18, the source electrode18a, and the drain electrode19are formed on the second interlayer insulating layer17. The source electrode18ais electrically coupled to the source section14S (refer toFIG. 3) of the semiconductor layer14via the contact hole H1provided to the second interlayer insulating layer17and the gate insulating layer15. The drain electrode19is electrically coupled to the drain section14D (refer toFIG. 3) of the semiconductor layer14via the contact hole H2provided to the second interlayer insulating layer17and the gate insulating layer15.

A third interlayer insulating layer20is formed on the second interlayer insulating layer17so as to cover the signal line18, the source electrode18a, and the drain electrode19. The common electrode21is formed on the third interlayer insulating layer20. A capacitance insulating layer22is formed on the third interlayer insulating layer20so as to cover the common electrode21. The pixel electrode23is formed on the capacitance insulating layer22. The pixel electrode23is electrically coupled to the drain electrode19via the contact hole H3provided to the capacitance insulating layer22and the third interlayer insulating layer20. A first orientation film24is formed on the capacitance insulating layer22so as to cover the pixel electrode23. A first polarizing plate25is adhesively bonded on the outer surface side (a side opposite the liquid crystal layer40) of the first base11.

As illustrated inFIG. 5, a capacitance component Cs0is formed in a region where the pixel electrode23and the common electrode21face each other with the capacitance insulating layer22interposed therebetween. The capacitance component Cs0serves as a holding capacitor that holds a video display voltage applied between the pixel electrode23and the common electrode21. In a region where the pixel electrode23and the common electrode21do not face each other, an electric field is generated across a region from the common electrode21through the capacitance insulating layer22and the liquid crystal layer40to the pixel electrode23. The orientation of the liquid crystal layer40is controlled by the electric field. Letting capacitance components in the capacitance insulating layer22and the liquid crystal layer40formed along the electric field denote a capacitance component Cs1and a capacitance component Clc, respectively, the total capacitance component including the capacitance components Cs1and Clc serves as a capacitance component Cs (refer toFIG. 2) between the pixel electrode23and the common electrode21.

Referring back toFIG. 4, the second substrate30includes a platy second base31. A color filter layer32, an overcoat layer (not illustrated), and a second orientation film33are sequentially formed on the inner side (on the liquid crystal layer40side) of the second base31. A second polarizing plate34is adhesively bonded on the outer surface side (a side opposite the liquid crystal layer40) of the second base31.

The color filter layer32includes a color filter32aand a black matrix32b. The black matrix32bis formed in a grid shape so as to overlap with the scan line16, the signal line18, the semiconductor layer14, and the light blocking layer12(the first light blocking layer12aand the second light blocking layer12b). The belt-like electrodes23a, the first joining section23b1, and the second joining section23b2do not overlap with the black matrix32b. The region rimmed by the black matrix32b(the opening of the black matrix32b) is the sub pixel PX.

FIGS. 6 and 7are schematic diagrams illustrating exemplary distribution of the transient leak current of the thin film transistor provided to each sub pixel PX. In the specification, a period from the end of the supply of the gate signal (on voltage) to the gate electrode of the thin film transistor to the start of the next supply of the gate signal to the gate electrode is defined as an off period. The transient leak current is a current that flows between the semiconductor layer and the drain electrode of the thin film transistor during the off period.

The distribution of the transient leak current means the distribution of the value of the transient leak current of each sub pixel PX, which is obtained in a state where a voltage difference between the potential of the source electrode and the potential of the gate electrode of the thin film transistor is equalized in all of the sub pixels PX. The value of the transient leak current of each sub pixel PX is obtained by measuring a current that flows between the semiconductor layer and the drain electrode of the thin film transistor when a video signal of a maximum gradation (e.g., a gradation value of 255) is written into the sub pixel PX. Specifically, a voltage signal equivalent to that when a video signal of a maximum gradation (e.g., a gradation value of 255) is written into the sub pixel PX is applied, and the transient leak current flowing in the drain terminal is measured while probes are attached to the source (the signal line), the gate (the scan line), and the drain (the pixel electrode) of the thin film transistor.

As illustrated inFIG. 6, the liquid crystal display device100includes the display region110A including a plurality of the sub pixels PX. Each sub pixel PX includes the pixel electrode and the thin film transistor electrically coupled to the pixel electrode. The display region110A has a polygonal shape having a plurality of corner sections (a first corner section COA1, a second corner section COA2, a third corner section COA3, and a fourth corner section COA4), for example. In each of the corner sections of the display region110A, the sub pixels PX are arranged each of which includes the thin film transistor having a transient leak current smaller than that of the thin film transistor included in each of the sub pixels PX located in a central section CE of the display region110A. The sub pixels PX located in the central section CE means the sub pixels PX provided at the nearest positions from the center of the display region110A.

The end portion of the display region110A including the respective corner sections of the display region110A is a low leak region LLA in which the sub pixels are arranged that include thin film transistors each having a transient leak current smaller than that of the thin film transistor included in each of the sub pixels PX located in the central section CE. In the low leak region LLA, a plurality of the sub pixels PX are arranged. The central portion of the display region110A including the central section CE of the display region110A is a high leak region HLA in which the sub pixels are arranged that include thin film transistors each having a transient leak current larger than that of the thin film transistor included in each of the sub pixels PX provided in the low leak region LLA. The high leak region HLA includes at least half the sub pixels PX in the display region110A, for example. In the display region110A, the region excluding the low leak region LLA is the high leak region HLA. The border between the low leak region LLA and the high leak region HLA is curved toward the corner at each of the corners (a first corner CO1, a second corner CO2, a third corner CO3, and a fourth corner CO4) of the display region110A.

In the embodiment, the low leak region LLA is selectively provided to only each corner section of the display region110A. The position of the low leak region LLA is not limited to the example. As illustrated inFIG. 7, the low leak region LLA may be provided continuously to the whole circumference of the display region110A. In the example illustrated inFIG. 7, the low leak region LLA is expanded to respective side sections (a first side section SEA1, a second side section SEA2, a third side section SEA3, and a fourth side section SEA4) each connecting the adjacent corner sections.

The following describes a reason why the low leak region LLA is provided to the end portion of the display region110A.

As illustrated inFIG. 1, the seal material39that seals the outer circumference of the liquid crystal layer40(refer toFIG. 4) is provided to the periphery of the display region110A. If a sealability of the seal material39is insufficient, moisture enters the liquid crystal layer40via interfaces between the seal material39and the first substrate10and the second substrate30, for example, when the liquid crystal display device100is left in a high temperature and high humidity environment. Even if the sealability of the seal material39is sufficient, when the interlayer insulating layers and the overcoat layer of the color filter have moisture permeability, moisture may pass through the layers to enter the liquid crystal layer40.

When moisture enters the liquid crystal layer40, a resistance of the liquid crystal layer40is reduced, thereby causing the deterioration of the holding characteristic of the liquid crystal layer40. The absolute value of a video display voltage applied to the liquid crystal layer40is attenuated in the off period, thereby reducing luminance of a video image. The reduction of luminance occurs regardless of the polarity of the frame. Although the voltage is averaged between the multiple frames by the dot inversion driving or the column inversion driving, the luminance change remains without being cancelled, thereby causing the occurrence of periodical luminance change (flicker). Particularly, when the frame frequency is smaller than 40 Hz after the low frequency driving or the intermittent driving, the visibility of the flicker becomes high. As a result, the flicker is easily viewed by a user.

The flicker becomes larger at the end portion of the display region110A near the seal material39. Particularly, at the corner sections of the display region110A, the holding ratio is easily reduced because moisture enters the corner section from the two sides adjacent to each other with the corner section therebetween. In the embodiment, the reduction of the holding ability caused by the reduction of the holding ratio of the liquid crystal layer40is compensated by increasing the holding ability of the thin film transistors. The thin film transistor having a small transient leak current has a high holding ability of holding the video display voltage. Although a voltage leak occurs via the liquid crystal layer due to the reduction of the holding ratio, the change in voltage is reduced as a whole by reducing the voltage leak via the thin film transistors. As a result, the flicker is hardly viewed even at the end portion of the display region110A.

The following describes the structures of the thin film transistors in the central portion and in the end portion in the display region.FIG. 8is a schematic diagram illustrating the thin film transistor SW provided in the high leak region HLA.FIG. 9is a schematic diagram illustrating the thin film transistor SW provided in the low leak region LLA. In the following description, the thin film transistor SW provided in the high leak region HLA is described as a first thin film transistor SW1while the thin film transistor SW provided in the low leak region LLA is described as a second thin film transistor SW2in some cases.

In the embodiment, the value of the transient leak current of the thin film transistor SW is controlled by the size of the light blocking layer12. On the backlight120side (refer toFIG. 1) of the semiconductor layer14, the light blocking layer12is provided that overlaps with the semiconductor layer14. As the size of the light blocking layer12becomes larger, illumination light from the backlight120(refer toFIG. 1) illuminating the display region110A is blocked more effectively.

Letting an area of the region where the light blocking layer12overlaps with the semiconductor layer14denote a light blocking area of the thin film transistor SW, the light blocking area of the second thin film transistor SW2provided in the low leak region is larger than that of the first thin film transistor SW1provided in the high leak region. When the light blocking layer12includes a plurality of separated light blocking layers (a first separated light blocking layer12aand a second separated light blocking layer12b) as illustrated inFIGS. 8 and 9, the light blocking area is calculated as the sum of the light blocking areas of the respective separated light blocking layers (the areas of overlaps of the semiconductor layer14and the respective separated light blocking layers).

As for the light blocking layer12that covers the second thin film transistor SW2, it is preferable that the area of the first separated light blocking layer12ais larger than that of the second separated light blocking layer12b. It is preferable that the area of the first separated light blocking layer12athat covers the second thin film transistor SW2is larger than that of the first separated light blocking layer12athat covers the first thin film transistor SW1. The light blocking layer12may be integrated without being separated.

As illustrated inFIG. 1, the backlight120that illuminates the display region110A is provided on the rear side of the liquid crystal panel110. The light blocking layer12is provided on the backlight120side of the semiconductor layer14. The light blocking layer12blocks illumination light from the backlight120, thereby reducing the transient leak current. As the light blocking area blocked by the light blocking layer12is increased, an amount of illumination light entering the channel section14C of the thin film transistor SW is reduced, resulting in the transient leak current being smaller. That the transient leak current is small means that the transient leak current of one thin film transistor is smaller than that of the other thin film transistor in a plurality of the thin film transistors in each of which potential Vs of the source electrode and potential Vg of the gate electrode are equal to each other.

FIG. 10is a schematic diagram illustrating an exemplary distribution of the light blocking area of the thin film transistor in order to achieve the distribution of the transient leak current illustrated inFIG. 6. The abscissa axis ofFIG. 10represents the position of the sub pixel in the display region110A illustrated inFIG. 6while the ordinate axis ofFIG. 10represents the light blocking area of the thin film transistor. The position of the sub pixel means the position on a diagonal line of the display region110A. The diagonal line passes through the first corner CO1, the central section CE, and the third corner CO3, which are illustrated inFIG. 6. Although illustration is omitted, the distribution of the light blocking area along the other diagonal line of the display region110A, the diagonal line passing through the second corner CO2, the central section CE, and the fourth corner CO4, is the same as that illustrated inFIG. 10.

As illustrated inFIG. 10, in the low leak region LLA, the more the position of the sub pixel close to the outer circumference of the display region, the larger the light blocking area of the thin film transistor is. In the low leak region LLA, therefore, the more the position of the sub pixel close to the outer circumference of the display region, the smaller the transient leak current of the thin film transistor is. In the high leak region HLA, the light blocking area of the thin film transistor is constant regardless of the position of the sub pixel. This structure reduces the luminance change at the border between the low leak region LLA and the high leak region HLA, thereby causing the border to be hardly viewed. In the example illustrated inFIG. 10, the light blocking area is continuously increased as the position of the sub pixel gets closer to the outer circumference of the display region. The distribution of the light blocking area is, however, not limited to the example. For example, the line representing the distribution of the light blocking area may be shaped in a step-like manner as the position of the sub pixel gets closer to the outer circumference of the display region.

In the low leak region LLA, the more the position of the sub pixel close to the outer circumference of the display region, the larger the increasing ratio of the light blocking area is, for example. The increasing ratio of the light blocking area is a ratio of an amount of change in light blocking area to an amount of change in position in the display region. For example, the ratio is a gradient of a tangent to the curve representing the distribution of the light blocking area. InFIG. 10, the line representing the distribution of the light blocking area in the low leak region LLA is the downward convex curve, for example. As a result of the comparison of the light blocking areas of the thin film transistors among the sub pixels, the increasing ratio of the light blocking area in relation to the thin film transistors on a side near the central section CE of the display region110A is larger than that in relation to the thin film transistors on a side near the end portion of the display region110A.

FIG. 11is a schematic diagram illustrating an exemplary distribution of the light blocking area of the thin film transistor in order to achieve the distribution of the transient leak current illustrated inFIG. 7. The abscissa axis ofFIG. 11represents the position of the sub pixel in the display region110A illustrated inFIG. 7while the ordinate axis ofFIG. 11represents the light blocking area of the thin film transistor. The position of the sub pixel means the position on a horizontal line of the display region110A. The horizontal line passes a second midpoint SE2, the central section CE, and a fourth midpoint SE4, which are illustrated inFIG. 7. Although illustration is omitted, the distribution of the light blocking area along a vertical line of the display region110A, the vertical line passing through a first midpoint SE1, the central section CE, and a third midpoint SE3, is the same as that illustrated inFIG. 11. The distribution of the light blocking area along the diagonal line of the display region110A is the same as that illustrated inFIG. 10.

The first midpoint SE1is the point equidistant from the first corner CO1and the second corner CO2on the side connecting the first corner CO1and the second corner CO2. The second midpoint SE2is the point equidistant from the second corner CO2and the third corner CO3on the side connecting the second corner CO2and the third corner CO3. The third midpoint SE3is the point equidistant from the third corner CO3and the fourth corner CO4on the side connecting the third corner CO3and the fourth corner CO4. The fourth midpoint SE4is the point equidistant from the fourth corner CO4and the first corner CO1on the side connecting the fourth corner CO4and the first corner CO1.

In the distribution illustrated inFIG. 11, in the low leak region LLA, the more the position of the sub pixel close to the outer circumference of the display region, the larger the light blocking area of the thin film transistor is. The luminance change at the border between the low leak region LLA and the high leak region HLA along the horizontal direction is, thus, reduced. The distribution illustrated inFIG. 11is effective when the holding ratio of the liquid crystal at each side section (the first side section SEA1, the second side section SEA2, the third side section SEA3, and the fourth side section SEA4) of the display region110A is larger than that of the liquid crystal at the central section CE.

The light blocking area of the thin film transistor near each of the second midpoint SE2and the fourth midpoint SE4is smaller than that of the thin film transistor near each of the first corner CO1and third corner CO3illustrated inFIG. 10. The light blocking area of the thin film transistor near the midpoint of each side (the first midpoint SE1, the second midpoint SE2, the third midpoint SE3, and the fourth midpoint SE4) of the display region is smaller than that of the thin film transistor near each corner (the first corner CO1, the second corner CO2, the third corner CO3, and the fourth corner CO4) of the display region, although those relations are not illustrated. In the distribution of the light blocking area along each side of the display region, the light blocking area is minimum near the midpoint and as the position of the sub pixel is nearer the corner from the midpoint, the light blocking area is increased.

In this structure, the light blocking area near the midpoint where the holding ability of the liquid crystal is relatively high in the low leak region LLA is smaller than that at the corner section where the holding ability of the liquid crystal is the lowest. As a result, the luminance change in the low leak region is also reduced, thereby further enhancing display quality.

The following describes the operations and effects of the liquid crystal display device100in the embodiment.

FIG. 12is a schematic diagram illustrating a temporal change in the potential Vs of the source electrode, potential Vd of the drain electrode, potential Vc of the channel section, and the potential Vg of the gate electrode of the first thin film transistor SW1.FIG. 13is a schematic diagram explaining an occurrence mechanism of the transient leak current generated during the off period.FIG. 14is a schematic diagram illustrating the transient leak current of the first thin film transistor SW1. FIG.15is a schematic diagram illustrating a temporal change in the potential Vs of the source electrode, the potential Vd of the drain electrode, the potential Vc of the channel section, and the potential Vg of the gate electrode of the second thin film transistor SW2.FIG. 16is a schematic diagram illustrating the transient leak current of the second thin film transistor SW2.

As illustrated inFIG. 12throughFIG. 14, upon supply of a gate signal (on voltage Vg0) to the gate electrode of the first thin film transistor SW1at time0, the potential Vc of the channel section14C is increased from potential Vc0and the potential Vd of the drain electrode19is increased from Vd0(Vc0=Vd0), and each potential reaches a substantially constant value at time t0before the end of the supply of the gate signal. Just before time t1at which the supply of the gate signal ends (just before the start of the off period), the potential Vs of the source electrode18a, the potential Vc of the channel section14C, and the potential Vd of the drain electrode19are equal to one another, and are equal to potential Vs0of the video signal supplied from the signal line18.

Upon the end of the supply of the gate signal at time t1, the potential Vc of the channel section14C just below the gate electrode is markedly reduced due to capacity coupling. The potential of the drain electrode19, to which the pixel capacitance Cs (refer toFIG. 2) having a large value is coupled, is reduced as time elapses due to capacity coupling. The potential Vs of the source electrode18ais not reduced because the potential Vs is fixed to the potential of the video signal supplied from the signal line18.

During the off period (from time t1onward), the transient leak current flows between the semiconductor layer14and the drain electrode19, thereby causing a voltage applied to the liquid crystal layer to be changed. The transient leak current is generated by electrons e and holes h generated at the border between the source section14S and the channel section14C or at the border between the channel section14C and the drain section14D due to the influence of illumination light L0or heat from the backlight.

The channel section14C is charged by the transient leak current, resulting in the potential Vc of the channel section14C being increased. The transient leak current at the border between the channel section14C and the drain section14D is considered as charge reallocation between the channel section14C and the drain section14D. With the increase in potential Vc of the channel section14C, the potential vd of the drain electrode19is, thus, slightly decreased. The change of the potential Vd of the drain electrode19causes the voltage applied to the liquid crystal layer to be changed, resulting in the occurrence of the flicker.

As illustrated inFIGS. 15 and 16, the transient leak current is generated also in the second thin film transistor SW2just after the start of the off period. In the second thin film transistor SW2, the transient leak current is small because the light blocking layer12is larger than that in the first thin film transistor SW1. As a result, the change ratio of the potential Vd of the drain electrode19is smaller than that of the first thin film transistor SW1. InFIG. 15, the bold broken line depicted below the curve representing the temporal change in the potential Vd is the same as the curve representing the temporal change in the potential Vd illustrated inFIG. 12while the thin broken line depicted above the curve representing the temporal change in the potential Vc is the same as the curve representing the temporal change in the potential Vc illustrated inFIG. 12.FIG. 15illustrates those curves together for the comparison purpose.

FIG. 17is a schematic diagram illustrating an exemplary distribution of a luminance change amount of the sub pixel due to the reduction of the holding ratio of the liquid crystal layer.FIG. 18is a schematic diagram illustrating an exemplary distribution of a luminance change amount of the sub pixel due to the transient leak current.FIGS. 17 and 18each illustrates the distribution of the luminance change amount in the diagonal direction of the display region110A illustrated in each ofFIGS. 6 and 7.

As illustrated inFIG. 17, the reduction of the holding ratio of the liquid crystal layer increases toward the end of the display region110A, which is near the seal material. In the central portion of the display region110A, the holding ratio is substantially constant. The luminance change amount of the sub pixel due to the reduction of the holding ratio of the liquid crystal layer is, thus, large in the low leak region LLA in which the second thin film transistors SW2are arranged and substantially constant in the high leak region HLA. The holding ratio decreases toward the outer circumference of the display region110A. The luminance change amount of the sub pixel, thus, increases toward the outer circumference of the display region110A.

As illustrated inFIG. 10, the light blocking area in the low leak region LLA is larger than that in the high leak region HLA. Hence, as illustrated inFIG. 18, the luminance change amount of the sub pixel due to the transient leak current in the low leak region LLA is smaller than that in the high leak region HLA. The light blocking area increases toward the outer circumference of the display region110A. The luminance change amount of the sub pixel, thus, decreases toward the outer circumference of the display region110A.

The liquid crystal display device100in the embodiment makes it possible for the sub pixel PX in the low leak region LLA to have the luminance change amount substantially similar to that of the sub pixel in the high leak region HLA. The distribution of the light blocking area illustrated inFIG. 11is employed when the distribution of the luminance change amount similar to that illustrated inFIG. 17is achieved at each side section (the first side section SEA1, the second side section SEA2, the third side section SEA3, and the fourth side section SEA4) of the display region110A, although the distribution achieved at each side section is not illustrated.

If the light blocking areas of all of the thin film transistors SW provided in the display region110A are sufficiently large, the occurrence of the flicker can be reduced. The design that employs the distribution of the area of the light blocking layer12in the display region110A, such as that in the embodiment, can reduce the occurrence of the flicker at the end portion of the display region110A while maintaining the aperture ratio.

The liquid crystal display device100according to the embodiment includes the low leak region LLA at the end portion of the display region110A. The liquid crystal display device100can, thus, reduce the occurrence of the flicker at the end portion of the display region110A, where the holding ability of liquid crystal is relatively low.

The shape of the display region110A is not limited to a polygonal shape. The shape of the display region110A may be a shape having a curve such as an elliptical shape or a circular shape. In this case, the central portion including the center serving as the center of gravity of the display region110A is the high leak region HLA. The low leak region LLA is provided to the region that includes the end of the display region110A, which is disposed on the line passing through the center of the display region110A. It is preferable that the low leak region LLA be provided to the region including both ends of the display region110A, which are arranged on the line passing through the center of the display region110A. It is preferable that the sub pixel PX located farthest from the center of the display region110A is included in the low leak region LLA.

In the embodiment, the transient leak current of each thin film transistor included in at least the sub pixels PX at both ends of the display region110A among the plurality of the sub pixels PX provided on the lines passing through the center of the display region110A in plan view is smaller than that of each thin film transistor included in the sub pixels PX in the central section CE including the center. The transient leak current is controlled by the light blocking area, for example. The light blocking area of each of the sub pixels PX at both ends of the display region110A among the plurality of the sub pixels PX provided on the lines passing through the center of the display region110A in plan view is larger than that of each of the sub pixels PX in the central section CE including the center. When the display region110A is shaped in a shape having corner sections, it is preferable that the sub pixels PX each including the thin film transistor having a transient leak current smaller than that of each thin film transistor included in the sub pixels PX located at the center section CE are arranged in the corner sections including both ends of the display region110A.

Second Embodiment

FIG. 19is a schematic diagram illustrating the first thin film transistor SW1in a liquid crystal display device according to a second embodiment.FIG. 20is a schematic diagram illustrating the second thin film transistor SW2in the liquid crystal display device according to the second embodiment. In the second embodiment, the same structures as in the first embodiment are labeled with the same reference numerals, and detailed descriptions thereof are, thus, omitted.

The second embodiment differs from the first embodiment in that the value of the transient leak current of the thin film transistor is adjusted by electric resistance of the source section14S or the drain section14D. The electric resistance of the source section14S is the electrical resistance of a current flowing from the source electrode to the region that faces the gate electrode in the semiconductor layer14while the electric resistance of the drain section14D is the electrical resistance of a current flowing from the drain electrode to the region that faces the gate electrode in the semiconductor layer14.

Letting the ratio (Rs/Rd) of an electrical resistance Rs of the source section14S of the semiconductor layer14and an electrical resistance Rd of the drain section14D of the semiconductor layer14denote a source drain resistance ratio of the thin film transistor SW, the source drain resistance ratio of the second thin film transistor SW2provided in the low leak region is smaller than that of the first thin film transistor SW1provided in the high leak region. For example, a width W3of the source section14S of the second thin film transistor SW2provided in the low leak region is larger than a width W1of the source section14S of the first thin film transistor SW1provided in the high leak region. A width W2of the drain section14D of the second thin film transistor SW2provided in the low leak region is equal to a width W2of the drain section14D of the first thin film transistor SW1provided in the high leak region.

The width (W1or W3) of the source section14S is the width of the source section14S at the intersection between the edge of the first light blocking layer12aand the source section14S. The width (W2) of the drain section14D is the width of the drain section14D at the intersection between the edge of the second light blocking layer12band the drain section14D. InFIGS. 19 and 20, W1, W2, and W3are located at the positions shifted from the positions of the respective edges of the light blocking layers for clear illustration.

In the embodiment, the source drain resistance ratio is controlled by enlarging the width of the source section14S (reducing the electrical resistance of the source section14S) of the second thin film transistor SW2. The way to control the source drain resistance ratio is not limited to this manner. For example, the source drain resistance ratio may be controlled by adjusting the length and/or the impurity concentration of the source section14S. The source drain resistance ratio may be controlled by adjusting the width, the length, and/or the impurity concentration of the drain section14D.

FIG. 21is a schematic diagram illustrating a temporal change in the potential Vs of the source electrode, the potential Vd of the drain electrode, the potential Vc of the channel section, and the potential Vg of the gate electrode of the second thin film transistor SW2.FIG. 22is a schematic diagram illustrating the transient leak current of the second thin film transistor SW2. InFIG. 21, the bold broken line depicted below the curve representing the temporal change in the potential Vd is the same as the curve representing the temporal change in the potential Vd illustrated inFIG. 12while the thin broken line depicted above the curve representing the temporal change in the potential Vc is the same as the curve representing the temporal change in the potential Vc illustrated inFIG. 12.FIG. 21illustrates those curves together for the comparison purpose.

The transient leak current of the first thin film transistor SW1is the same as that described with reference toFIG. 12throughFIG. 14. The source drain resistance ratio of the second thin film transistor SW2is smaller than that of the first thin film transistor SW1. The transient leak current generated at the border between the source section14S and the channel section14C during the off period in the second thin film transistor SW2is larger than that in the first thin film transistor SW1. The channel section14C is, thus, charged faster, resulting in the potential Vc of the channel section14C being increased faster. The potential difference between the channel section14C and the drain section14D is, thus, reduced faster than that in the first thin film transistor SW1, resulting in the transient leak current between the channel section14C and the drain section14D being smaller than that in the first thin film transistor SW1. As a result, the temporal change ratio of the potential Vd of the drain electrode19is smaller than that of the first thin film transistor SW1.

As described above, in the embodiment, the source drain resistance ratio, which is the electrical resistance ratio of the source section to the drain section of the semiconductor layer, in the low leak region is smaller than that in the high leak region. In the low leak region, the transient leak current generated at the border between the source section14S and the channel section14C is larger than that generated at the border between the drain section14D and the channel section14C. As a result, the potential change in the drain electrode19is reduced. The mechanism of reduction of the potential change in the drain electrode19differs from that in the first embodiment. The second embodiment can, however, achieve the same effects as in the first embodiment. The second embodiment can also provide the liquid crystal display device having little flicker.

If the widths of the source sections14S of all of the thin film transistors SW provided in the display region110A are sufficiently large, the occurrence of the flicker can be reduced. The design that employs the distribution of the width of the source section14S in the display region110A, such as that in the embodiment, can reduce the occurrence of the flicker at the end portion of the display region110A while maintaining the aperture ratio. In the embodiment, the low leak region LLA is provided at the end portion of the display region110A, and the transient leak current of each thin film transistor included in at least the sub pixels PX at both ends of the display region110A among the plurality of the sub pixels PX provided on the lines passing through the center of the display region110A in plan view is smaller than that of each thin film transistor included in the sub pixels PX in the central section CE including the center. In the embodiment, the transient leak current is controlled by the source drain resistance ratio of the semiconductor layer14. The source drain resistance ratio of the semiconductor layer14of each of the sub pixels PX at both ends of the display region110A among the multiple sub pixels PX provided on the lines passing through the center of the display region110A in plan view is smaller than that of each of the sub pixels PX in the central section CE including the center.

Third Embodiment

FIG. 23is a schematic diagram illustrating the first thin film transistor SW1in a liquid crystal display device according to a third embodiment.FIG. 24is a schematic diagram illustrating the second thin film transistor SW2in the liquid crystal display device according to the third embodiment. In the third embodiment, the same structures as in the second embodiment are labeled with the same reference numerals, and detailed descriptions thereof are, thus, omitted.

The third embodiment differs from the second embodiment in that the thin film transistor SW has a lightly doped drain (LDD) structure, the thin film transistor SW has a single gate structure, and the size of the transient leak current of the thin film transistor SW is adjusted by the length of the lightly doped impurity region of the source section14S (by a difference in LDD structure). The length of the lightly doped impurity region means the length in the direction from the source electrode or the drain electrode toward the channel section14C.

In the source section14S, a lightly doped impurity region14S2is provided on a side near the channel section14C and a highly doped impurity region14S1is provided on a side far from the channel section14C. In the drain section14D, a lightly doped impurity region14D2is provided on a side near the channel section14C and a highly doped impurity region14D1is provided on a side far from the channel section14C.

A length W6of the lightly doped impurity region1452in the source section14S of the second thin film transistor SW2is longer than a length W4of the lightly doped impurity region14S2in the source section14S of the first thin film transistor SW1. A length W5of the lightly doped impurity region14D2in the drain section14D of the second thin film transistor SW2is equal to a length W5of the lightly doped impurity region14D2in the drain section14D of the first thin film transistor SW1.

As the length of the lightly doped impurity region14S2in the source section14S is increased, the absorption amount of illumination light from the backlight is increased. The transient leak current generated at the border between the channel section14C and the source section14S is, thus, increased, thereby reducing the potential change in the drain electrode19. The mechanism of reduction of the potential change in the drain electrode19differs from that in the second embodiment. The third embodiment can, however, achieve the same effects as in the second embodiment. The third embodiment can also provide the liquid crystal display device having little flicker.

Fourth Embodiment

FIG. 25is a schematic diagram illustrating the first thin film transistor SW1in a liquid crystal display device according to a fourth embodiment.FIG. 26is a schematic diagram illustrating the second thin film transistor SW2in the liquid crystal display device according to the fourth embodiment.FIG. 27is a cross sectional view of the thin film transistor SW. In the fourth embodiment, the same structures as in the second embodiment are labeled with the same reference numerals, and detailed descriptions thereof are, thus, omitted.

The fourth embodiment differs from the second embodiment in that the thin film transistor SW has the single gate structure, and the source drain resistance ratio of the thin film transistor SW is adjusted by the areas of a region where the source section14S of the semiconductor layer14overlaps with the source electrode18aand a region where the drain section14D of the semiconductor layer14overlaps with the drain electrode19.

As illustrated inFIG. 25, the semiconductor layer14is formed partially in the region where the branched section16bis formed. The branched section16bis formed with an area larger than that of the semiconductor layer14and is disposed so as to cover the whole of the backlight side of the semiconductor layer14. The branched section16bserves as the light blocking layer. Another layer (light blocking layer) having the light blocking function may be provided at the lower layer (the layer on the backlight120side) of the branched section16b. The branched section16boverlapped with the semiconductor layer14serves as the gate electrode of the thin film transistor SW. The source electrode18aand the drain electrode19of the thin film transistor SW overlap with the semiconductor layer14in the region where the semiconductor layer14and the branched section16b(the gate electrode of the thin film transistor SW) overlap with each other.

The source electrode18abranches from the signal line18and extends toward the semiconductor layer14side. The semiconductor layer14has a bombshell-like shape composed of three straight sides and one curved side. The semiconductor layer14is disposed such that the curved side faces the signal line18. The source electrode18ais formed in a U shape along the one curved side and the two sides adjacent to the one side of the semiconductor layer14. The drain electrode19includes a line section19athat extends in parallel with the scan line16. The line section19ais placed in the region surrounded by the source electrode18ain a U shape while a constant gap is kept between the line section19aand the source electrode18a.

The source electrode18aand the line section19aare layered above the semiconductor layer14with an ohmic contact layer52(refer toFIG. 27) interposed therebetween. The whole surfaces facing the semiconductor layer14of the source electrode18aand the line section19aare electrically coupled to the semiconductor layer14with the ohmic contact layer52interposed therebetween. The region of the semiconductor layer14that overlaps the source electrode18ais the source section14S. The region of the semiconductor14that overlaps the line section19a(the drain electrode19) is the drain section14D. The region of the semiconductor14disposed between the source section14S and the drain section14D is the channel section14C.

As illustrated inFIG. 27, the scan line16is formed on the first base11. The gate insulating layer51is formed on and above the first base11while covering the scan line16. The semiconductor layer14is formed on the gate insulating layer51. The ohmic contact layer52is formed on and above the gate insulating layer51so as to cover the semiconductor layer14. The source electrode18a, the signal line18, and the drain electrode19are laminated on the ohmic contact layer52.

The ohmic contact layer52is an impurity semiconductor layer for achieving a good ohmic contact between the semiconductor layer14and both of the source electrode18aand the drain electrode19. The ohmic contact layer52is patterned together with the source electrode18a, the signal line18, and the drain electrode19. The ohmic contact layer52is, thus, selectively formed in only the region where the source electrode18a, the signal line18, and the drain electrode19are formed.

Above the gate insulating layer51, the third interlayer insulating layer20is formed so as to cover the semiconductor layer14, the source electrode18a, the signal line18, and the drain electrode19. The structure on the upper side (on a side where the liquid crystal layer is provided) of the third interlayer insulating layer20is the same as that illustrated inFIG. 4.

As illustrated inFIGS. 25 and 26, in the embodiment, the source drain resistance ratio of the thin film transistor SW is adjusted by the areas of the region where the source section14S of the semiconductor layer14overlaps with the source electrode18aand the region where the drain section14D of the semiconductor layer14overlaps with the drain electrode19. The larger the area of the source section14S is, the smaller the electrical resistance of the source section14S while the larger the area of the drain section14D is, the smaller the electrical resistance of the drain section14D is.

A width W8of the source electrode18aformed along the three sides of the semiconductor layer14of the second thin film transistor SW2is larger than a width W7of the source electrode18aformed along the three sides of the semiconductor layer14of the first thin film transistor SW1. The area of the source section14S of the second thin film transistor SW2provided in the low leak region is, thus, larger than that of the source section14S of the first thin film transistor SW1provided in the high leak region. The area of the drain section14D of the second thin film transistor SW2is equal to that of the drain section14D of the first thin film transistor SW1. As a result, the source drain resistance ratio of the second thin film transistor SW2is smaller than that of the first thin film transistor SW1.

In the embodiment, the source drain resistance ratio is controlled by increasing the area of the source section14S (reducing the electrical resistance of the source section14S) of the second thin film transistor SW2. The way to control the source drain resistance ratio is not limited to this manner. For example, the source drain resistance ratio may be controlled by adjusting the impurity concentration of the source section14S. The source drain resistance ratio may be controlled by adjusting the area or the impurity concentration of the drain section14D.

The embodiment can also obtain the same effects as in the second embodiment. The fourth embodiment can also provide the liquid crystal display device having little flicker.

Fifth Embodiment

FIG. 28is a schematic diagram illustrating the first thin film transistor SW1in a liquid crystal display device according to a fifth embodiment.FIG. 29is a schematic diagram illustrating the second thin film transistor SW2in the liquid crystal display device according to the fifth embodiment. In the fifth embodiment, the same structures as in the third embodiment are labeled with the same reference numerals, and detailed descriptions thereof are, thus, omitted.

The fifth embodiment differs from the third embodiment in that the thin film transistor SW does not have the LDD structure, the value of the transient leak current of the thin film transistor is adjusted by electric resistance of the source section14S or the drain section14D. The electric resistance of the source section14S is the electrical resistance of a current flowing from the source electrode to the region of the semiconductor layer14that faces the gate electrode in the semiconductor layer14while the electric resistance of the drain section14D is the electrical resistance of a current flowing from the drain electrode to the region of the semiconductor layer14that faces the gate electrode.

In the embodiment, a width W11of the source section14S of the second thin film transistor SW2provided in the low leak region is larger than a width W9of the source section14S of the first thin film transistor SW1provided in the high leak region, for example. A width W10of the drain section14D of the second thin film transistor SW2provided in the low leak region is equal to a width W10of the drain section14D of the first thin film transistor SW1provided in the high leak region. As a result, the source drain resistance ratio of the second thin film transistor SW2provided in the low leak region is smaller than that of the first thin film transistor SW1provided in the high leak region. The fifth embodiment, thus, provides the liquid crystal display device having little flicker.

The width (W9or W11) of the source section14S is the width of the source section14S at the intersection between the edge of the gate electrode (the branched section16b) and the source section14S. The width (W10) of the drain section14D is the width of the drain section14D at the intersection between the edge of the gate electrode and the drain section14D. InFIGS. 28 and 29, W9, W10, and W11are located at the positions shifted from the positions of the respective edges of the gate electrode for clear illustration.

In the embodiment, the source drain resistance ratio is controlled by enlarging the width of the source section14S (reducing the electrical resistance of the source section14S) of the second thin film transistor SW2. The way to control the source drain resistance ratio is not limited to this manner. For example, the source drain resistance ratio may be controlled by adjusting the length and/or the impurity concentration of the source section14S. The source drain resistance ratio may be controlled by adjusting the width, the length, and/or the impurity concentration of the drain section14D.

Material of Liquid Crystal Layer

In the first through fourth embodiments, the liquid crystal layer40is made of a negative liquid crystal material having negative dielectric anisotropy. The liquid crystal material used satisfies the following conditions.
|Δε|≤4.0  (1)
Δn≤0.11  (2)
γ1≥100 where |Δε|=3.5  (3)

Δε represents the dielectric anisotropy of the liquid crystal material when the temperature of the liquid crystal material is 25° C. An represents refractive index anisotropy (at wavelength λ is 589 nm) of the liquid crystal material when the temperature of the liquid crystal material is 25° C. γ1 represents a rotation viscosity coefficient of the liquid crystal material when the temperature of the liquid crystal material is 20° C.

The use of the negative liquid crystal material easily causes the generation of the flicker when the low frequency driving or the intermittent driving is performed. The cause may be impedance mismatching due to the low resistivity of the liquid crystal material. Particularly in a high temperature environment, the occurrence of the flicker tends to increase. The inventor found a strong correlation between the physical property values of the liquid crystal material and a flicker level and thus conceived conditions (1) to (3) described above. The embodiment uses the liquid crystal material satisfying conditions (1) to (3), thereby making it possible to effectively reduce the flicker in a high temperature environment.

FIGS. 30 and 31are schematic diagrams illustrating the relation between the easiness of the generation of the flicker (the flicker level) in a high temperature environment versus Δε, Δn, and γ1. InFIGS. 30 and 31, “double circle”, “circle”, “square”, “filled triangle”, and “cross” symbols represent the flicker level from a low level (hard to generate the flicker) in this order.

The absolute value of the dielectric anisotropy |Δε| and the refractive index anisotropy Δn were found as the physical property values correlated to the flicker level. The experiment result (refer toFIG. 30) was obtained that as the values of both physical property values are reduced, the flicker level becomes lower (better). In addition, it was found that when the values of |Δε| and Δn are too small, a modulation ratio and a response speed, which are other characteristics than the flicker, deteriorate (refer to the broken line inFIG. 30). It was found that it is effective for achieving a good flicker characteristic at high temperatures to use the liquid crystal material satisfying conditions (1) and (2). In addition, it was found that the liquid crystal display device satisfying flicker, modulation ratio, and the response speed characteristics and employing the low frequency driving method can be achieved using the liquid crystal material having values of |Δε| and Δn limited to those determined by the broken line illustrated inFIG. 30.

From the inventor's experiment, it was found that the flicker characteristic deteriorates when the liquid crystal material having a small rotation viscosity coefficient γ1 is used. For example, the flicker level was the level denoted by the cross symbol when the liquid crystal having γ1 (20° C.) was 99 mPa·s where Δn was 0.1 and |Δε| was 3.5. For achieving a good flicker level, condition (3) was necessary. Because γ1 depends on |Δε|, condition (3) was specified when |Δε| was 3.5.

The present invention is not limited to the embodiments described above. The disclosed contents in the embodiments are examples. The embodiments can be changed in various ways without departing from the spirit of the invention. The appropriate changes performed without departing from the spirit of the invention also belong to the technical scope of the invention.