JP4293434B2 - Liquid crystal display - Google Patents

Description

  The invention disclosed in this specification relates to a liquid crystal display device. In particular, the present invention relates to a liquid crystal display device in which an active matrix circuit is composed of thin film transistors.

  Currently, cathode ray tubes are the mainstream display devices. Advantages of a cathode ray tube include high contrast and high resolution. On the other hand, there are drawbacks such as a heavy container for holding a vacuum and a single light source that occupies depth. As one of new display devices that solve these problems, a liquid crystal display has been developed.

  Liquid crystal displays are lighter and thinner than CRTs. However, high contrast cannot be obtained with the simple matrix driving method used in the early stages of development. Therefore, a method using an active matrix circuit is attracting attention in order to drive a liquid crystal display.

  In the active matrix circuit, a liquid crystal is sandwiched between a pixel electrode and a counter electrode to form a capacitor, and electric charges entering and leaving the capacitor are controlled by a thin film transistor (hereinafter abbreviated as TFT). In order to display an image stably, it is required that the voltage between both electrodes of the capacitor be kept constant.

  However, it has been difficult to display an image stably for several reasons. The biggest problem is that charge leaks even when the TFT is off. In addition, electric charge may leak inside the capacitor, but in general, the leakage from the former TFT is about one digit larger. When this leak is severe, a phenomenon called flicker occurs in which the brightness of an image changes at the same cycle as the frame frequency.

  There has been considered a method in which a capacitor is formed of a wiring metal in parallel with a capacitor sandwiching liquid crystal so that the leaked charge becomes a negligible amount. This method has disadvantages such as a decrease in the aperture ratio of the pixel, an increase in the time for charging the capacitor, and a reduction in the operation speed.

  In order to improve the characteristics of TFTs used for active matrix circuits, a technique for crystallizing an active layer of a semiconductor film is employed. The main methods include a solid phase growth method by heating and a method of lowering the crystallization barrier energy by some catalytic action. The latter method is suitable for use in TFTs, diodes, and resistors because the active layer of the semiconductor film undergoes continuous crystal growth by catalysis and thus can be regarded as a substantially single crystal. .

  In this crystallization method, since the catalytic element moves in the progress direction of crystal growth, the catalytic element is present at a high concentration in the region where it was first added and in the end point of the crystal growth. Considering the time required for crystallization, a catalytic element exists in the vicinity of the channel region. However, if the catalytic element is present in a channel portion that should be an intrinsic semiconductor, the characteristics of the TFT are deteriorated. Therefore, it is necessary to control the direction of crystal growth by selectively adding a catalyst.

  Using the above active matrix circuit manufacturing technique, the liquid crystal driving circuit is manufactured on the same substrate as the TFT constituting the pixel. In this case, in addition to the pixel matrix, the signal line driving circuit and the scanning line driving circuit are constituted by TFTs. The line sequential scanning signal line driver circuit shown in FIG. 1 includes a shift register circuit, a sampling circuit, a transfer circuit, and an analog buffer circuit. In the shift register, a start pulse synchronized with the video signal (101) is input to the input terminal (102) and sequentially shifted by the clock pulse (103). The output of the shift register is input to the sampling circuit via the inverter type buffer circuit (104).

  The sampling circuit includes a switch (105) called a transmission gate and a holding capacitor (106). The transmission gate is controlled to be turned on and off by the buffer circuit (104). In the on state, the video signal line (101) and the holding capacitor (106) are short-circuited, and electric charge is stored in the holding capacitor (106). . When the start pulse passes through the shift register, the output of the buffer circuit (104) is inverted and the switch (105) is turned off. The charge in the storage capacitor (106) is held as it is, and the potential is maintained until the switch (105) is turned on next time.

  The transfer signal is input from the transfer signal input terminal (107) until sampling for one line is completed and the next sampling is started. As a result, the switch (108) is turned on, the holding capacitor (106) and the holding capacitor (109) holding the input potential of the analog buffer are short-circuited, and the potential is transmitted. At this time, if the value of the storage capacitor (106) is sufficiently larger than the storage capacitor (109), the potential change due to the short circuit is small. An analog buffer is connected to the storage capacitor (109), and the signal lines (110) to (112) are driven through the analog buffer. The analog buffer is necessary for driving the signal lines (110) to (112) without affecting the potential on the input side.

  Here, a large leakage current of the TFT means that the potential of the storage capacitor (109) cannot be maintained, which leads to deterioration in image quality. Furthermore, the leakage current of the TFT becomes a noise generation source in the analog buffer.

  If the leakage current of the TFT is reduced as described above, it is possible to display an image stably without connecting a capacitor in parallel with the liquid crystal. Further, by reducing the leakage current of the TFT, it is effective in improving the image quality even when the liquid crystal driving circuit is manufactured on the same substrate as the TFT constituting the pixel.

  Hereinafter, a source of leakage current will be described. For example, the pixel in the active matrix circuit has the structure shown in FIG. (201) is a metal wiring constituting the gate electrode and the wiring, and (202) is a metal wiring connected to the source electrode. (203) is an active layer of the semiconductor film, and (204) is a transparent electrode forming one side of the capacitor electrode sandwiching the liquid crystal. Reference numeral (205) denotes a contact hole.

  When the TFT portion is enlarged, it becomes as shown in FIG. (301) is a gate electrode, and (302) is an active layer of a semiconductor film. Here, the leak current is assumed to occur at the edge portion (303). As a cause of this, at the edge portion (303), the gate electrode (301) causes a short circuit between the source electrode and the drain electrode due to poor insulation of the gate electrode (301). Alternatively, the periphery of the active layer (302) of the semiconductor film does not have a crystal structure due to damage caused by etching or ion doping. Hereinafter, a short circuit by the gate electrode will be described.

  As shown in FIG. 4A, consider a case where the gate insulating film (402) completely covers the active layer (401) of the semiconductor film. When a voltage equal to or lower than the threshold is applied to the gate electrode (403) in a state where no channel is formed, almost no current flows because the active layer (401) of the semiconductor film has a high resistance. Therefore, no current flows between the drain electrode (for example, the front area) and the source electrode (for example, the back area).

  On the other hand, consider the case where the gate insulating film (405) does not completely cover the active layer (404) of the semiconductor film as shown in FIG. 4 (b). When the insulating film (405) is formed on the surface of the active layer (404) in order to insulate the gate electrode (406) from the active layer (404) of the semiconductor film, the side surface (407) The insulating film (405) is difficult to be formed, and the side surface (407) may be exposed. In this state, the gate electrode (406) and the active layer (404) of the semiconductor film are short-circuited on the side surface (407). For this reason, when a voltage lower than the threshold value is applied to the gate electrode (406), the drain electrode and the source electrode are always short-circuited by the gate electrode even when the channel is not formed, and a leak current is generated. End up.

  In general, because of the manufacturing process, it is difficult to form a thin film on the side surface of the step portion of the active layer. Therefore, the side surface of the active layer (404) is completely covered with the insulating film (405) as shown in FIG. It is easy to happen. For this reason, the leak current flows through the edge portion (303) in FIG. Conversely, the leakage current can be reduced if there is no structural portion such as the edge portion (303) in FIG.

  An object of the present invention is to solve the above-described problems and provide a liquid crystal display device capable of reducing a leakage current in a TFT and stably displaying with good contrast. In particular, a method for suppressing the source of leakage current by devising the electrode structure of the TFT is proposed.

  Another object of the present invention is to devise the TFT electrode structure to suppress the source of leakage current and selectively add a catalyst to crystallize the active layer of the TFT. This is to improve the performance of the liquid crystal display device.

  In order to solve the above problems, the present invention reduces the leakage current by adopting a structure in which the edge of the active layer of the semiconductor film does not coincide with the line connecting the source electrode and the drain electrode. Specifically, a TFT electrode structure as shown in FIG.

  In FIG. 5A, the gate electrode (502), the electrodes (501), and (503) are arranged so that the center of the active layer (504) composed of a crystalline silicon thin film or the like is the center of symmetry. ing. Also, by making the outer shapes of these electrodes (501) to (503) substantially similar to each other, one of the electrodes (501) and (503) is used as a source electrode, and the other is used as a drain electrode. The edge of the active layer (504) is configured not to exist on the line connecting the source electrode and the drain electrode. Accordingly, the drain electrode and the source electrode are not short-circuited by the gate electrode, and the leakage current can be reduced.

  The present invention is characterized in that a pixel thin film transistor includes a thin film transistor having a structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. .

  The present invention has a pixel thin film transistor having a structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. To do.

  The present invention is characterized in that a pixel thin film transistor includes a thin film transistor having a structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode.

  The present invention has a pixel thin film transistor having a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode. To do.

  The present invention is an electrode structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to substantially surround the gate electrode, and the drain electrode is made of a transparent conductor. The thin film transistor thus configured is provided as a pixel thin film transistor.

  The present invention provides an electrode structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to substantially surround the gate electrode, and the drain electrode is a transparent conductor. The pixel thin film transistor is a pixel thin film transistor.

  The present invention is an electrode structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode, and the drain electrode is formed of a transparent conductor. The thin film transistor is provided as a pixel thin film transistor.

  The present invention is an electrode structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode, and the drain electrode is made of a transparent conductor. The thin film transistor thus configured is provided as a pixel thin film transistor.

  In the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed on the outside of the gate electrode so as to substantially surround the gate electrode. It is characterized by that.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. It is characterized by doing.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode. It is characterized by.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a drain electrode, and a source electrode is disposed outside the gate electrode so as to surround the gate electrode. It is characterized by that.

  The present invention has a pixel thin film transistor having a thin film transistor having an electrode structure in which a gate electrode is disposed so as to surround a source electrode and a drain electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. And

  According to the present invention, a pixel thin film transistor includes a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. Features.

  The present invention includes a pixel thin film transistor having a thin film transistor having an electrode structure in which a gate electrode is disposed so as to surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to surround the gate electrode. And

  The present invention is characterized in that a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a source electrode and a drain electrode is disposed outside the gate electrode so as to surround the gate electrode is provided as a pixel thin film transistor. And

  According to the present invention, a peripheral driving circuit is configured by a thin film transistor having a structure in which a gate electrode is disposed so as to surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. Features.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to substantially surround the gate electrode. It is characterized by doing.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to surround the gate electrode. It is characterized by.

  According to the present invention, a peripheral driving circuit is constituted by a thin film transistor having an electrode structure in which a gate electrode is disposed so as to substantially surround a source electrode, and a drain electrode is disposed outside the gate electrode so as to surround the gate electrode. It is characterized by that.

  Further, the present invention has an electrode structure in which the gate electrode, the drain electrode, and the source electrode are arranged substantially concentrically in the thin film transistor included in the pixel thin film transistor or the thin film transistor constituting the peripheral driver circuit. It is characterized by.

  In the thin film transistor included in the pixel thin film transistor or the thin film transistor included in the peripheral driver circuit, the gate electrode, the drain electrode, and the source electrode may have a rectangular shape with a substantially symmetrical point, or It has a polygonal electrode structure.

  Further, according to the present invention, the thin film transistor included in the pixel thin film transistor or the thin film transistor constituting the peripheral driving circuit is crystallized by a catalyst addition region having a shape similar to or close to the drain electrode or the source electrode. Features.

  In the liquid crystal display device according to the present invention, even if the gate electrode crosses the edge portion of the active layer of the semiconductor film in the TFT electrode structure, the potential of the edge portion of the semiconductor film crossed by the gate electrode is made equal, and this Since the edge portion is not on the line connecting the source electrode and the drain electrode and the gate electrode prevents the source electrode and the drain electrode from being short-circuited, leakage current generated at the edge portion of the semiconductor film can be reduced. In the TFT, by reducing the leakage current, the capacitor in the pixel portion can surely hold the potential. If the potential can be maintained, the input state of the video signal can be constantly maintained until the next potential update. Therefore, in the liquid crystal display device, it is possible to avoid a decrease in contrast and occurrence of flicker.

FIG. 5A and FIG. 5B are plan configuration diagrams of the TFT of Example 1. FIG.
In FIG. 5A, the TFT electrodes are concentrically arranged, and the electrode to be the drain electrode or the source electrode is surrounded by the gate electrode. In the active layer (504) of the rectangular semiconductor film, a gate electrode (502) and an annular electrode (503) are disposed so as to surround the outside of the circular electrode (501). Note that one of the electrode (501) and the electrode (503) may be a source electrode and the other may be a drain electrode. These electrodes (501) to (503) are arranged concentrically with the center at the center of the active layer (504).

  The electrode (501) is arranged in a layer different from the wiring metal constituting the gate electrode. For this reason, the electrode (501) and the gate electrode (502) may overlap. On the other hand, the electrode (501) and the electrode (503) are made of the same layer of wiring metal. Therefore, in order to avoid a short circuit, the electrode (503) has a shape in which a part of the ring is missing.

  In the above electrode structure, there is a portion where the wiring metal constituting the gate electrode (502) crosses the edge of the active layer (504) of the semiconductor film. However, since both sides of the edge of the active layer (504) of the semiconductor film to be short-circuited are electrically at the same potential, there is no problem.

  For example, when a TFT having the electrode structure of this embodiment is used as a pixel TFT, a protective film is formed after forming the TFT and wiring, and the pixel electrode is formed of a transparent conductor. In FIG. 5A, the electrode (501), the gate electrode (502), and the electrode (503) are wired in different layers, and the electrode (501) is made of a transparent conductor using the electrode (501) as a drain electrode. A pixel TFT can be formed by forming a wiring metal using the electrode (503) as a source electrode. By adopting such a structure, the leakage current can be reduced, and the wiring capacitance between the electrode (501) used as the drain electrode and the gate electrode (502) can be reduced.

  Reduction of the leakage current is effective in reliably holding charges in a structure in which TFTs are used as charge control switches and charges are stored in capacitors. If the loss of charge is reduced, the capacity can be reduced to the allowable potential displacement, leading to area reduction and microfabrication. Further, when the electrode (501) is a source electrode and the electrode (503) is a drain electrode, the contact area between the source region and the channel formation region (contact area between the source region and the channel formation region) is compared. Since the contact area between the drain region and the channel formation region can be increased, the electric field strength between the drain and the channel can be substantially reduced. Therefore, the effect of reducing the leakage current can be obtained as in the case where the LDD structure is adopted.

  When the three-layer metal wiring in which the gate electrode, the wiring metal, and the transparent conductor are formed in different layers as described above and the wiring capacity can be ignored, the source electrode and the drain electrode Therefore, a TFT as shown in FIG. 5B can be formed without forming a notch in the electrode (503).

  An annular gate electrode (506) and an annular electrode (507) are arranged substantially concentrically so as to surround the outer side of the circular electrode (505). Since the electrodes (505) and (507) are formed in different layers, notches are not formed in the electrode (507). Note that one of the electrodes (505) and (507) may be used as a source electrode and the other may be used as a gate electrode. In particular, when a TFT having a structure in which the electrode (507) is formed of a metal wiring as a source electrode and the electrode (505) is formed of a transparent conductor as a drain electrode is used as a pixel TFT, it is effective against leakage current.

  6A and 6B are plan structural views of the TFT of Example 2. FIG. This embodiment is characterized in that the electrodes are arranged concentrically.

  In FIG. 6A, on the active layer (604) of the semiconductor film, an annular gate electrode (602) and an annular electrode are concentrically formed so as to surround the outer side of the circular electrode (601). (603) is arranged. The electrode (601) is composed of a metal wiring layer different from the wiring metal constituting the gate electrode (602), and the electrode (601) and the electrode (603) are composed of the same layer of wiring metal.

  Here, since the capacitor is formed when the electrode (601) and the gate electrode (602) overlap, in order to avoid the formation of this capacitor, the shape of the gate electrode (602) is a part of the ring. The shape lacks. Therefore, [Embodiment 1] may be used when this capacitor can be ignored, and this embodiment may be used when the capacitor cannot be ignored.

  Further, in order to avoid a short circuit between the electrode (601) and the electrode (603), the electrode (603) also has a shape in which a part of the ring is missing. Note that one of the electrode (601) and the electrode (603) may be a source electrode and the other may be a drain electrode.

  As shown in FIG. 6A, there is a portion where the wiring metal constituting the gate electrode (602) crosses the edge of the active layer (604) of the semiconductor film. However, there is no problem since both sides of the edge of the active layer (604) of the semiconductor film to be short-circuited are electrically at the same potential.

  For example, when the TFT of this embodiment is used as a pixel TFT, a protective film is formed after the TFT and wiring are formed, and the electrode is formed of a transparent conductor. In FIG. 6A, an electrode (601) is made of a transparent conductor as a drain electrode, an electrode (603) is made of a wiring metal as a source electrode, and the electrode (601), gate electrode (602), electrode A pixel TFT can be formed by wiring (603) in different layers. By adopting such an electrode structure, the leakage current can be reduced, and the wiring capacity between the electrode (601) used as the drain electrode and the gate electrode (602) can also be reduced.

  When a three-layer metal wiring can be formed as described above and the wiring capacity can be ignored, a TFT having an electrode structure as shown in FIG. 6B can be configured. A gate electrode (606) and an annular electrode (607) are arranged substantially concentrically so as to surround the outer side of the circular electrode (605). Since the electrodes (605) and (607) are formed in different layers, the electrode (607) is not formed with a notch. Note that either one of the electrodes (605) and (607) may be used as a source electrode and the other may be used as a gate electrode. In particular, the electrode (607) can be used as a pixel TFT by forming the electrode (607) with a metal wiring as a source electrode and the electrode (605) with a transparent conductor as a drain electrode, and has such an electrode structure. The pixel TFT is effective against leakage current.

  In addition, when doping an impurity into the active layer of the TFT, if the impurity is doped using the gate electrode (602) as a mask, the drain electrode and the active layer (604) of the semiconductor film doped with the impurity are connected. The source electrode is short-circuited. For this reason, it is necessary to devise such as adding a pattern for separating the active layer (604) of the semiconductor film into the drain region and the source region to the mask.

  As shown in FIGS. 5 and 6, the electrodes of [Example 1] and [Example 2] have substantially similar outer shapes and are arranged substantially concentrically. It is required to form the channel region concentrically with the electrode. Furthermore, in order to improve the characteristics of the TFT, it is also required to increase the crystallinity of the semiconductor material. Here, as a method of forming a semiconductor material having good crystallinity, a method of selectively adding a catalytic element for promoting crystallization to the active layer is employed. Based on FIGS. 7A and 7B, a method of selectively adding a catalytic element for promoting crystallization to an active layer to grow a crystal and forming a channel region concentric with an electrode explain.

  FIG. 7A is a schematic diagram for explaining the crystallization of the active layer of the semiconductor. The semiconductor film is etched to form an island-shaped active layer (704). A catalytic element for promoting crystallization is added into a circular region (701) in the center of the active layer (704). By heat treatment or the like, crystal growth of the active layer (702) proceeds in a direction in which the radius of the concentric circle increases as indicated by an arrow (702).

  On the other hand, FIG. 7B is another schematic diagram for explaining the crystallization of the active layer of the semiconductor. The semiconductor film is etched to form an island-shaped active layer (708). A catalytic element that promotes crystallization is added to an annular region (705) centered at the center of the island-shaped active layer (708) at the outer edge of the active layer (708). By heat treatment or the like, crystal growth proceeds in a direction in which the radius of the region (705) decreases as indicated by an arrow (706).

  From the crystal growth indicated by these arrows (702) and (706), it is clear that the catalyst element is present at a high concentration at the start and end points of crystal growth. Since it is necessary to form the channel region while avoiding the region where the catalyst element is present at a high concentration, the regions (703) and (707) serving as the channel are formed approximately in the middle between the starting point and the ending point of the crystal growth. Is formed in an annular shape so that it has a symmetrical center at the center of the active layers (704) and (708) and is substantially similar to the electrodes shown in FIGS.

  Even when the semiconductor material is crystallized using a catalytic element as shown in FIGS. 7A and 7B, an annular channel region (703) avoiding a region where the catalytic element is present at a high concentration is avoided. , (707). Therefore, a process of selectively crystallizing a semiconductor material using a catalytic element that promotes crystallization may be employed in the manufacturing process of a TFT having the electrode structure of [Example 1] and [Example 2]. Is possible.

  In addition, the TFT having the electrode structure of [Embodiment 1] or [Embodiment 2] can be used not only as a pixel TFT but also as a TFT constituting a peripheral drive circuit. For example, as shown in FIG. 8, in the analog buffer circuit diagram used in the signal line driver circuit, it can be used for the TFTs (801) and (802) of the operational amplifier circuit input stage. If the TFTs (801) and (802) at the input stage of the operational amplifier circuit have a large leakage current, they become sources of noise. Therefore, the performance of the analog buffer can be improved by using TFTs having the electrode structure of [Embodiment 1] or [Embodiment 2] as the TFTs (801) and (802).

  FIG. 9 is a plan view showing the wiring of the operational amplifier input stage. The TFTs (801) and (802) are composed of TFTs (901) and (902) having the electrode structure shown in FIG. I try to take measures against noise.

  Further, in a structure in which the TFT is a charge control switch and charges are accumulated in the capacitor as in the pixel portion, it is possible to reliably hold the charge in the capacitor by reducing the leakage current. If the loss of charge is reduced, the capacity can be reduced up to the allowable potential displacement, leading to area reduction and microfabrication.

  FIG. 10A and FIG. 10B are plan configuration diagrams of the TFT of Example 3. FIG.

  FIG. 10A is characterized in that the gate electrode is arranged so as to surround the entire periphery of the source electrode or the drain electrode. In the active layer (1004) of the semiconductor film, the gate electrode (1002) and the electrode (1003) are arranged so that the center of the electrode (1001) is a symmetrical center so as to surround the outside of the rectangular electrode (1001). In addition, the electrode (1001), the gate electrode (1002), and the electrode (1003) each have a substantially similar rectangular shape, and are formed to have a symmetrical center at the center of the active layer (1004). In this embodiment, an example in which the outer shape of the electrode is rectangular has been described, but the outer shape of the electrode may be a polygon.

  Since the electrode (1001) is formed in a layer different from the wiring metal constituting the gate electrode (1002), it may overlap with the gate electrode (1002). Since the electrode (1001) and the electrode (1003) are made of the same layer of wiring metal, the electrode (1003) has a shape with a part of a rectangle cut away in order to avoid a short circuit. Note that one of the electrodes (1001) and (1003) may be used as a source electrode and the other as a drain electrode.

  In the above configuration, there is a portion where the wiring metal constituting the gate electrode (1002) crosses the edge of the active layer (1004) of the semiconductor film. However, there is no problem because both sides of the edge of the active layer of the semiconductor film to be short-circuited are electrically at the same potential.

  For example, when the TFT of this embodiment is used as a pixel TFT, a protective film is formed after the TFT and wiring are formed, and the electrode is formed of a transparent conductor. In FIG. 10A, the electrode (1001), the gate electrode (1002), and the electrode (1003) are wired in different layers, and the electrode (1001) is formed of a transparent conductor as a drain electrode. Is formed of a wiring metal as a source electrode, whereby a pixel TFT can be formed. By adopting such an electrode structure, the leakage current can be reduced and the wiring capacity between the electrode (1001) used as the drain electrode and the gate electrode (1002) can be reduced.

  When the three-layer metal wiring can be formed as described above and the wiring capacitance can be ignored, the source electrode and the drain electrode are not short-circuited, so that the notch is not formed in the electrode (1003). A TFT having an electrode structure as shown in FIG. In the semiconductor active layer (1008), a gate electrode (1006) and a rectangular ring-shaped electrode (1007) are arranged so as to surround the outside of the rectangular electrode (1005), and the electrode (1005) and the gate electrode (1006). The electrodes (1007) each have a substantially similar rectangular shape, and are formed so as to have a symmetric center at the center of the active layer (1008).

  Further, since the electrodes (1005) and (1007) are formed in different layers, the electrode (1007) is not formed with a notch. Note that one of the electrodes (1005) and (1007) may be used as a source electrode and the other may be used as a gate electrode. In particular, if a TFT having a structure in which (1007) is formed of a metal wiring as a source electrode and (1005) is formed of a transparent conductor as a drain electrode is used as a pixel TFT, it is effective against leakage current.

  When the TFT is used as a charge control switch and the charge is accumulated in the capacitor, reducing the leakage current has an effect of reliably holding the charge in the capacitor. If the loss of charge is reduced, the capacity can be reduced to the allowable potential displacement, leading to area reduction and microfabrication.

  FIGS. 11A and 11B are plan views of the TFT of this example. In the active layer (1104) of the semiconductor, the gate electrode (1102) and the electrode ( 1103). The electrode (1101), the gate electrode (1102), and the electrode (1103) each have a substantially similar rectangular shape, and are formed so as to have a symmetric center at the center of the active layer (1104). In this embodiment, an example in which the outer shape of the electrode is rectangular has been described, but the outer shape of the electrode may be a polygon.

  Since the electrode (1101) is arranged in a layer different from the wiring metal constituting the gate electrode (1102), a capacitor is formed by overlapping the gate electrode (1102). In order to avoid the formation of this capacitor, the electrode (1102) has a shape in which a part of the rectangular ring is missing so as to avoid overlapping with the electrode (1101). The TFT of [Example 3] may be used when this capacitor can be ignored, and the TFT of [Example 4] may be used when the capacitor cannot be ignored.

  The electrode (1101) and the electrode (1103) are made of the same layer of wiring metal. Therefore, in order to avoid a short circuit, the electrode (1103) has a shape in which a part of a rectangle is missing. One of the electrodes (1101) and (1103) may be used as a source electrode and the other may be used as a drain electrode.

  In the above electrode structure, there is a portion where the wiring metal constituting the gate electrode (1102) crosses the edge of the active layer (1104) of the semiconductor film. However, since both sides of the edge of the active layer (1104) of the semiconductor film to be short-circuited are electrically at the same potential, there is no problem.

  Note that in the case of doping an impurity into the active layer (1004), when the impurity is doped using the gate electrode (1102) as a mask, the drain is passed through the active layer (1104) of the semiconductor film doped with the impurity. The electrode and the source electrode are short-circuited. For this reason, it is necessary to devise such as adding to the mask a pattern for separating the active layer (1104) of the semiconductor film into the drain region and the source region.

  When the TFT of this embodiment is used as a pixel TFT, a protective film is formed after the TFT and wiring are formed, and the electrode is formed of a transparent conductor. In FIG. 11A, the electrode (1101), the gate electrode (1102), and the electrode (1103) are wired in different layers, and the electrode (1101) is made of a transparent conductor as a drain electrode. ) To form a pixel TFT by using a wiring metal as a source electrode. By adopting such an electrode structure, the leakage current can be reduced, and the wiring capacity between the electrode (1101) used as the drain electrode and the gate electrode (1102) can be reduced.

  When the three-layer metal wiring can be formed as described above and the wiring capacitance can be ignored, the source electrode and the drain electrode are not short-circuited. A TFT having an electrode structure as shown in FIG. In the semiconductor active layer (1108), a gate electrode (1006) is arranged so as to surround the outside of the rectangular electrode (1105), and a rectangular annular electrode (1107) is arranged. Since the electrodes (1105) and (1107) are formed in different layers, notches are not formed in the electrode (1107).

  Note that one of the electrodes (1105) and (1107) may be used as a source electrode and the other may be used as a drain electrode. In particular, when a TFT having a structure in which (1107) is formed of a metal wiring as a source electrode and (1105) is formed of a transparent conductor as a drain electrode is used as a pixel TFT, it is effective against leakage current.

  As shown in FIGS. 10 and 11, the electrode structures of [Embodiment 3] and [Embodiment 4] are arranged so that rectangular annular electrodes have the same point as the center of symmetry. It is necessary to form the channel region substantially similar to the electrode. Furthermore, in order to improve the characteristics of the TFT, it is also required to increase the crystallinity of the semiconductor material. Here, as a method of forming a semiconductor material having good crystallinity, a method of selectively adding a catalytic element for promoting crystallization to the active layer is employed. Based on FIGS. 12 (a) and 12 (b), a method of selectively adding a catalytic element for promoting crystallization to an active layer to grow a crystal and to form a channel region similar to an electrode. explain.

  FIG. 12A is a schematic diagram for explaining the crystallization of the active layer of the semiconductor. The semiconductor film is etched to form an island-shaped active layer (1204). A catalytic element that promotes crystallization is added into a rectangular region (1201) at the center of the active layer (1204). The crystal growth of the active layer (1204) proceeds in the direction in which the rectangular region (1201) expands as shown by the arrow (1202) by heat treatment or the like.

  On the other hand, FIG. 12B is another schematic diagram for explaining the crystallization of the active layer of the semiconductor. The semiconductor film is etched to form an island-shaped active layer (1208). A catalytic element that promotes crystallization is added to a rectangular annular region (1205) centered at the center of the active layer (1208) in the vicinity of the outer edge of the active layer (1208). As shown by an arrow (1206), crystal growth proceeds in a direction in which the rectangular annular region (1205) is reduced by heat treatment or the like.

  From the crystal growth indicated by the arrows (1202) and (1206), it is clear that the catalyst element is present at a high concentration at the start and end points of crystal growth. Therefore, since it is necessary to form the channel region avoiding this region, the regions (1203) and (1207) to be the channels are formed approximately in the middle of the starting point and the ending point of the crystal growth, and the shape is an island-shaped active region. The layers (1204) and (1208) have a center of symmetry, and are formed in a rectangular ring shape so as to be substantially similar to the electrodes shown in FIGS.

  As shown in FIGS. 12A and 12B, even when a semiconductor material is crystallized using a catalytic element, a rectangular ring similar to an electrode is avoided, avoiding a region where the catalytic element is present at a high concentration. Channel regions (1203) and (1207) can be formed. Therefore, a process of selectively crystallizing the active layer using a catalyst element that promotes crystallization may be employed in the manufacturing process of the TFT having the electrode arrangement of [Example 3] and [Example 4]. It becomes possible.

It is a block circuit diagram of a signal line scanning circuit of a conventional example. It is a plane block diagram of the conventional TFT. It is a plane block diagram of the conventional TFT, and is an explanatory view showing a place where leakage current is generated. It is explanatory drawing of the generation mechanism of the conventional leak current. 1 is a configuration diagram of a TFT of Example 1. FIG. 4 is a configuration diagram of a TFT of Example 2. FIG. In Example 1 and Example 2, it is a schematic diagram explaining crystallization of the active layer of a semiconductor. It is a block diagram of an analog buffer circuit. It is an element arrangement diagram of an analog buffer circuit. 4 is a configuration diagram of a TFT of Example 3. FIG. 6 is a configuration diagram of a TFT of Example 4. FIG. In Example 3 and Example 4, it is a schematic diagram explaining crystallization of the active layer of a semiconductor.

Explanation of symbols

(501), (503), (505), (507)... Electrode (502), (506)... Gate electrode (504), (508)... Active layer of semiconductor film

(601), (603), (605), (607)... Electrode (602), (606)... Gate electrode (604), (608)... Active layer of semiconductor film

(701), (705) ... catalyst addition region (702), (706) ... growth direction (703), (707) ... gate electrodes (704), (708) ... of the semiconductor film Active layer

Claims (10)

Having first and second thin film transistors;
The first and second thin film transistors are thin film transistors in which no edge of the semiconductor film exists in the channel region,
The first thin film transistor is disposed in a pixel portion;
The second thin film transistor is disposed in a peripheral circuit;
The thin film transistor in which the edge of the semiconductor film does not exist in the channel region has a source electrode, a drain electrode, and a gate electrode,
One of the source electrode or the drain electrode has an annular shape or a shape lacking an annular portion,
One of the source electrode or the drain electrode is provided so as to surround the other of the source electrode or the drain electrode,
The channel region is provided between one of the source electrode or the drain electrode and the other of the source electrode or the drain electrode,
The semiconductor film is formed by selectively adding a catalytic element that promotes crystallization of a semiconductor and growing the crystal in an annular region centering on a central portion of the semiconductor film. Liquid crystal display device. In claim 1,
The liquid crystal display device, wherein the first thin film transistor is connected to a capacitor. In claim 1 or claim 2,
An analog buffer circuit is arranged in the peripheral circuit,
The liquid crystal display device, wherein the second thin film transistor is used in an input stage of an operational amplifier circuit of the analog buffer circuit. In any one of Claims 1 thru | or 3,
Said first and second thin film transistor, said being a gate insulating film on the semiconductor film is formed, a liquid crystal display device characterized by having the gate electrode is formed structure on the gate insulating film . In any one of Claims 1 thru | or 4,
One of the source electrode and the drain electrode is an annular shape, a rectangular shape, or a polygonal shape, and a liquid crystal display device. In any one of Claims 1 thru | or 4,
One of the source electrode or the drain electrode, a liquid crystal, wherein the shape with the cut part of the annular shape missing part of the rectangular ring or polygonal ring that portion of which is chipped shape Display device. In any one of Claims 1 thru | or 6,
The liquid crystal display device, wherein the channel region has an annular shape, a rectangular shape, or a polygonal shape. In any one of Claims 1 thru | or 6,
The liquid crystal display device according to claim 1, wherein the channel region has a shape in which a part of an annular shape is missing, a shape in which a part of a rectangular shape is missing, or a shape in which a part of a polygonal shape is missing. In any one of Claims 1 thru | or 8,
The liquid crystal display device, wherein the gate electrode has an annular shape, a rectangular shape, or a polygonal shape. In any one of Claims 1 thru | or 8,
The liquid crystal display device, wherein the gate electrode has a shape in which a part of an annular shape is missing, a shape in which a part of a rectangular shape is missing, or a shape in which a part of a polygonal shape is missing.

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