JP2009237447A - Liquid crystal device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal device, preventing image persistence after the power-off. <P>SOLUTION: This liquid crystal device 1 includes a first substrate having a pixel electrode 14, a second substrate disposed opposite to the first substrate, a liquid crystal layer held between the first substrate and the second substrate, and a depression type pixel transistor 13 for switching an electric signal supplied to a pixel electrode 14. According to this configuration, while a voltage is not applied to a gate electrode of the pixel transistor 13, a sub-threshold current can be caused to flow through a channel region of the pixel transistor 13. Accordingly, charges stored in the pixel electrode 14 can be decreased in a short time according to an electric signal so that an electric field generated by the charges is weakened in a short time. Thus, an electric field applied to an alignment layer and the liquid crystal layer can be weakened in a short time to prevent image persistence of the alignment layer and liquid crystal layer. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal device.

  Conventionally, liquid crystal devices have been widely used as display units for personal computers, mobile phones, television receivers, and the like. The liquid crystal device includes, for example, a liquid crystal layer sandwiched between an element substrate and a color filter substrate, a pixel electrode and a common electrode that apply an electric field to the liquid crystal layer, an alignment film that controls the alignment of liquid crystal molecules in the liquid crystal layer, and a pixel electrode A thin film transistor (TFT) or the like for switching an electric signal supplied to the device is provided. When an electric signal is transmitted to the pixel electrode, an electric field is applied to the liquid crystal layer, and the azimuth angle of the liquid crystal molecules changes. Then, the light passing through the liquid crystal layer changes its polarization state, and a part of the light is absorbed by the polarizing plate, becomes light of a predetermined gradation, and is emitted from the liquid crystal device.

  As a driving method of liquid crystal molecules, a vertical electric field method such as a TN method and a VA method and a horizontal electric field method such as an FFS method and a VA method are known. In general, in the vertical electric field method, a pixel electrode is provided on an element substrate and a common electrode is provided on a color filter substrate to generate an electric field (vertical electric field) in the thickness direction of the substrate. In the horizontal electric field method, a pixel electrode and a common electrode are provided on the element substrate to generate an electric field (lateral electric field) in the surface direction of the element substrate. In either method, an alignment film is provided between the liquid crystal layer and the electrode in order to rotate the liquid crystal molecules in a predetermined direction.

  By the way, when the same image is displayed for a long time in the liquid crystal device, an afterimage or image sticking may occur. A similar phenomenon may occur after the liquid crystal device is turned off. This is because when the power is turned off, the TFT is also turned off, the electric charge accumulated in the pixel electrode for holding the electric field in the liquid crystal layer is held, and the electric field between the common electrode is held. The alignment film is charged by this electric field, and ions contained in the liquid crystal layer are localized, and the alignment of the liquid crystal molecules changes from a desired state. Then, in the initial stage where the current is supplied again, the liquid crystal molecules are not satisfactorily controlled and flicker (flickering) occurs. In particular, in the horizontal electric field method, since the distance between the pixel electrode and the common electrode is shorter than in the vertical electric field method, a strong electric field acts on the alignment film and the liquid crystal layer, and such inconvenience is likely to be remarkable.

As a technique capable of reducing image sticking in an IPS (horizontal electric field) liquid crystal device, a technique disclosed in Patent Document 1 can be cited. In the horizontal electric field method, a comb-shaped electrode is often used, and in Patent Document 1, an upper insulating film is provided on the comb-shaped electrode. By satisfying the predetermined relationship between the dielectric constant of the interlayer insulating film between the electrodes and the dielectric constant of the upper interlayer insulating film, plastic deformation of the alignment film due to electric field concentration is reduced, and seizure due to plastic deformation can be prevented. It is said that.
JP 2003-29247 A

However, image sticking after the power of the liquid crystal device is turned off is caused by applying a substantially constant electric field to the alignment film or the liquid crystal layer for a long time. .
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a liquid crystal device that prevents seizure after power-off.

A liquid crystal device according to the present invention includes a first substrate having a pixel electrode, a second substrate disposed opposite to the first substrate, and a liquid crystal layer sandwiched between the first substrate and the second substrate. And a depletion type pixel transistor for switching an electric signal supplied to the pixel electrode.
In this way, since the pixel transistor is a depletion type, it is possible to flow a subthreshold current to the channel region of the pixel transistor in a state where no voltage is applied to the gate electrode of the pixel transistor (off state). Therefore, the charge accumulated in the pixel electrode by the electric signal is reduced in a shorter time than when the pixel transistor is an enhancement type, and the electric field generated by this charge becomes weak in a short time. Therefore, the electric field applied to the alignment film or the liquid crystal layer becomes weak in a short time, and the alignment film or the liquid crystal layer is prevented from being burned.

  Preferably, a transmission gate electrically connected to the source region of the pixel transistor is provided, and the transmission gate is composed of an enhancement type N-type transistor and a depletion type P-type transistor. In this case, a drive circuit electrically connected to the transmission gate is provided, and the drive circuit has an inverter circuit including an enhancement type N-type transistor and an enhancement type P-type transistor. preferable.

  In this case, since the transmission gate electrically connected to the source region of the pixel transistor has the depletion type P-type transistor, the charge on the source region side of the pixel transistor is transferred to the liquid crystal via the P-type transistor. It can escape outside the circuit of the device. Therefore, the charge is prevented from being saturated on the source region side of the pixel transistor, and the charge accumulated in the pixel electrode can be released well.

  By the way, the amplitude of the electric signal (voltage waveform) needs to be within the range of the gate voltage of the pixel transistor and within the range of the gate voltage of the transmission gate. Normally, the pixel transistor is N-type, and is turned off when the gate voltage becomes GND, and turned on when the gate voltage becomes high. This positive voltage is set sufficiently high to ensure a margin with respect to the amplitude of the electric signal. Further, the gate voltage of the transmission gate has a low potential slightly higher than GND in order to ensure a margin, and the high potential is slightly lower than the high potential of the gate voltage of the pixel transistor. If the N-type transistor is made to be a depletion type, the threshold voltage shifts in the negative direction and falls outside the margin range of the gate voltage with respect to GND. If an attempt is made to secure a margin at this threshold voltage, there will be inconveniences such as a low level electric signal can be written and the driving method of the liquid crystal device becomes complicated.

  On the other hand, since the P-type transistor is turned off at a high potential and there is a sufficient margin on the high potential side, if the N-type transistor is an enhancement type and the P-type transistor is a depletion type, the driving method is complicated while ensuring the margin. Can be avoided. As described above, by adopting the above configuration, it is possible to favorably release the charge accumulated in the pixel electrode.

  If such a transmission gate is provided, the charge on the source region side of the pixel transistor can be released by adjusting the characteristics of the P-type transistor of the transmission gate. No need to adjust. Therefore, an enhancement type inverter circuit composed of an N-type transistor and a P-type transistor can be obtained, and there is no inconvenience such as a through electrode flowing through the inverter circuit. Therefore, a drive circuit having an inverter circuit can be stably operated, and a liquid crystal device that can be prevented from being burned and can be stably operated is obtained.

Preferably, both the pixel electrode and the common electrode are provided on the first substrate.
In this way, an electric field (lateral electric field) along the surface direction of the first substrate can be generated between the pixel electrode and the common electrode. That is, a horizontal electric field type liquid crystal device can be obtained, and a viewing angle can be made wider than that of a vertical electric field type.
On the other hand, in the horizontal electric field method, since the electric field between the pixel electrode and the common electrode is stronger than in the vertical electric field method, the alignment film and the liquid crystal layer are more likely to be seized. However, according to the present invention, since the electric field can be weakened in a short time, seizure of the alignment film and the liquid crystal layer can be prevented and the viewing angle can be widened.

The active layer of the pixel transistor is preferably made of polycrystalline silicon.
In this way, the electrical characteristics of the pixel transistor can be improved compared to the case where a transistor whose active layer is made of amorphous silicon is employed.

In addition, it is preferable that a current value of a subthreshold current flowing in a channel region of the pixel transistor when the pixel transistor is in an off state is 1 pA or more and 20 pA or less due to charges accumulated in the pixel electrode by the electric signal.
If the current value of the subthreshold current is 1 pA or more, the charge accumulated in the pixel electrode can be released in a short time to such an extent that the alignment film and the liquid crystal layer are not burned. Further, if the current value of the subthreshold current is 20 pA or less, it is possible to prevent the charge of the pixel electrode from leaking in a short time during the operation of the liquid crystal device.

  Hereinafter, although one embodiment of the present invention is described, the technical scope of the present invention is not limited to the following embodiment. In the following description, various structures are illustrated using drawings, but in order to show the characteristic parts of the structures in an easy-to-understand manner, the structures in the drawings are shown in different sizes and scales from the actual structures. There is.

  The liquid crystal device according to the present embodiment performs image display by controlling the azimuth angle of liquid crystal molecules by a lateral electric field orthogonal to the traveling direction of light. As such a method, an FFS (Fringe Field Switching) method, an IPS (In Plane Switching) method, and the like are known. Here, description will be made based on a liquid crystal device capable of full color display in the FFS method.

FIG. 1 is a schematic diagram showing a circuit configuration of the liquid crystal device 1 of the present embodiment. As shown in FIG. 1, the liquid crystal device 1 includes a plurality of scanning lines 11 extending in one direction and a plurality of data lines (drain lines) 12 extending perpendicular to the one direction. ing. Each of the plurality of scanning lines 11 is electrically connected to a scanning line driving circuit 40 that supplies a scanning signal, and each of the plurality of data lines 12 is a data line driving circuit that supplies an image signal (electric signal). 60 is electrically connected.
Hereinafter, the XYZ orthogonal coordinate system shown in FIG. 1 is set, and the positional relationship of the members will be described based on this. In this XYZ orthogonal coordinate system, the X direction is the extending direction of the scanning lines 11, the Y direction is the extending direction of the data lines 12, and the Z direction is the thickness direction of the liquid crystal device 1.

  Each of the plurality of regions partitioned into the scanning lines 11 and the data lines 12 is one subpixel. Each sub-pixel is provided with a pixel transistor (hereinafter referred to as a pixel TFT) 13 that switches an image signal based on a scanning signal. Each subpixel is provided with a pixel electrode 14 that is electrically connected to the pixel TFT 13, a common electrode 15 that forms a capacitance between the pixel electrode 14, and the like. The capacitance between the pixel electrode 14 and the common electrode 15 is, for example, about 0.2 pF.

  In the row of subpixels arranged in the Y direction, the source region of each pixel TFT 13 is electrically connected to one data line 12. One end of the data line 12 is electrically connected to the data line driving circuit 60 via the transmission gate 50. The other end of the data line 12 is electrically connected to a conductive portion or the like having a ground potential via a switching element (precharge) or the like (not shown). The capacitance (parasitic capacitance) of one data line 12 between the precharge and the transmission gate 50 is, for example, about 20 pF.

  The transmission gate 50 includes a P-type transistor (hereinafter referred to as P-type TFT) 51 and an N-type transistor (hereinafter referred to as N-type TFT) 52. In this embodiment, the P-type TFT 51 is a depletion type and the N-type TFT 52 is an enhancement type. The data line driving circuit 60 has an inverter circuit (not shown), and this inverter circuit is composed of an enhancement type P-type TFT and an enhancement type N-type TFT.

FIG. 2A is a plan view showing an enlarged pixel, and FIG. 2B is a plan view showing a configuration of sub-pixels.
The liquid crystal device 1 of the present embodiment has a large number of pixels, and as shown in FIG. 2A, the pixel P includes three sub-pixels Pr, Pg, and Pb. The sub-pixels Pr, Pg, and Pb emit red light, green light, and blue light, respectively, and these are mixed and visually recognized, thereby enabling full color display. Here, the light shielding region D is between the sub-pixels Pr, Pg, and Pb and between the plurality of pixels P.

  As shown in FIG. 2B, the sub-pixel Pg has a pixel TFT 13, a pixel electrode 14, and a common electrode 15. The pixel TFT 13 is disposed in the vicinity of the intersection of the scanning line 11 and the data line 12 and has an active layer provided so as to overlap the scanning line 11. A portion of the scanning line 11 overlapped with the active layer functions as a gate electrode, and the source region of the active layer is electrically connected to the data line 12, and the drain region of the active layer is electrically connected to the pixel electrode 14, respectively. Has been. The pixel electrode 14 of the present embodiment has a substantially rectangular shape that is long in the Y direction, and has a slit-like opening extending in the X direction. The pixel electrode 14 is disposed so as to overlap the common electrode 15 in the Z direction, and an insulating film (not shown) is interposed between the pixel electrode 14 and the common electrode 15. The sub-pixels Pr and Pb have the same configuration.

  FIG. 3 is a cross-sectional view taken along the line A-A ′ of FIG. As shown in FIG. 3, the liquid crystal device 1 is sandwiched between an element substrate (first substrate) 10 having a pixel electrode 14 and a common electrode 15, a color filter substrate 20 disposed to face the element substrate 10, and these substrates. The liquid crystal layer 30 is provided. Here, an illumination device (not shown) such as a backlight is provided in the negative Z direction, and an image is displayed toward the positive Z direction.

  The element substrate 10 uses a transparent substrate 10A made of glass, quartz, plastic, or the like as a base. The scanning line 11 and the common electrode 15 are provided on the inner side (the liquid crystal layer 30 side) of the transparent substrate 10A, and an insulating film 16 is provided so as to cover them. On the insulating film 16, the data line 12, the active layer 131 of the pixel TFT 13, the source electrode 135, and the like are provided, and a part of the data line 12 is branched and brought into contact with the source region of the active layer 131. . The active layer 131 is made of polycrystalline silicon, and its channel region is doped with impurities. The pixel TFT 13 is of a depletion type by adjusting the dose amount. The drain region of the active layer 131 is in contact with the source electrode 135. A part of the source electrode 135 is disposed so as to overlap the common electrode 15, and this part constitutes a capacitor that holds an image signal for a certain period when energized.

  An interlayer insulating film 17 is provided so as to cover the data line 12, the TFT 13, and the source electrode 135. The interlayer insulating film 17 is provided with a through hole that exposes the source electrode 135, and the pixel electrode 14 is provided across the inside of the through hole and a predetermined region on the interlayer insulating film 17. The pixel electrode 14 is in contact with the source electrode 135 inside the through hole. An alignment film 18 is provided so as to cover the pixel electrode 14, and the alignment film 18 controls the alignment direction of the liquid crystal molecules of the liquid crystal layer 30 together with the alignment film 22 described later. A polarizing plate 19 is provided outside the transparent substrate 10A, and an illumination device (not shown) such as a backlight for illuminating the polarizing plate 19 side of the liquid crystal device 1 is provided.

  The color filter substrate 20 of this embodiment uses a glass substrate 20A as a base. A color filter layer 21 is formed on the inner side (the liquid crystal layer 30 side) of the glass substrate 20A. The color filter layer 21 has a color material portion corresponding to each of the sub-pixels Pr, Pg, and Pb, although a detailed configuration thereof is not illustrated. For example, the sub-pixel Pr is provided with a color material portion that absorbs light in a wavelength band other than red light, and the light passing therethrough is emitted as red light. Between these color material portions, a light shielding portion corresponding to the light shielding region D (see FIG. 2A) is provided. An alignment film 22 is provided inside the color filter layer 21 and a polarizing plate 23 is provided outside the glass substrate 20A.

  In the liquid crystal device 1 configured as described above, the image signal supplied from the data line driving circuit 60 is supplied to the source region of the pixel TFT 13 via the transmission gate 50 and the data line 12. The scanning signal supplied from the scanning line driving circuit 40 turns on and off the channel region of the pixel TFT 13 via the scanning line 11 at a predetermined timing. As a result, the image signal is transmitted to the pixel electrode 14 via the drain region of the pixel TFT 13 and the source electrode 135, and an electric field corresponding to the image signal is generated between the pixel electrode 14 and the common electrode 15. Of this electric field, the azimuth angle of the liquid crystal molecules of the liquid crystal layer 30 is controlled by a component parallel to the XY plane, that is, a horizontal electric field.

  On the other hand, the light from the illuminating device is incident on the liquid crystal layer 30 with the polarization direction aligned by the polarizing plate 19, and the polarization state changes according to the azimuth angle of the liquid crystal molecules and is emitted to the color filter substrate 20 side. The light incident on the color filter substrate 20 passes through the color filter layer 21 to become a predetermined color light and enters the polarizing plate 25. The light incident on the polarizing plate 25 is absorbed by the polarizing plate 25 according to the polarization state, and is emitted as colored light having a predetermined gradation. Light emitted from each of the sub-pixels Pr, Pg, and Pb is mixed and visually recognized to form a minimum unit for full-color display. Such light is emitted from each of a large number of pixels, so that a desired image can be displayed.

  When the power supply is turned off in such a liquid crystal device 1, the pixel TFT 13 is turned off in a state where charges are accumulated between the pixel electrode 14 and the common electrode 15. As a result, an electric field is maintained between the pixel electrode 14 and the common electrode 15, but in the present invention, since the pixel TFT 13 is a depletion type, this electric field can be eliminated in a short time. The mechanism will be described below.

  4A is a graph showing the difference between the depletion type and the enhancement type with respect to the characteristics of the N-type TFT. FIG. 4B is a graph showing the difference between the depletion type and the enhancement type with respect to the characteristics of the P-type TFT. It is a graph which shows. 4A and 4B, the horizontal axis represents the gate voltage Vgs which is the potential of the gate electrode with respect to the source region, and the vertical axis represents the logarithm of the subthreshold current value flowing in the channel region.

  In a conventional liquid crystal device, an enhancement type N-type TFT is used as a pixel TFT or the like. In the enhancement type, holes due to P-type impurities (for example, boron) doped in the channel region become carriers in the negative region of the gate voltage. As shown in FIG. 4A, a depletion layer spreads in the channel region as the gate voltage approaches 0 V from the negative region, and the subthreshold current hardly flows here. In the positive region of the gate voltage, the inversion layer spreads in the channel region and electrons become carriers. When the gate voltage is further increased, the subthreshold current increases exponentially.

  Such characteristics are controlled by adjusting the dose of boron. If the dose of boron is reduced, the number of holes due to boron decreases, so that the depletion layer and the inversion layer spread at a gate voltage lower than that of the enhancement type. That is, a depletion type in which a sub-threshold current flows when the gate voltage is 0V. As described above, since the subthreshold current value due to the inversion layer increases exponentially with respect to the gate voltage, even if the amount by which the threshold voltage is shifted in the negative direction is small, the subthreshold current value is only a significant amount. Can be changed. In other words, it is possible to control the subthreshold current value without significantly changing the gate voltage or the like. In this embodiment, the pixel TFT 13 is a depletion type N-type TFT. Here, the dose amount of the active layer 131 of the pixel TFT 13 is adjusted so that the subthreshold current value at the gate voltage Vgs = 0 V is 1 pA or more and 20 pA or less.

On the other hand, in the enhancement type P-type TFT, in the region where the gate voltage Vgs is positive, holes due to N-type impurities (for example, phosphorus) doped in the channel region become carriers. As shown in FIG. 4B, a depletion layer spreads in the channel region as the gate voltage approaches 0 V from the positive region, and the subthreshold current hardly flows there. In the negative region of the gate voltage, the inversion layer spreads in the channel region and electrons become carriers. When the gate voltage is further increased, the subthreshold current increases exponentially. When an active layer of such an enhancement type P-type TFT is doped with a P-type impurity (boron), the number of carriers due to phosphorus is taken in and reduced. Then, the depletion layer and the inversion layer are spread at a gate potential Vgs higher than that of the enhancement type. In the present embodiment, the P-type TFT 51 of the transmission gate 50 is a depletion type.
Such adjustment of the dose amount can be performed, for example, by the following method.

  FIG. 5 is a table showing an example of a method for adjusting the dose in channel dope. In the liquid crystal device 1 of the present embodiment, various transistors are formed using a low-temperature polysilicon technique. In this example, the pixel TFT 13, the P-type TFT 51, the N-type TFT 52, or the peripheral circuit TFTs, etc., adjust the dose amount by making the process common, thereby improving the process efficiency. FIG. 5 shows a P-type TFT and an N-type TFT included in the peripheral circuit for comparison, both of which are enhancement type.

  Prior to channel doping, a polycrystalline silicon film serving as an active layer is formed in a predetermined region. First, an N-type impurity (for example, phosphorus) is doped into a portion that becomes an active layer of a P-type TFT. Then, the active layer of the P-type TFT and the active layer of the N-type TFT are collectively doped with P-type impurities (here, boron), and the first channel doping is performed. The dose amount for the first time is an amount in which the N-type TFT becomes a depletion type. Further, by adjusting the phosphorus dose, the P-type TFT becomes an enhancement type. Then, the active layer 131 of the pixel TFT 13 and the active layer of the P-type TFT in the peripheral circuit are protected with a mask or the like, and the second channel doping is performed. The dose amount for the second time is a total amount with the dose amount for the first time so that the N-type TFT becomes an enhancement type. As described above, the active layer 131 of the pixel TFT 13 has a boron dose smaller than that of the enhancement type, and the P-type TFT 51 of the transmission gate 50 has a boron dose larger than that of the enhancement type.

  In the liquid crystal device 1 having the above configuration, when the power is turned off, the power of the scanning line driving circuit 40 and the data line driving circuit 60 is turned off, and the scanning line 11, the data line 12, and the common electrode 15 become GND. Therefore, the pixel TFT 13 is turned off, and the pixel electrode 14 has a potential of, for example, about several hundred mV to several V with respect to the common electrode 15. In the present invention, since the pixel TFT 13 is a depletion type, the subthreshold current of the pixel TFT 13 is larger than that of the enhancement type in a state where the scanning line 11, that is, the gate electrode is GND. As a result, the charge accumulated in the pixel electrode 14 (positive charge here) is carried to the data line 12 side by the subthreshold current and leaks in a short time. Therefore, the electric field between the pixel electrode 14 and the common electrode 15 is weakened in a short time after the power is turned off, and charging of the alignment film 18 and the like is remarkably reduced. Accordingly, uneven distribution of ions in the liquid crystal layer 30 due to charging of the alignment film 18 is prevented, and liquid crystal molecules are prevented from being aligned in an unexpected direction after the power is turned off.

  In the present embodiment, since the subthreshold current value is set to 1 pA or more, the charge accumulated in the pixel electrode 14 is leaked within a period in which image sticking does not occur. Since the subthreshold current value is 20 pA or less, the charge accumulated in the pixel electrode 14 does not leak during the display period. For example, the period in which image sticking does not occur is about several to several tens of minutes, for example, and the display period of one frame is several tens of ms or less (17 ms at 60 fps). As described above, the period during which charges are to be retained is much shorter than the period during which leakage is performed, so that charges can be retained within the display period and the period during which no image sticking occurs.

  In addition, since the P-type TFT 51 of the transmission gate 50 is a depletion type, the charge carried to the data line 12 side after the power is turned off becomes a subthreshold current passing through the active layer of the P-type TFT 51, and the data line driving circuit 60 is To the outside of the circuit. As a result, the charge in the data line 12 is prevented from being saturated, and the charge in the pixel electrode 14 can be released well. Further, since the N-type TFT 52 is an enhancement type, the transmission gate 50 can be operated normally without complicating the driving method. Hereinafter, a driving method of the liquid crystal device 1 will be schematically described.

  FIG. 6 is a timing chart schematically showing an example of a driving method of the liquid crystal device 1. FIG. 6 shows gate voltages for the pixel TFT 13, the P-type TFT 51 of the transmission gate 50, and the N-type TFT 52 of the transmission gate 50, and also shows changes in the voltage of the pixel electrode 14 due to image signals. In the driving method of this example, it is possible to write a positive voltage / negative voltage image signal to the pixel electrode 14, but the case of a positive voltage will be described here.

  As shown in FIG. 6, in this example, the gate voltage of the pixel TFT 13 is a low potential of −5V and a high potential of 8V. By the scanning signal supplied from the scanning line driving circuit 40, the gate voltage of the pixel TFT 13 becomes 8V at time t1, and the pixel TFT 13 is turned on. As the gate voltage of the P-type TFT 51 and the N-type TFT 52, the high potential is set lower than 8V to ensure a margin with respect to the gate voltage of the pixel TFT 13, and the low potential is set to GND to ensure a margin with respect to GND. Higher than that. At time t2 after time t1, the gate voltage of the P-type TFT 51 becomes low and the gate voltage of the N-type TFT 52 becomes high. As a result, both the P-type TFT 51 and the N-type TFT 52 are turned on at time t2. The image signal (voltage waveform) supplied from the data line driving circuit 60 has an amplitude that can ensure a margin with respect to the gate voltage of the transmission gate 50. That is, the low potential of the image signal is higher than the low potential of the gate voltage of the transmission gate 50. Such an image signal is written to the pixel electrode 14 via the transmission gate 50, the data line 12, and the pixel TFT 13 after time t2.

  If the N-type TFT 52 is made to be a depletion type, the threshold voltage shifts in the negative direction and falls outside the margin of the N-type TFT 52. Therefore, if a margin is to be secured, it becomes necessary to adjust the gate voltage of the transmission gate 50, the gate voltage of the pixel TFT 13, or the voltage of the image signal, and the driving method becomes complicated.

  If the P-type TFT 51 is made to be a depletion type as in this embodiment, the threshold voltage shifts in the positive direction, so that the subthreshold current value can be adjusted within the margin of the P-type TFT 51. In addition, since the high potential (about 8V) of the gate voltage of the P-type TFT 51 has a sufficient margin with respect to the high potential (about 5V) of the image signal, normal operation can be performed even on the high potential side. .

  As described above, the transmission gate 50 operates well and the charge of the pixel electrode 14 can be released through the transmission gate 50. Therefore, the characteristics of the inverter circuit of the data line driving circuit 60 can be obtained for the purpose of releasing the charge. No need to adjust. The inverter circuit needs to adjust the characteristics of the P-type TFT and the N-type TFT with high accuracy so as not to cause a problem such as a through current flowing through the P-type TFT and the N-type TFT. Since it is not necessary to adjust the characteristics of the inverter circuit as described above, both the N-type TFT and the P-type TFT in the inverter circuit of the data line driving circuit 60 can be made enhancement type. As a result, it is possible to operate the inverter circuit normally so that the data driving circuit 60 can be stably operated, and it is possible to release the charge of the pixel electrode 14 to the outside of the circuit via the data line driving circuit 60.

  As described above, in the present invention, since the pixel transistor (pixel TFT) 13 is a depletion type, the electric field between the pixel electrode 14 and the common electrode 15 is reduced after the power of the liquid crystal device 1 is turned off. It becomes weak in a short time. Accordingly, the orientation film 18 is charged by the electric field and the ions of the liquid crystal layer 30 are not unevenly distributed due to the charge of the orientation film 18, and seizure after the power is turned off is prevented. Therefore, the liquid crystal molecules of the liquid crystal layer 30 are normally aligned in a state where the power is turned on again, and display quality is not deteriorated due to flicker or the like. Thus, according to the present invention, a good liquid crystal device capable of obtaining a high-quality display is obtained.

It is a schematic diagram which shows the circuit structure of the liquid crystal device of this invention. (A) is an enlarged plan view of a pixel, and (b) is a plan configuration diagram of a sub-pixel. FIG. 3 is a cross-sectional view taken along the line A-A ′ of FIG. (A) is a characteristic graph of an N-type TFT, and (b) is a characteristic graph of a P-type TFT. It is a table | surface which shows an example of the adjustment method of the dose amount in channel dope. 3 is a timing chart schematically showing an example of a method for driving a liquid crystal device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Liquid crystal device, 10 ... Element board | substrate (1st board | substrate), 13 ... Pixel transistor, 131 ... Active layer, 14 ... Pixel electrode, 20 ... Color filter board | substrate (2nd Substrate), 30 ... Liquid crystal layer, 50 ... Transmission gate, 51 ... P-type TFT (P-type transistor), 52 ... N-type TFT (N-type transistor), 60 ... Data line drive Circuit (Drive circuit)

Claims (6)

A first substrate having a pixel electrode;
A second substrate disposed opposite the first substrate;
A liquid crystal layer sandwiched between the first substrate and the second substrate;
A liquid crystal device comprising: a depletion type pixel transistor that switches an electric signal supplied to the pixel electrode.   2. The transmission gate according to claim 1, further comprising a transmission gate electrically connected to a source region of the pixel transistor, wherein the transmission gate includes an enhancement type N-type transistor and a depletion type P-type transistor. The liquid crystal device described.   3. A drive circuit electrically connected to the transmission gate, wherein the drive circuit has an inverter circuit composed of an enhancement type N-type transistor and an enhancement type P-type transistor. The liquid crystal device according to 1.   The liquid crystal device according to claim 1, wherein both the pixel electrode and the common electrode are provided on the first substrate.   The liquid crystal device according to claim 1, wherein an active layer of the pixel transistor is made of polycrystalline silicon.   2. The current value of a subthreshold current that flows in a channel region of the pixel transistor in an off state of the pixel transistor due to charges accumulated in the pixel electrode by the electrical signal is 1 pA or more and 20 pA or less. The liquid crystal device according to claim 1.

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