JP3441337B2 - Active matrix type liquid crystal display - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device used as a display device for computers, word processors and the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, a simple matrix method and an active matrix method have been known as methods for displaying characters and images using liquid crystals. The former simple matrix system is a system in which a scanning signal wiring and a pixel signal wiring are formed so as to vertically intersect each other with a liquid crystal interposed therebetween, and a voltage is applied to a portion where both wirings intersect to display a pixel. The latter active matrix method is a method in which a switching element is provided for each unit of display pixels, and has a faster response than a simple matrix method and is suitable for displaying a moving image.

In the active matrix type liquid crystal display device, a transistor using an amorphous semiconductor material as a switching element has been the mainstream. Recently, a technique has been developed in which a driving circuit is formed over the same substrate as a liquid crystal display device by using a polycrystalline semiconductor material having higher field effect mobility than an amorphous semiconductor. As a technique related to the polycrystalline semiconductor, a so-called low-temperature process polycrystallization technique, which makes it possible to use an inexpensive glass substrate by realizing crystallization at a low temperature, has been particularly noted.

[0004]

There are a thermal crystallization method and a laser crystallization method as a method of crystallizing a semiconductor film in the above-mentioned low temperature process polycrystallization technique. Each of these two crystallization methods has advantages and disadvantages.

In the thermal crystallization method, since crystallization is performed in a heating furnace, a transistor having uniform characteristics can be obtained, but a long heat treatment is required for crystallization, and heat shrinkage of the glass substrate is a problem. Becomes On the other hand, the laser crystallization method uses excimer laser light, and can be crystallized in a short time and has good crystallinity.
Since it is necessary to irradiate the laser beam many times on the sheet,
There is a drawback that variations in characteristics of switching elements occur due to variations between laser shots, and display unevenness occurs in a liquid crystal display device including the switching elements.

The problems associated with the above laser crystallization method will be described in detail below.

When a linear beam with a length of 150 to 300 mm is used for laser irradiation in the laser crystallization method,
This is compatible with the increase in area, and variations in transistor characteristics can be suppressed in the length direction of the linear beam. However, since the width of the laser is about 0.1 mm, it is necessary to irradiate a single substrate with a laser beam many times, resulting in variations between laser shots. Further, even if a method such as superposition is used, the boundary of the laser beam remains, resulting in lack of uniformity of crystallinity, resulting in variations in transistor characteristics as shown in FIG. 6, and even if the same signal is applied to different pixel transistors. Since the output was different, it eventually appeared as uneven display on the liquid crystal display device.

The present invention has been made to solve the problems of the prior art, and an object thereof is to provide an active matrix type liquid crystal display device having no display unevenness and a manufacturing method thereof.

[0009]

According to an active matrix type liquid crystal display device of the present invention, a plurality of gate bus lines and a plurality of source bus lines are arranged in an intersecting manner on an insulating substrate, and in the vicinity of the intersection of both bus lines. In an active matrix type liquid crystal display device in which a pixel transistor for driving a pixel electrode is arranged in a matrix, a pixel transistor corresponding to each gate bus line and linearly arranged with a pixel transistor connected to the gate bus line. And a compensating circuit that is provided corresponding to the compensating transistor and that controls a gate voltage applied to the gate bus line by an output signal from the compensating transistor. Thereby, the above object is achieved.

In the active matrix type liquid crystal display device of the present invention, the compensation circuit comprises a constant voltage power source for supplying a constant voltage to a source electrode of the compensation transistor, and a fixed resistor for converting the output of the compensation transistor into a voltage. And a differential amplifier that receives a gate signal from a gate driver and an output signal of the compensation transistor, outputs a properly adjusted gate voltage, and supplies the gate voltage to the gate bus line. Good.

In the active matrix type liquid crystal display device of the present invention, the compensating circuit supplies a constant voltage power source for supplying a constant voltage to a gate electrode and a source electrode of the compensating transistor, and outputs the voltage from the compensating transistor as a voltage. A fixed resistor for converting to, a comparator for comparing the output from the compensation transistor with a reference voltage, a gate signal from the gate driver and an output signal from the comparator are input, and a properly adjusted gate voltage is input. A level shifter for outputting and supplying to the gate bus line may be configured.

In the active matrix type liquid crystal display device of the present invention, the crystallinity of a semiconductor film used as an element region of the pixel transistor connected to any of the gate bus lines and the above-mentioned crystallinity corresponding to the gate bus line. The crystallinity of the semiconductor film used as the element region of the compensation transistor may be substantially the same.

In the active matrix type liquid crystal display device of the present invention, the compensation transistor and the pixel transistor are formed in the same step, and the gate length and the gate width of each of the pixel transistor and the compensation transistor are formed. May have substantially the same size.

[0014]

The operation of the present invention will be described below.

In the present invention, the semiconductor films of the compensation transistor and the pixel transistor, which are linearly arranged in the gate bus line direction, are simultaneously irradiated so that the uniformity of the linear laser beam in the length direction can be utilized. It is arranged as possible. Therefore, when a linear laser beam is applied to the semiconductor films of the compensation transistor and the pixel transistor arranged in the straight line in the gate bus line direction,
The pixel transistor and the compensating transistor have the same characteristics. Therefore, the characteristic variation of the pixel transistor can be detected by the compensation transistor.

By detecting the signal of the compensating transistor and adjusting the gate voltage of the pixel transistor arranged in the gate bus line direction by the compensating circuit, the output variation of the pixel transistor can be suppressed. Therefore, uniform display characteristics of the liquid crystal display device can be obtained.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the active matrix type liquid crystal display device of the present invention will be described in detail below with reference to the drawings.

(Embodiment 1) FIG. 1 is a plan view schematically showing the overall structure of an active matrix type liquid crystal display device according to Embodiment 1.

In this active matrix type liquid crystal display device, a source bus line 302 and a gate bus line 306 are provided so as to intersect each other on an insulating substrate, and an image signal from the source driver 301 is supplied to each source bus line 302. The gate signal is supplied to each gate bus line 306 from the gate driver 303. Further, a compensating circuit 304 having a configuration described later including a compensating transistor 305 is provided between the gate driver 303 and each gate line 306.

The source driver 301 is composed of a shift register circuit, sends an image signal to each source bus line 302 in sequence, and supplies the pixel transistor 30.
An electric field is applied to the liquid crystal through a pixel electrode (not shown) connected to the source electrode of No. 7. The gate driver 303 is composed of a shift register circuit and is connected to each gate bus line 306 through a compensation circuit 304,
The gate signal of the pixel transistor 307 is sequentially turned on and then O
FF. Further, the compensation transistor 305 and the pixel transistor 307 arranged in a line, which are provided for each gate bus line 306, are linearly arranged.

An alignment film is formed on the liquid crystal driving substrate having the above structure and a counter substrate having a counter electrode, the two substrates are bonded together, and a liquid crystal material is injected between the two substrates.

FIG. 2 shows a compensation transistor 305 in the active matrix type liquid crystal display device according to the first embodiment.
3 is a cross-sectional view showing a portion near a pixel transistor 307 adjacent thereto and FIG.

The structure of the liquid crystal drive substrate will be described below in the order of manufacturing steps.

The compensating transistor 305 and the pixel transistor 307 have an island-shaped semiconductor film 202 on an insulating substrate 201. This semiconductor film 202
Has a thickness of 30 to 150 nm. As a method of manufacturing the island-shaped semiconductor film 202, in the case of a silicon semiconductor, an amorphous silicon film is formed by a plasma CVD method using SiH ₄ gas and H ₂ gas at a substrate temperature of 200 ° C. to 300 ° C. To film. Here, as the source gas used for forming the amorphous silicon film, Si ₂ H ₆ can be used in addition to SiH ₄ . Further, as a material of the semiconductor film, silicon (S
Other than i), silicon germanium (SiGe) or the like can be used. The silicon semiconductor film thus manufactured is patterned by etching to form an island-shaped semiconductor film 202.

Next, the processed semiconductor film 202 made of amorphous silicon is irradiated with a laser beam 203 to become polycrystalline silicon. Here, the laser light 203 for irradiation is
XeCl excimer laser, shape is 300mm x
A laser beam of 0.1 mm was used to form a laser beam.
Irradiation is performed sequentially in steps of 02 mm. At this time, the linear laser light and the gate bus line 306 are aligned in parallel so that the pixel transistor 307 and the compensation transistor 305, which are arranged linearly in the direction of the gate bus line 306, are simultaneously irradiated. By doing so, irradiation is performed within the same laser shot. Here, the laser light used was an XeCl (308 nm) excimer laser, but KrF (248 nm), ArF (193 nm),
An excimer laser such as KrCl (222 nm) may be used.

Next, the insulating film 204 is formed to a thickness of 50 to 150.
to be nm. As the insulating film, a SiO ₂ film formed by atmospheric pressure CVD method at 430 ° C. using SiH ₄ gas and O ₂ gas was used. Although the atmospheric pressure CVD method is used here, the film thickness is 50 nm formed by any one of the sputtering method, the low pressure CVD method, the plasma CVD method, and the remote plasma CVD method.
It goes without saying that a SiO ₂ film of up to 150 nm may be used. TEOS (Tetra-) with good step coverage
Ethyl-Ortho-Silicate, Si (O
C ₂ H ₅ ) ₄ ) Atmospheric pressure CVD method using gas, plasma CV
SiO ₂ film may be used by the D method. Although SiO ₂ is used here, SiNx, Al ₂ O ₃ , Ta ₂ O ₅ or a combination thereof may be used.

Next, the gate bus line 306 and the gate electrode 205 are formed by the sputtering method. Film thickness is 20
It is set to 0 to 400 nm. Pixel transistor 307
It is preferable that the compensation transistor 305 and the compensation transistor 305 have substantially the same performance. Therefore, it is desirable that the semiconductor film 202 and the insulating film 204 have the same film thickness, the same gate length, and the same gate width. . The material is Al, Al
A metal containing Si, Ta, Nb, Ti or the like as a main component may be used.
A metal containing aluminum as a main component is preferable because it can form a low resistance electrode wiring.

Next, impurity ions are implanted into the source and drain portions of the semiconductor film 202 using an ion doping apparatus. Ions containing a phosphorus element are implanted into the source and drain portions of the N-channel transistor, and ions containing a boron element are implanted into the source and drain portions of the P-channel transistor, respectively, using a resist mask. Although an ion doping apparatus is used here, an ion implantation apparatus may be used. Needless to say, ions forming N and P channels may be used as the impurity ions other than phosphorus and boron.

Next, the impurity ions are activated by laser irradiation. Although laser irradiation is performed here, impurity ions may be activated by thermal annealing.

Next, the first interlayer insulating film 206 is coated with TEOS (Tetra-Ethyl-Orth) with good step coverage.
Si by atmospheric pressure CVD method using o-Silicate, Si (OC ₂ H ₅ ) ₄ ) gas, or plasma CVD method
An O ₂ film was formed. This is the first interlayer insulating film may be formed by a plasma CVD method SiNx film other than SiO ₂ film, or may have a two-layer structure of SiO ₂ film and the SiNx film.

Next, contact holes are formed, a metal film is formed by a sputtering method, and then patterning is performed to form source bus lines 302 and lead electrodes 207 such as source and drain electrodes. Similar to the gate electrode, this metal film is made of Al, AlSi, Ta, Nb,
A metal containing Ti or the like as a main component may be used. A metal containing aluminum as a main component is preferable because it can form a low resistance electrode wiring.

Next, as the second interlayer insulating film 208, a SiNx film was formed by the plasma CVD method. In addition to the SiNx film, the second interlayer insulating film may be SiO _{2 formed} by a normal pressure CVD method using TEOS gas or a plasma CVD method.
A film may be formed, and a SiO ₂ film and a SiNx film may be used.
It may have a layered structure. As described above, the pixel transistor 307 including the thin film transistor and the compensation transistor 305 were manufactured.

After that, a contact hole is formed, a transparent electrode is formed by a sputtering method, and patterning is performed to form a pixel electrode 209.

Note that the driver circuits such as the source driver 301 and the gate driver 303 and the compensating circuit 304 having the structure shown in FIG. 3 are manufactured by using another step or the same step performed in the middle of the above steps.

As described above, a liquid crystal drive substrate having a drive circuit and a compensation circuit, a compensation transistor, and a pixel transistor is manufactured.

After that, an alignment film is formed, bonding with a counter electrode is performed, and a liquid crystal material is injected to complete the liquid crystal display device shown in FIG.

FIG. 3 shows an example of a compensation circuit 304 including a compensation transistor 305 provided for each gate bus line 306.

A constant voltage 402 is applied to the source electrode of the compensating transistor 305, and an output current from the drain electrode is made to flow through a fixed resistor 404 to be voltage-converted and input to a differential amplifier 401. The signal from the gate driver is input to the other input of the differential amplifier 401, and by adjusting the gate voltage of the compensation transistor 305 so that the voltage-converted output of the compensation transistor 305 becomes the same voltage. , The gate voltage of the pixel transistors 307 arranged on the same gate bus line 306 is adjusted, and the output currents of the pixel transistors 307 arranged in a matrix in the substrate can be made uniform for each gate bus line 306. Uniform display characteristics can be obtained by eliminating display unevenness due to variations between laser shots.

(Embodiment 2) FIG. 4 is a plan view schematically showing the overall structure of an active matrix type liquid crystal display device according to Embodiment 2. As shown in FIG.

In this active matrix type liquid crystal display device, a source bus line 302 and a gate bus line 306 are provided so as to intersect each other on an insulating substrate, and an image signal from the source driver 301 is supplied to each source bus line 302. The gate signal is supplied to each gate bus line 306 from the gate driver 303. Further, a compensating circuit 510 having a configuration described later including a compensating transistor 305 is provided between the gate driver 303 and each gate line 306.

The source driver 301 is composed of a shift register circuit, sends an image signal to each source bus line 302 in sequence, and supplies the pixel transistor 30.
An electric field is applied to the liquid crystal through a pixel electrode (not shown) connected to the source electrode of No. 7. The gate driver 303 is composed of a shift register circuit, and is connected to each gate bus line 306 through a compensation circuit 510,
The gate signal of the pixel transistor 307 is sequentially turned on and then O
FF. Further, the compensation transistor 305 and the pixel transistor 307 arranged in a line, which are provided for each gate bus line 306, are linearly arranged, and therefore, the gate bus line 306 is provided in the compensation transistor 305. It is diverted at the place.

An alignment film is formed on the liquid crystal driving substrate having the above structure and the counter substrate having the counter electrode, the both substrates are bonded together, and the liquid crystal material is injected between the two substrates.

The compensating transistor 305, the pixel transistor 307 and the vicinity thereof in the active matrix type liquid crystal display device of the present embodiment have the same structure as in the first embodiment, and the structure of the liquid crystal driving substrate will be described with reference to FIG. Will be described below in the order of manufacturing steps.

The compensating transistor 305 and the pixel transistor 307 have an island-shaped semiconductor film 202 on an insulating substrate 201. This semiconductor film 202
Has a thickness of 30 to 150 nm. As a method of manufacturing the island-shaped semiconductor film 202, in the case of a silicon semiconductor, an amorphous silicon film is formed by a plasma CVD method using SiH ₄ gas and H ₂ gas at a substrate temperature of 200 ° C. to 300 ° C. To film. Here, as the source gas used for forming the amorphous silicon film, Si ₂ H ₆ can be used in addition to SiH ₄ . Further, as a material of the semiconductor film, silicon (S
Other than i), silicon germanium (SiGe) or the like can be used. The silicon semiconductor film thus manufactured is patterned by etching to form an island-shaped semiconductor film 202.

Next, the processed semiconductor film 202 made of amorphous silicon is irradiated with a laser beam 203 to become polycrystalline silicon. Here, the laser light 203 for irradiation is
XeCl excimer laser, shape is 300mm x
A laser beam of 0.1 mm was used to form a laser beam.
Irradiation is performed sequentially in steps of 02 mm. At this time, the linear laser light and the gate bus line 306 are aligned in parallel so that the pixel transistor 307 and the compensation transistor 305, which are arranged linearly in the direction of the gate bus line 306, are simultaneously irradiated. By doing so, irradiation is performed within the same laser shot. Here, the laser light used was an XeCl (308 nm) excimer laser, but KrF (248 nm), ArF (193 nm),
An excimer laser such as KrCl (222 nm) may be used.

Next, the insulating film 204 is formed to a thickness of 50 to 150.
to be nm. As the insulating film, a SiO ₂ film formed by atmospheric pressure CVD method at 430 ° C. using SiH ₄ gas and O ₂ gas was used. Although the atmospheric pressure CVD method is used here, the film thickness is 50 nm formed by any one of the sputtering method, the low pressure CVD method, the plasma CVD method, and the remote plasma CVD method.
It goes without saying that a SiO ₂ film of up to 150 nm may be used. TEOS (Tetra-) with good step coverage
Ethyl-Ortho-Silicate, Si (O
C ₂ H ₅ ) ₄ ) Atmospheric pressure CVD method using gas, plasma CV
SiO ₂ film may be used by the D method. Although SiO ₂ is used here, SiNx, Al ₂ O ₃ , Ta ₂ O ₅ or a combination thereof may be used.

Next, the gate bus line 306 and the gate electrode 205 are formed by the sputtering method. Film thickness is 20
It is set to 0 to 400 nm. Pixel transistor 307
It is preferable that the compensation transistor 305 and the compensation transistor 305 have substantially the same performance. Therefore, it is desirable that the semiconductor film 202 and the insulating film 204 have the same film thickness, the same gate length, and the same gate width. . The material is Al, Al
A metal containing Si, Ta, Nb, Ti or the like as a main component may be used.
A metal containing aluminum as a main component is preferable because it can form a low resistance electrode wiring.

Next, impurity ions are implanted into the source and drain portions of the semiconductor film 202 using an ion doping apparatus. Ions containing a phosphorus element are implanted into the source and drain portions of the N-channel transistor, and ions containing a boron element are implanted into the source and drain portions of the P-channel transistor, respectively, using a resist mask. Although an ion doping apparatus is used here, an ion implantation apparatus may be used. Needless to say, ions forming N and P channels may be used as the impurity ions other than phosphorus and boron.

Next, the impurity ions are activated by laser irradiation. Although laser irradiation is performed here, impurity ions may be activated by thermal annealing.

Next, the first interlayer insulating film 206 is coated with TEOS (Tetra-Ethyl-Orth) with good step coverage.
Si by atmospheric pressure CVD method using o-Silicate, Si (OC ₂ H ₅ ) ₄ ) gas, or plasma CVD method
An O ₂ film was formed. This is the first interlayer insulating film may be formed by a plasma CVD method SiNx film other than SiO ₂ film, or may have a two-layer structure of SiO ₂ film and the SiNx film.

Next, a contact hole is formed, a metal film is formed by a sputtering method, and then patterning is performed to form a source bus line 302 and a lead electrode 207 such as a source electrode and a drain electrode. Similar to the gate electrode, this metal film is made of Al, AlSi, Ta, Nb,
A metal containing Ti or the like as a main component may be used. A metal containing aluminum as a main component is preferable because it can form a low resistance electrode wiring.

Next, a SiNx film was formed as the second interlayer insulating film 208 by the plasma CVD method. In addition to the SiNx film, the second interlayer insulating film may be SiO _{2 formed} by a normal pressure CVD method using TEOS gas or a plasma CVD method.
A film may be formed, and a SiO ₂ film and a SiNx film may be used.
It may have a layered structure. As described above, the pixel transistor 307 including the thin film transistor and the compensation transistor 305 were manufactured.

After that, a contact hole is formed, a transparent electrode is formed by a sputtering method, and patterning is performed to form a pixel electrode 209.

Note that the driver circuits such as the source driver 301 and the gate driver 303 and the compensating circuit 510 having the structure shown in FIG. 5 are manufactured by using another step or the same step performed in the middle of the above steps.

As described above, a liquid crystal drive substrate having a drive circuit and a compensation circuit, a compensation transistor, and a pixel transistor is manufactured.

After that, an alignment film is formed, bonding with a counter electrode is performed, and a liquid crystal material is injected, whereby the liquid crystal display device shown in FIG. 4 is manufactured.

FIG. 5 shows an example of a compensation circuit 510 provided for each gate bus line 306 and including a compensation transistor 305.

A constant voltage 502 is applied to the gate and source electrodes of the compensating transistor 305, and the output current from the drain electrode is made to flow through the fixed resistor 504 for voltage conversion, and the comparator 5
01a, 501b, 501c. Comparator 50
1a, 501b, 501c, a constant voltage 502 is applied to the resistor 50.
The reference voltage divided by 3a, 503b, and 503c is input to the other side, and compared with the input from the voltage-converted compensation transistor 305, and the comparator 501
The output voltage of a, 501b, 501c is changed stepwise. The outputs from the comparators 501a, 501b, and 501c are connected to the level shifter 508 via the diode 507, and the pixel transistors 307 arranged on the gate bus line 306 by the signal from the gate driver.
A gate voltage adjusted by the compensating circuit is applied to. Here, three comparators are connected to adjust in three steps, but it is possible to make fine adjustments by increasing the number of comparators. By thus making the output currents of the pixel transistors 307 arranged in the substrate uniform for each gate bus line 306, it is possible to eliminate display unevenness due to variations between laser shots, and to provide uniform display characteristics. Is obtained.

[0060]

As described in detail above, according to the present invention, a compensation transistor arranged linearly in the gate bus line direction so that the uniformity of the linear laser beam in the length direction can be utilized. Since the respective semiconductor films of the pixel transistor and the pixel transistor are arranged so that they can be simultaneously irradiated, a linear laser beam is applied to the semiconductor film of the compensation transistor and the pixel transistor arranged in the straight line in the gate bus line direction. When is irradiated with, the pixel transistor and the compensation transistor have the same characteristics. Therefore, the characteristic variation of the pixel transistor can be detected by the compensation transistor.

By detecting the signal of the compensating transistor and adjusting the gate voltage of the pixel transistor arranged in the gate bus line direction by the compensating circuit, the output variation of the pixel transistor can be suppressed. Therefore, uniform display characteristics of the liquid crystal display device can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view schematically showing the overall configuration of an active matrix liquid crystal display device according to a first embodiment.

FIG. 2 is a cross-sectional view showing a portion in the vicinity of a compensation transistor and a pixel transistor adjacent thereto in the active matrix liquid crystal display device of the first embodiment.

FIG. 3 is a circuit diagram showing a compensating circuit, a compensating transistor, and a pixel transistor included in the active matrix liquid crystal display device according to the first embodiment.

FIG. 4 is a plan view schematically showing the overall configuration of an active matrix liquid crystal display device according to a second embodiment.

FIG. 5 is a circuit diagram showing a compensating circuit, a compensating transistor, and a pixel transistor included in the active matrix liquid crystal display device according to the second embodiment.

FIG. 6 is a diagram showing a voltage-current characteristic example of a pixel transistor due to variations between laser shots in a conventional active matrix type liquid crystal display device.

[Explanation of symbols]

201 Insulating substrate 202 semiconductor film 203 laser light 204 insulating film 205 gate electrode 206 First interlayer insulating film 207 Extraction electrode 208 Second interlayer insulating film 209 pixel electrode 301 Source driver 302 Source bus line 303 gate driver 304 Compensation circuit 305 Compensation transistor 306 Gate bus line 307 Pixel transistor 401 differential amplifier 402 constant voltage 404 fixed resistance 501a, 501b, 501c comparator 502 constant voltage 503a, 503b, 503c resistance 504 fixed resistance 507 diode 508 level shifter 510 Compensation circuit

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Claims (5)

(57) [Claims] 1. A plurality of gate bus lines and a plurality of source bus lines are arranged in a cross shape on an insulating substrate, and a pixel transistor for driving a pixel electrode is arranged in the vicinity of an intersection of both bus lines. In the active matrix type liquid crystal display device, a compensation transistor corresponding to each gate bus line and provided in a linear arrangement with the pixel transistor connected to the gate bus line, and the compensation transistor An active matrix type liquid crystal display device comprising: a compensation circuit which is provided correspondingly and controls a gate voltage applied to the gate bus line by an output signal from the compensation transistor. 2. The compensation circuit includes a constant voltage power source for supplying a constant voltage to a source electrode of the compensation transistor, a fixed resistor for converting an output of the compensation transistor into a voltage, a gate signal from a gate driver, and The active matrix type liquid crystal display device according to claim 1, further comprising a differential amplifier which receives an output signal of the compensation transistor, outputs an appropriately adjusted gate voltage, and supplies the gate voltage to the gate bus line. . 3. The compensation circuit includes a constant voltage power source for supplying a constant voltage to a gate electrode and a source electrode of the compensation transistor, a fixed resistor for converting an output from the compensation transistor into a voltage, and the compensation circuit. The comparator for comparing the output from the transistor for use with the reference voltage, the gate signal from the gate driver and the output signal from the comparator are input, and a properly adjusted gate voltage is output and supplied to the gate bus line. 2. The active matrix type liquid crystal display device according to claim 1, wherein the active matrix type liquid crystal display device comprises a level shifter. 4. The crystallinity of a semiconductor film used as an element region of the pixel transistor connected to the arbitrary gate bus line, and used as an element region of the compensation transistor corresponding to the gate bus line. 4. The active matrix type liquid crystal display device according to claim 1, wherein the crystallinity of the semiconductor film is substantially the same. 5. The compensation transistor and the pixel transistor are formed in the same step, and the gate length and the gate width of each of the pixel transistor and the compensation transistor are substantially the same size. 4. An active matrix liquid crystal display device according to any one of items 1 to 3.