KR100573108B1 - Flat panel display with TFT - Google Patents

Abstract

According to the present invention, according to the difference in the thickness and the grain size of the active layer of each driving TFT, and white balance is achieved without changing the size of the active layer of the driving TFT, an appropriate luminance is obtained by supplying an optimum current to each subpixel, In order not to shorten the lifespan, the organic electroluminescent device includes pixels including a plurality of subpixels including an organic electroluminescent device, and a semiconductor active layer provided in each of the subpixels and having a channel region. The present invention relates to a flat panel display device comprising a driving thin film transistor having a thickness of a channel region of the active layer that is connected to each other to supply a current.

Description

Flat panel display with TFT

1 is a plan view for explaining the structure of a thin film transistor active layer of an active matrix organic electroluminescent display device according to an exemplary embodiment of the present invention;

2 is a plan view showing a crystal structure of a second active layer of a driving TFT of red, green and blue subpixels;

3 is a cross sectional view taken along the line I-I of FIG. 2, showing different thicknesses of the second active layers of the driving TFTs of the red, green, and blue subpixels;

4 is a graph showing a relationship between grain size and current mobility;

5 is a graph showing the relationship between energy density and grain size in the ELA crystallization method;

6 is a partially enlarged plan view illustrating one subpixel in FIG. 1;

7 is an equivalent circuit diagram of a unit pixel of FIG. 6;

8 is a cross-sectional view taken along line II-II of FIG. 6;

9 is a cross-sectional view taken along line III-III of FIG. 6;

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix flat panel display device having a thin film transistor, and more particularly, to a polycrystalline silicon as an active layer, and to a thin film transistor having a different thickness and grain size of a channel region for each display pixel. A flat panel display is provided.

Thin Film Transistors (TFTs), which are used in flat panel displays such as liquid crystal display devices, organic electroluminescent display devices, or inorganic electroluminescent display devices, are used as switching elements for controlling the operation of each pixel and as driving elements for driving pixels. do.

The thin film transistor has a semiconductor active layer having a drain region and a source region doped with a high concentration of impurities on a substrate, and a channel region formed between the drain region and the source region, and the gate insulating film and the active layer formed on the semiconductor active layer. The semiconductor active layer is divided into amorphous silicon and polycrystalline silicon according to the crystal state of silicon.

Thin film transistors using amorphous silicon have the advantage of being capable of low temperature deposition. However, recently, polycrystalline silicon has been used a lot since electrical properties and reliability are deteriorated, and the large area of the display device is difficult. Polycrystalline silicon has a high current mobility of tens to hundreds of cm 2 /V.s, and has a high frequency operation characteristic and a low leakage current value, making it suitable for use in high-definition and large-area flat panel display devices.

On the other hand, as described above, the thin film transistor is used as a switching element or a driving element of a pixel in a flat panel display device, and an active matrix active matrix (AM) type organic electroluminescent display device is a Two thin film transistors (hereinafter referred to as "TFT") are provided per pixel.

The organic electroluminescent device has a light emitting layer made of an organic material between an anode electrode and a cathode electrode. In this organic electroluminescent device, as the anode and cathode voltages are applied to these electrodes, holes injected from the anode electrode are moved to the light emitting layer via the hole transport layer, and electrons are transferred from the cathode electrode through the electron transport layer. Is injected into the light emitting layer, and electrons and holes recombine in the light emitting layer to generate excitons, and as the excitons change from the excited state to the ground state, the fluorescent molecules in the light emitting layer emit light to form an image. In the case of a full color organic light emitting display device, full color is realized by including pixels emitting three colors of red (R), green (G), and blue (B) as the organic light emitting diode.

However, in the organic light emitting display device as described above, the luminous efficiency of the red, green, and blue light emitting layers emitting the respective colors is different for each of the colors. When the same current is applied, some colors are inferior in luminance according to the luminous efficiency, and in some colors, the luminance is increased and it is difficult to obtain a proper color balance or white balance. For example, since the luminous efficiency of the green light emitting layer is 3 to 6 times higher than that of the red light emitting layer and the blue light emitting layer, more current needs to be applied to the red and blue light emitting layers in order to achieve white balance.

As described above, as a conventional method for adjusting the white balance, Japanese Patent Laid-Open No. Hei 5- 107561 discloses a method of differently applying a voltage supplied through a driving line, that is, a Vdd value for each pixel.

In addition, Japanese Patent Laid-Open No. 2001-109399 discloses a method of adjusting the white balance by adjusting the size of the driving TFT. That is, when the channel width of the channel region of the driving TFT is W and the channel length is L, the ratio W / L is designed differently for each pixel of red, green, and blue, so that each organic field of red, green, and blue is made. The amount of current flowing through the light emitting device is controlled.

Japanese Patent Laid-Open No. 2001-290441 discloses a method of matching white balance by forming each pixel in a different size. That is, the light emitting area of the green light emitting area having the highest luminous efficiency is formed to be the smallest compared to the light emitting areas of the red and blue light emitting areas to achieve white balance and long life. This difference in light emitting area can be made possible by the area of the anode electrode.

In addition, a method of controlling luminance by controlling a current amount by varying a voltage range applied through a data line for each pixel of red, green, and blue is known.

By the way, the above methods do not consider the crystal structure of the TFT of a flat panel display device using polycrystalline silicon. That is, the crystal grains of the TFT active layer may have various shapes and sizes depending on the crystallization method, and the current mobility may vary according to the shape and size of the crystal grains. In this case, the white balance may be adjusted by the above methods. Problems that can't be solved can occur.

On the other hand, in the organic EL device, when the amount of current flowing through the organic EL device per sub-pixel exceeds the threshold value, the luminance per unit area is greatly increased by the amount of current exceeding the threshold value. The lifespan of is drastically reduced. Therefore, the optimum amount of current per subpixel must be supplied for the lifetime of the device.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide a flat panel display that can balance white balance according to a difference in thickness and grain size of an active layer of each driving TFT.

Another object of the present invention is to provide a flat panel display that can achieve white balance even under the same driving voltage applied without changing the size of the active layer of the driving TFT.

It is still another object of the present invention to provide a flat panel display device which obtains an appropriate brightness by supplying an optimum current to each subpixel and does not shorten the lifetime.

In order to achieve the above object, the present invention includes a pixel including a plurality of subpixels having an organic electroluminescent element, and a semiconductor active layer provided in each of the subpixels and having a channel region. The present invention provides a flat panel display device comprising a driving thin film transistor having a thickness of a channel region of the active layer differently provided for each subpixel, respectively, connected to an electroluminescent element to supply a current.

According to another feature of the invention, the sub-pixels may be provided to have two different colors.

According to another feature of the present invention, the thickness of each channel region may be provided differently for each color of each subpixel.

According to another feature of the present invention, the thickness of each channel region may be determined to be inversely proportional to the current value flowing through each of the subpixels under the same driving voltage.

According to another feature of the present invention, the thickness of each channel region may be determined to be inversely proportional to the current mobility value of the channel region of the active layer of the respective subpixels.

According to another feature of the present invention, the subpixels are provided to have red, green, and blue colors, and the thickness of the channel region of the driving thin film transistors of the green subpixels is the channel of the driving thin film transistors of the red and blue subpixels. It may be provided thicker than the thickness of the region.

According to another feature of the present invention, the subpixels are provided to have red, green, and blue colors, and the thickness of the channel region of the driving thin film transistor of the red subpixels is the channel of the driving thin film transistor of the green and blue subpixels. It may be provided thinner than the thickness of the region.

According to another feature of the present invention, the subpixels may be provided to have red, green, and blue colors, and the thickness of each channel region may be provided to be thin in the order of green, blue, and red subpixels.

According to another feature of the present invention, the semiconductor active layer may be formed of polycrystalline silicon, and the size of crystal grains of the channel region of the driving thin film transistor may be different from each other.

According to another feature of the present invention, the size of the crystal grains of each channel region may be determined to be inversely proportional to the current value flowing through each of the subpixels under the same driving voltage.

According to another feature of the present invention, the size of the grains of each channel region may be determined to be inversely proportional to the current mobility value of the channel region of the active layer of each of the subpixels.

According to another feature of the present invention, the subpixels are provided to have red, green, and blue colors, and the size of the crystal grains of the channel region of the driving thin film transistors of the green subpixels is the driving thin film transistor of the red and blue subpixels. It may be provided smaller than the size of the grains of the channel region of.

According to another feature of the present invention, the subpixels are provided to have red, green and blue colors, and the size of the crystal grains of the channel region of the driving thin film transistors of the red subpixels is the driving thin film transistor of the green and blue subpixels. It may be provided larger than the size of the grains of the channel region of.

According to another feature of the present invention, the subpixels may be provided to have red, green, and blue colors, and the size of crystal grains of each channel region may be provided to increase in the order of green, blue, and red subpixels. .

According to another feature of the invention, the semiconductor active layer is provided with polycrystalline silicon, the polycrystalline silicon may be formed by a crystallization method by a laser.

According to another feature of the present invention, the channel regions of the respective subpixels may be formed by simultaneously performing laser irradiation.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

1 is a plan view illustrating a thin film transistor active layer structure of an active matrix organic light emitting display device according to an exemplary embodiment of the flat panel display device according to the present invention. As shown in FIG. 1, each pixel of the organic light emitting display device has a sub-pixel of red (R), green (G), and blue (B) in a vertical direction (vertical direction in FIG. 1). ) To be arranged repeatedly. However, the configuration of the pixels is not necessarily limited thereto, and the subpixels of each color may be arranged in various patterns such as mosaics or grids to form pixels, and the colors may be variously composed of two or more different colors. Of course you can. Further, the mono color flat display device may be used instead of the full color flat display device shown in FIG. 1.

In such an organic light emitting display device, a plurality of gate lines 51 are disposed in a horizontal direction (left and right directions in FIG. 1), and a plurality of data lines 52 are disposed in a vertical direction. In addition, a drive line 53 for supplying electric power is also arranged in the longitudinal direction. These gate lines 51, data lines 52, and drive lines 53 are provided to surround one subpixel.

Meanwhile, in the above configuration, each of the subpixels of the red (R), green (G), and blue (B) pixels is a first thin film transistor (hereinafter referred to as a "first TFT") and a second thin film transistor. Two thin film transistors (hereinafter referred to as "second TFTs") are provided, wherein the first TFTs 10r, 10g, 10b control the operation of the device in accordance with the signal of the gate line 51. It may be a switching thin film transistor, and the second TFT 20r (20g) 20b may be a driving thin film transistor for driving an element. Of course, the number and arrangement of the thin film transistors may vary depending on the characteristics of the display, the driving method, and the like, and the arrangement may also exist in various ways.

These first TFTs 10r, 10g, 10b and second TFTs 20r, 20g, 20b are respectively the first active layers 11r, 11g, 11b and the second active layer 21r, which are semiconductor active layers. (21g) 21b, these active layers each have channel regions as described below, although not shown in the figure. The channel region is a region located substantially at the center of the first active layers 11r, 11g, 11b and the second active layers 21r, 21g, 21b formed in the longitudinal direction, and the gate electrode is insulated through the top thereof. Corresponds to the formed area.

In such an organic light emitting display device, the thickness of the channel region of the second active layers constituting the driving TFT may be provided differently for each subpixel. In a preferred embodiment of the present invention, the thickness of each channel region of these second active layers may be formed differently for each color. That is, the channel region of the second active layer 21r constituting the red pixel R, the channel region of the second active layer 21g constituting the green pixel G, and the second active layer constituting the blue pixel B ( 21b) to have different thicknesses for each channel region. And, if the color of each sub-pixel is composed of a color other than red, green, blue, it may be arranged by varying the thickness of the channel region for each color.

Meanwhile, according to a preferred embodiment of the present invention, the first active layer 11r (11g) 11b and the second active layer 21r (21g) 21b may be formed of a polycrystalline silicon thin film. As shown in FIG. 3, the thicknesses of the second active layers 21r, 21g, and 21b, in particular, the thickness of the channel region are red (R), green (G), and blue (B). Each pixel of the color is provided differently. At this time, it is sufficient that the thickness of the channel region, which is the center portion of the second active layers 21r, 21g, 21b, is different from each other. However, due to the complexity of the structural design, the thickness of the entire second active layer is different from each other. It was. In the preferred embodiments of the present invention to be described below, the types of crystal grains are different for each subpixel, but the present invention is not limited thereto, and the first active layers 11r and 11g of each display pixel are not limited thereto. All of (11b) may have the same crystal structure.

According to an exemplary embodiment of the present invention, the channel region of the active layer of the second TFT used as the driving thin film transistor is made to have a different thickness for each display pixel of red, green, and blue, that is, the total size of the active layer, that is, Therefore, the planar size can be the same and white balance can be achieved even for the same driving voltage. Hereinafter, this principle will be described in more detail.

As described above, in the organic electroluminescent display, the sub-pixels of red, green, and blue differ in luminance due to the difference in the luminous efficiency of the light emitting layer, and accordingly, the white balance is adjusted for the same current value. I could not. Table 1 shows the current values to be applied to each of the red, green, and blue subpixels in order to satisfy the luminous efficiency and white balance of the red, green, and blue organic light emitting layers commonly used in the organic light emitting display. It was.

Red green blue  Efficiency (Cd / A) 6.72 23.37 4.21  Display pixel current (㎂) 0.276 0.079 0.230  Display pixel current ratio 3.5 One 2.9

As can be seen in Table 1 above, the current values that must flow to satisfy the white balance are the greenest subpixel, the next blue subpixel, and the largest red subpixel should flow. have.

This difference in current value can be achieved by varying the thickness of the channel region of the active layer of the second TFT 20r (20g) 20b of FIG. 1, which is a driving thin film transistor that supplies current to the light emitting device.

Due to the change in the thickness of the channel region of the TFT active layer, there are many differences in the TFT characteristics. When the thickness of the channel region of the active layer is thin, the current mobility in the channel region is increased, and thus, excellent TFT characteristics can be obtained. . Therefore, when the thickness of the channel region of the active layer of the TFT requiring a higher current mobility value is formed thinner, better TFT characteristics can be obtained. This effect is true not only for polycrystalline silicon but also for amorphous silicon.

Accordingly, according to an exemplary embodiment of the present invention as shown in FIG. 3, the thickness of the channel region of each of the second active layers 21r, 21g, and 21b is a current flowing through each subpixel under the same driving voltage. You can adjust it to be inversely proportional to the value. That is, as described above, when the red, green, and blue subpixels are provided, the second active layer 21r of the red subpixel, which requires the most current to flow in order to achieve the white balance, is the thinnest and the smallest current The thickest is the second active layer 21g of the green sub-pixel that should flow. Accordingly, the thickness of the channel region of the second active layer may be thinned in the order of green, blue, and red.

Further, the thickness of the channel region of each of the second active layers 21r, 21g, 21b can be adjusted in inverse proportion to the current mobility value of the channel region. That is, as described above, when the red, green, and blue subpixels are provided, the red subpixel, which requires the most current to flow in order to achieve the white balance, should have the largest current mobility value. The greenest sub-pixel to which 21r) is the thinnest and the smallest current should flow has the smallest current mobility value, so that the second active layer 21g is the thickest. Accordingly, the thickness of the channel region of the second active layer may be thinned in the order of green, blue, and red. The thickness adjustment of the active layer channel region as described above is the same even when the active layer is formed of amorphous silicon.

On the other hand, by adjusting the thickness of the channel region of each active layer as described above, when the crystallization from amorphous silicon to polycrystalline silicon, the crystal size can be different, and thus the current mobility can be different. In addition, without adjusting the crystal size by a separate process, for example, when crystallized by the ELA method, it is possible to obtain active layers having different crystal sizes from each other by laser irradiation simultaneously over three regions.

FIG. 2 shows a state in which red, green, and blue subpixels form a second active layer of a second TFT which is a driving TFT as a crystal structure of a different polycrystalline silicon thin film for each of the red, green, and blue subpixels. This polycrystalline silicon thin film is obtained by crystallizing an amorphous silicon thin film according to a known excimer laser annealing (ELA, hereinafter referred to as "ELA") method. 3 is a cross-sectional view taken along the line II of FIG. 2.

As shown in FIG. 3, the second active layer 21r of the red subpixel is formed in the silicon thin film having the first crystal structure 61 having the thinnest thickness d1 and the second active layer 21g of the green subpixel. Is formed on the silicon thin film having the second crystalline structure 62 having the thickest thickness d2, and the second active layer 21b of the blue subpixel is silicon having the third crystalline structure 63 having the intermediate thickness d3. As shown in FIG. 2, the second active layer 21r of the red subpixel is formed in the first crystal structure 61 having the largest size, and the second active layer 21g of the green subpixel is formed in the thin film. ) Is formed in the second crystal structure 62 of the smallest size, and the second active layer 21b of the blue subpixel is formed in the third crystal structure 63 of the medium size. The preparation is possible by varying the thickness of amorphous silicon, and then making it into polycrystalline silicon by crystallization and patterning it, as will be described later.

As described above, as the size of the crystal grains is changed according to the thickness of the silicon thin film, the difference in current mobility can be obtained. As shown in FIG. 4, as the size of the crystal grains increases, the current mobility is increased. It becomes large and becomes a nearly linear relationship.

Therefore, in the preferred embodiment of the present invention as shown in FIG. 2, the size of the crystal grains of the channel region of the second active layer of the driving TFT of the red subpixel requiring a larger current mobility is larger than this. The driving TFT of the green subpixel that requires the smallest current mobility so that the size of the crystal grains of the channel region of the second active layer of the blue TFT subpixel requiring the small current mobility is smaller than that of the red subpixel. The size of the crystal grains in the channel region of the second active layer of was minimized.

This means that the thinner the silicon thin film, the higher the energy density of the laser that amorphous silicon receives, thereby obtaining larger grains.

On the other hand, when the amorphous silicon thin film is subjected to an excessively high energy density laser, it may be completely melted and the size of its crystal grains may be rather small. Therefore, the second active layer 21r of the driving TFT of the red subpixel requiring larger crystal grains is formed. It is preferable not to make the thickness d1 of the silicon thin film of the first crystal structure 61 to be made too thin. FIG. 5 shows the difference in grain size according to the energy density of the irradiated laser when crystallizing an amorphous silicon thin film of 500 kV by the ELA method.

Therefore, according to an exemplary embodiment of the present invention, the thickness of the channel region of the second active layer of each of the subpixels may be thin in the order of green, blue, and red. The thickness of the silicon thin film may be changed by a half-tone method using a known photolithography method, and the red, green, and blue subpixels may be formed in regions where the second active layers are to be formed. Correspondingly, a layer having different thicknesses can be formed in a single process by exposure and development using an optical mask having a different light transmittance for each and then etching.

As such, the thickness d1 of the silicon thin film on which the second active layer 21r of the red subpixel is to be formed is thinner than the thickness d3 of the silicon thin film on which the second active layer 21b of the blue subpixel is to be formed, and the blue subpixel is formed. The thickness d3 of the silicon thin film on which the second active layer 21b is to be formed is thinner than the thickness d2 of the silicon thin film on which the second active layer 21g of the green subpixel is to be formed. The current mobility is green and blue by making the second active layer 21r of the pixel largest, the second active layer 21b of the blue subpixel next larger, and the second active layer 21g of the green subpixel smallest. And red subpixels in order, and consequently, larger currents can flow in the order of green, blue, and red subpixels under the same driving voltage. Therefore, white balance can be easily achieved even under the same driving voltage. In addition, in the present invention, by using such a thickness difference, the size of the crystal grains can be different from one laser irradiation, so that the manufacturing process can be very simplified.

Meanwhile, each sub-pixel of the organic light emitting display device as described above has a structure as shown in FIGS. 6 to 9.

First, FIG. 6 is a partially enlarged plan view of a subpixel of one of the subpixels of FIG. 1, and FIG. 7 is an equivalent circuit diagram of the subpixel of FIG. 6.

Referring to FIG. 7, each sub-pixel of an active matrix organic light emitting display device according to an exemplary embodiment of the present invention may have two sub-pixels: a first TFT 10 for switching and a second TFT 20 for driving. A thin film transistor, one capacitor 30 and one organic electroluminescent element (hereinafter referred to as "EL element", 40). The number of thin film transistors and capacitors as described above is not necessarily limited thereto, and of course, a larger number of thin film transistors and capacitors may be provided.

The first TFT 10 is driven by a scan signal applied to the gate line 51 to transfer a data signal applied to the data line 52. The second TFT 20 determines the amount of current flowing into the EL element 40 according to the data signal transmitted through the first TFT 10, that is, the voltage difference Vgs between the gate and the source. The capacitor 30 stores a data signal transmitted through the first TFT 10 for one frame.

In order to implement such a circuit, an organic light emitting display device having a structure as shown in FIGS. 6, 8, and 9 is formed, which will be described in more detail as follows.

6, 8, and 9, the buffer layer 2 is formed on the insulating substrate 1 of glass material, and the first TFT 10 and the second TFT 20 are formed on the buffer layer 2 above. , A capacitor 30 and an EL element 40 are provided.

6 and 8, the first TFT 10 is formed on the gate electrode 13 connected to the gate line 51 to apply the TFT on / off signal, and on the gate electrode 13. And a source electrode 14 which is connected to the data line 52 and supplies a data signal to the first active layer 11, and the first TFT 10 and the capacitor 30 are connected to supply power to the capacitor 30. Is composed of a drain electrode 15. A gate insulating film 3 is provided between the first active layer 11 and the gate electrode 13.

The charging capacitor 30 is located between the first TFT 10 and the second TFT 20 to store a driving voltage required to drive the second TFT 20 for one frame. FIGS. 6 and 8. As can be seen from, the first electrode 31 is connected to the drain electrode 15 of the first TFT 10, the upper portion of the first electrode 31 is formed so as to overlap the first electrode 31, the power is applied It may be provided with a second electrode 32 and an interlayer insulating film 4 formed between the first electrode 31 and the second electrode 32 and electrically used as a dielectric. . Of course, the structure of the charging capacitor 30 is not necessarily limited thereto, and the silicon thin film of the TFT and the conductive layer of the gate electrode may be used as the first and second electrodes, and the gate insulating layer may be used as the dielectric layer. It can be formed by various methods.

As shown in FIGS. 6 and 9, the second TFT 20 is connected to the first electrode 31 of the capacitor 30 to supply a TFT on / off signal and a gate electrode 23. Source electrode 24 formed on the top of the circuit board and connected to the driving line 53 to supply a reference voltage for driving to the second active layer 21, the second TFT 20 and the EL element 40. ) Is connected to the drain electrode 25 to apply driving power to the EL element 40. The gate insulating film 3 is provided between the second active layer 21 and the gate electrode 23. Here, as described above, the channel region of the second active layer 21 may have a shape or size of crystal grains different according to the color of the pixel.

On the other hand, the EL element 40 emits red, green, and blue light according to the flow of current to display predetermined image information. As shown in FIGS. 6 and 9, the drain of the second TFT 20 is shown. An anode electrode 441 connected to the electrode 25 and receiving positive power therefrom, a cathode electrode 43 provided to cover the entire pixel to supply negative power, and these anode electrodes 441 and It consists of the organic light emitting film 42 arrange | positioned between the cathode electrodes 43, and emitting light. In the drawing, reference numeral 5 denotes an insulating passivation film made of SiO ₂ , and the like, and 6 denotes an insulating planarization film made of acryl or the like.

As described above, the layer structure of the organic light emitting display device according to the preferred embodiment of the present invention is not necessarily limited to the above description, and the present invention may be applied to any other structure.

Such an organic light emitting display device has a channel region of the second active layer of the second TFT, which is the driving TFT, having a different thickness for each subpixel color, and thus has a different crystal grain size. The current mobility is different for each display pixel, and different current values flow even at the same driving voltage, so that the white balance can be balanced as a whole. In addition, since the current value of an appropriate value required for the EL elements of the respective subpixels can be supplied, durability of the element can be improved.

Next, an embodiment of a method of manufacturing an organic light emitting display device according to an exemplary embodiment of the present invention having the structure as described above will be described.

First, as shown in FIGS. 10 and 11, the buffer layer 2 is formed on the insulating substrate 1 of glass material. The buffer layer 2 may be formed of SiO ₂ , and may be deposited by PECVD, APCVD, LPCVD, ECR, or the like. The buffer layer 2 can be deposited to about 3000 mW.

An amorphous silicon thin film may be deposited on the buffer layer 2. The amorphous silicon thin film may be collectively patterned by a photolithography method using a half-tone mask to form a silicon thin film layer having different thicknesses. The second active layer of each of the red, green, and blue subpixels is formed. As shown in FIG. 3, the thicknesses of the regions where they are to be formed are formed to be d1, d2, d3. This difference in thickness can be formed by one exposure by making a difference in the light transmittance of the photomask. After the photoresist is applied to a region where the second active layers of each of the red, green, and blue subpixels are to be formed, By exposing and etching by using masks having different light transmittances, it is possible to obtain regions in which first active layers having different thicknesses are to be formed.

The amorphous silicon thin film thus formed may be crystallized into a polycrystalline silicon thin film by various methods. In this case, as shown in FIGS. 2 and 3, the crystallized polycrystalline silicon thin film may have different grain sizes for each color of each subpixel or for a portion corresponding to the second active layer of the second TFT of each subpixel. Will have

As described above, in order to obtain a crystal structure as shown in Figs. 2 and 3, laser irradiation is performed simultaneously by the ELA method. That is, the crystal structure having grains of different sizes can be obtained even by the single scanning of the laser due to the thickness difference as described above.

After the polycrystalline silicon thin film is formed, as shown in FIGS. 1 and 2, the first active layers 11r, 11g, and 11b of the first TFTs 10r, 10g, and 10b are formed for each subpixel. ) And the second active layers 21r (21g) 21b of the second TFTs 20r (20g) 20b are patterned. Of course, the patterning of such an active layer can be performed simultaneously when forming the thickness difference in the above-mentioned amorphous silicon thin film, and can also be collectively patterned after depositing the gate insulating film and gate electrode which are mentioned later.

After the active layer is patterned, a gate insulating film is formed by SiO _{2 or the} like by PECVD, APCVD, LPCVD, ECR, etc., and a conductive film is formed by MoW, Al / Cu, etc., and then patterned to form a gate. Form an electrode. The active layer, gate insulating film, and gate electrode may be patterned by various procedures and methods.

After patterning of the active layer, gate insulating film, and gate electrode, N-type or P-type impurities are doped into the source and drain regions.

After the doping process is completed, as shown in FIGS. 8 and 9, the interlayer insulating film 4 and the passivation film 5 are formed, and then the source electrodes 14, 24 and the drain electrode 15 are formed through contact holes. 25 is connected to the active layers 11 and 21 to form a planarization film 6.

On the other hand, the EL element 40 connected to the second TFT 20 can be formed by various methods. First, an anode electrode connected to the drain electrode 15 of the second TFT 20 by ITO. After forming 441, patterning is carried out to form an organic film 42 thereon. In this case, the organic film 42 may be a low molecular or polymer organic film.

First, in the case of using a low molecular organic film, a hole injection layer, a hole transport layer, an organic light emitting layer, an electron transport layer, an electron injection layer, etc. may be formed by stacking a single or a complex structure, and the usable organic material may also be formed of copper phthalocyanine (CuPc: copper phthalocyanine). ), N, N-di (naphthalen-1-yl) -N, N'-diphenyl-benzidine (N, N'-Di (naphthalene-1-yl) -N, N'-diphenyl-benzidine: NPB) And tris-8-hydroxyquinoline aluminum (Alq3). These low molecular weight organic films are formed by the vacuum deposition method.

The polymer organic film may have a structure including a hole transporting layer (HTL) and a light emitting layer (EML). In this case, PEDOT is used as the hole transporting layer, and polyvinyl vinylene (PPV) and polyfluorene (PPV) are used as the light emitting layer. Polymer organic materials such as polyfluorene) are used and are formed by screen printing or inkjet printing.

After the organic film is formed in this manner, the cathode electrode 43 may be formed by depositing or patterning the cathode electrode 43 on Al / Ca or the like. The upper portion of the cathode electrode 43 is sealed by a glass or metal cap.

What has been described above is a case where the present invention is applied to an organic electroluminescent display, but the present invention is not limited thereto, and the present invention is applicable to any structure that can use TFT as a pixel driving element such as a liquid crystal display or an inorganic electroluminescent display. Of course it can.

According to the present invention as described above, the following effects can be obtained.

First, white balance can be achieved even with an active layer of the same size without changing the size of the active layer or the driving voltage of the TFT.

Second, since the appropriate current is supplied for each subpixel, appropriate luminance can be obtained and life degradation can be prevented.

Third, by controlling only the amount of current flowing through the element without increasing the area occupied by the driving TFT per subpixel, it is possible to solve the problem of decreasing the aperture ratio and to improve the reliability.

Although described above with reference to a preferred embodiment of the present invention, those skilled in the art will be variously modified and changed within the scope of the invention without departing from the spirit and scope of the invention described in the claims below I can understand that you can.

Claims (16)

Pixels including a plurality of subpixels having an organic EL device; And A driving thin film transistor provided in each of the subpixels and having a semiconductor active layer having a channel region, the driving thin film transistor being connected to each of the organic electroluminescent elements to supply a current; In the subpixels having different colors among the subpixels, the thickness of the channel region of the active layer of the driving thin film transistor is provided differently for each color of the subpixels, The semiconductor active layer is made of polycrystalline silicon, And the polycrystalline silicon is formed by a crystallization method by a laser. delete delete The method of claim 1, And the thickness of each channel region is determined to be inversely proportional to a current value flowing through the subpixels under the same driving voltage. The method of claim 1, And the thickness of each channel region is determined to be inversely proportional to the current mobility value of the channel region of the active layer of each of the subpixels. The method of claim 1, The sub-pixels are provided to have a color of red, green, blue, And a channel area of the driving thin film transistors of the green subpixels is thicker than a channel area of the driving thin film transistors of the red and blue subpixels. The method of claim 1, The sub-pixels are provided to have a color of red, green, blue, And a channel area of the driving thin film transistors of the red subpixels is thinner than a thickness of the channel area of the driving thin film transistors of the green and blue subpixels. The method of claim 1, The sub-pixels are provided to have a color of red, green, blue, And the thickness of each of the channel regions is thinned in the order of green, blue, and red subpixels. delete delete delete delete delete delete delete The method of claim 1, And channel regions of each of the subpixels are formed by simultaneously performing laser irradiation.

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