KR20040069016A - Poly Si TFT LCD with High Reflection and Apparatus for Driving the Same - Google Patents

Abstract

PURPOSE: A TFT-LCD(Thin Film Transistor Liquid Crystal Display) having high reflectivity and an apparatus for driving the TFT-LCD are provided to increase reflectivity even when resolution of a fabrication process is low. CONSTITUTION: A TFT-LCD includes a common electrode layer(84), a pixel electrode layer(82), and a liquid crystal layer(83) interposed between the common electrode layer and pixel electrode layer. The TFT-LCD further includes a planarization layer(96) and a reflecting layer(97) formed between the liquid crystal layer and pixel electrode layer. The reflecting layer is formed in a manner that an insulating layer having high refractive index and an insulating layer having low refractive index are alternately formed.

Description

High-reflection thin film polycrystalline silicon liquid crystal display and its driving device {Poly Si TFT LCD with High Reflection and Apparatus for Driving the Same}

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a thin film transistor (poly TFT LCD) having a high reflectivity even with a large distance between adjacent pixel electrodes, and a driving device thereof.

A liquid crystal display device used in an LCD projector displays an image by applying a voltage to the liquid crystal layer using an active element such as a TFT for each pixel. The liquid crystal display device used in the liquid crystal projector is classified into a transmission type through which light passes through the pixel electrode and a reflection reflection type.

1 is an equivalent circuit diagram of a unit pixel of a liquid crystal display device using a TFT device. In the TFT liquid crystal display device, a TFT section 50 is attached to each pixel. During the selection period of the scan line 70, the voltage applied to the signal line 60 passes through the source-drain of the TFT portion 15 to pass through the capacitance C _LC of the liquid crystal layer and the capacitance C _ST of the storage capacitor. And the resistance between the source and the drain increases during the non-selection period of the scan line, so that the voltage applied to the liquid crystal layer and the storage capacitor is maintained during the selection period.

Fig. 2 is a sectional view of a conventional reflective TFT liquid crystal display device, which is composed of a common electrode layer, a pixel electrode layer and a liquid crystal layer therebetween. The common electrode layer includes a glass substrate 85, a common electrode 84 formed on the glass substrate 85, and an alignment layer 95 formed on the common electrode 84. The pixel electrode layer is formed by laminating pixel electrodes 82, TFT portions 50, light blocking films 81 and 85, and insulating films 90, 91, 92, 93, and 94 on the TFT glass substrate 80. An alignment film 95-1 is formed on the surface of the pixel electrode layer adjacent to the liquid crystal layer. On the other hand, the insulating film 93 formed between the light blocking film 86 and the pixel electrode 82 serves as a storage capacitor (Cst), as shown in FIG. 3 is a cross-sectional view illustrating the TFT unit 50 in detail, and includes a source electrode 51, a gate electrode 52, a drain electrode 53, and lightly doped drains (LDDs) 54 and 55 and LDDs connected thereto. It consists of a channel 56 formed between 54 and 55 and an insulating film 91 and 92. In the conventional liquid crystal display device, the pixel electrode 82 is made of aluminum (Al) having high light reflectivity and good processability, and the channel 56 of the TFT is made of polycrystalline silicon (Si) having high mobility. Make. Polycrystalline silicon reduces the leakage current by adding LDD (Lightly Doped Drain) structures 54 and 55 because the scan line has a large off current during the non-selection period. In the case of the liquid crystal projector, since the light incident on the TFT liquid crystal display is very bright, when light is irradiated onto the TFT channel, the leakage current increases and the image quality is deteriorated. Therefore, light blocking films 81 and 86 are placed above and below the TFT.

When such a liquid crystal display device is used in an LCD projector, the pixel has to be manufactured very finely with a pitch of 10 to 20 μm. For this purpose, strict process capability is required in the exposure and etching processes. Accordingly, the active element substrate of the liquid crystal display device used in the liquid crystal projector is made in a general semiconductor factory of a memory or a non-memory that requires a high resolution of 1 μm or less. In the general semiconductor process, the size of the substrate is limited to 12 kHz or 8 kHz, and mass production is difficult, resulting in very high production costs. On the other hand, the process of making a polysilicon thin film transistor by irradiating an amorphous silicon film with a laser. When making a liquid crystal display device for a liquid crystal projector, the size of the substrate is large, such as 370 x 470mm, The fill factor of the pixel electrode is low on the display screen at a resolution of about 2 to 3 μm, resulting in a low reflectance of light. 4 is a cross-sectional view for explaining the operation of FIG. 2, and the TFT portion is omitted. First, if the length of the pixel is P and the distance between the pixel electrode 82 and the pixel electrode 82 is R, the pitch of the pixel may be defined as (P + R). Light L2 incident on the pixel electrode 82 is reflected on the pixel electrode 82 and appears on the screen, but light L1 incident between the pixel electrode 82 and the pixel electrode 82 is lost. The fill factor F of the liquid crystal display device is as follows.

For example, if the pitch of the pixel is 15 mu m and the distance between the pixel electrodes is 3 mu m, the charge ratio F is 64%. If the distance between the pixel electrode and the pixel electrode is reduced, the charge ratio is increased, but there is a problem that the yield is reduced due to a lot of short circuit in the production process. On the contrary, when the distance between the pixel electrodes is increased to increase the yield, the light reflectivity decreases and the brightness of the screen decreases.

5 is a configuration diagram of a driving apparatus of a conventional liquid crystal display element. The liquid crystal display device used in the liquid crystal projector has a fine pitch of 10 to 20 μm, and thus it is difficult to directly connect the electrodes corresponding to the respective scan lines and the signal lines to the external driving IC. In 40, a TFT is made in each pixel, and a driving circuit is made outside the display area 40. The scan line driver 20 sequentially generates a selection signal and a non-selection signal and applies them to the scan line, and the signal line driver 30 applies a signal voltage to the pixel when the scan line is applied with the selection signal. The signal line driver and scan line driver are mainly composed of a shift register and a clock circuit.

SUMMARY OF THE INVENTION An object of the present invention is to provide a polycrystalline silicon liquid crystal display device having a high reflectance of light and a driving apparatus thereof by alternately coating an insulating film having a high refractive index and a low refractive index on a pixel electrode even when the resolution of the manufacturing process is low.

1 is an equivalent circuit diagram of a unit pixel of a TFT liquid crystal display element.

2 is a cross-sectional view of a conventional reflective TFT liquid crystal display device.

3 is a cross-sectional view showing the TFT portion in FIG. 2 in detail.

4 is a cross-sectional view illustrating the operation of FIG. 2.

5 is a configuration diagram of a driving apparatus of a conventional liquid crystal display element.

6 is a cross-sectional view of a reflective TFT liquid crystal display device of the present invention.

7 is a sectional view of a multiple reflecting film reflecting light of the present invention.

8 is a cross-sectional view for explaining the operation of the reflective TFT liquid crystal display device of the present invention.

9 is an embodiment of an equivalent circuit diagram of a polycrystalline silicon TFT liquid crystal display device of the present invention.

Fig. 10 is another embodiment of an equivalent circuit diagram of the polycrystalline silicon TFT liquid crystal display device of the present invention.

Fig. 11 is a drive line switching waveform of the present invention.

12 is a waveform of driving a signal line switch.

※ Explanation of codes for main parts of drawing

10 Control unit 20 Signal line driver 30 Scan line driver

40 Display area 50 Thin film transistor section 51 Source electrode

52 Gate Electrode 53 Drain Electrode 54,55 LDD

56 channel 60 signal line 70 scan line

80 TFT Glass Substrate 81,86 Light Blocker 82 Pixel Electrode

83 Liquid crystal layer 84 Common electrode 85 Common electrode Glass substrate

90,91,92,93,94 insulating film 95 alignment film 96 planarization film

97 Reflective film 200 Signal line switch 300 Scan line switch

310 Approved Period of Selection 320 Approved Period of Non-Selective Period

Fig. 6 is a sectional view of the reflective TFT liquid crystal display device of the present invention, which is composed of a common electrode layer, a pixel electrode layer, and a liquid crystal layer therebetween. The structure of FIG. 6 is the same as that of FIG. 2, but a planarization film 96 and a reflection film 97 are further formed between the liquid crystal layer and the pixel electrode layer.

7 is a cross-sectional view showing in detail the multiple reflective film reflecting light of the present invention. The insulating film H having a high refractive index and the insulating film L having a low refractive index are formed so that the product of the refractive index n and the thickness d is 1/4 of the central wavelength of visible light, and various layers are coated. Representative cases are SiO ₂ and Ti ₂ O ₅ . Such a reflective film is called a dielectric mirror.

The operation of the reflective TFT liquid crystal display device of the present invention configured as described above is as follows.

8 is a cross-sectional view for explaining the operation of the reflective TFT liquid crystal display device of the present invention. As shown, since the light L2 incident to the pixel electrode 82 and the light L1 incident between the pixel electrode and the pixel electrode are both reflected by the reflective film 97, the fill factor becomes 100%.

9 is an embodiment of an equivalent circuit diagram of a polycrystalline silicon liquid crystal display device of the present invention. Even when the amorphous silicon is irradiated with a laser to produce a polycrystalline TFT, when the screen mode is less than VGA, the driving frequency is low, so that peripheral circuits of the signal line driver and the scan line driver can be made as shown in FIG. However, when the screen modes are XGA and SXGA, it is difficult to make the shift register and the clock generator in the signal line driver and the scan line driver as shown in FIG. Although an n-channel TFT made of polysilicon by irradiating laser mobility 100~200cm ² / Vsec in the amorphous silicon, the mobility of the p-channel has less than 100cm ² / Vsec, the drive frequency is limited. In addition, when the signal line driver and the scan line driver are incorporated as shown in Fig. 8, a defect may occur in itself, and further processing is required so that the yield is reduced.

Therefore, as shown in FIG. 9, the signal input to the signal line switching unit 200 is a video signal V (1), V (2), ..., V (n) and control signals D (1), D (2). ), ..., D (n), and the scan line switching unit 300 receives signals G (1), G (2), ..., G (m) and a control signal to which voltage is directly applied to the scan line. S (1), S (2), ..., S (m), SL (1), SL (2), ..., SL (m) are input. The scan line switching unit 300 includes a selection period applying unit 310 and a non-selection period applying unit 320. FIG. 10 is an example of an equivalent circuit diagram of the polysilicon TFT liquid crystal display device according to the present invention. The selection period applying unit 310 and the non-selection period applying unit 320 of FIG. 9 are divided by the display area 40. will be. The circuit operations of Figs. 9 and 10 are the same. When the selection period applying unit 310 and the non-selection period applying unit 320 are separated, as shown in FIG. Can be. Since the switching elements are all n channels, when the control signal is subjected to high voltage (1), the resistance of the channel is low, and when the low voltage is applied (0), the channel is high.

If there are s of scan lines, the scan lines are divided into m regions. The relationship is as follows.

If m is not an integer, it is rounded up. 2 m control signals S and SL and m application signals G are required. When the scanning line region is divided as in Equation 2, the number of scanning line driving electrodes required is 3 m, and the number is minimum. If there are t signal lines, the signal lines are divided into n areas. The relationship is as follows.

If n is not an integer, it is rounded up. n control signals D and n video signals V are required. When the signal line region is divided as shown in Equation 3, the number of scanning line driving electrodes required is 2n, and the number is minimum.

In the case of UXGA, since there are 1600 signal lines and 1200 scanning lines, the conditions for reducing the number of driving electrodes to a minimum can be known from Equations 2 and 3 below. The signal line switching unit needs 40 image signals V and 40 control signals, respectively. Each of the scanning line switching units requires 35 control units S and SL, and also requires 35 scanning line applying signals G. The total connection terminals are 185.

11 is a scanning line drive waveform of the present invention. When the area 1 of the selection period applying unit of the scanning line is selected, the signal S (1) becomes 1, while the signals G1, G2, ... Gm are applied to the scanning lines 1, 2 while S (1) becomes 1, respectively. , ... m). While S (1) becomes 1, the V _H signals are sequentially applied to the scan lines 1, 2, ... m. In the period in which region 1 of the selection period applying unit 310 of the scanning line becomes non-selected, the SL1 signal of region 1 of the non-selecting period applying unit 320 becomes 1, and thus the scanning lines (1, 2, ..., m) It takes V _L voltage. When area 2 of the selection period applying unit 310 of the scanning line is selected, the signal S2 becomes 1, while the signals G1, G2, ... Gm are respectively applied to the scanning line m + 1, m while S2 becomes 1; +2, ... 2m). While S2 becomes 1, the V _H signal is sequentially applied to the scan lines m + 1, m + 2, ... 2m. In the period where region 2 of the selection period applying unit 310 of the scanning line becomes non-selected, the SL2 signal of region 2 of the non-selecting period applying unit 320 becomes 1, and the scanning lines (m + 1, m + 2, ... 2 m) takes the V _L voltage.

12 is a drive waveform of a signal line switching unit. The voltage of the signal line is applied to the pixel where the scan line is selected. The control signals D (1), D (2), ..., D (n) of the signal line switching unit 200 are sequentially scanned while V _{H is applied} to the scan line so that n video signals are simultaneously caught on the signal line. When the control signal S (r) of the scanning line is 1, G (p) is V _H , and the control signal D (q) of the signal line is 1, the scanning line to which the video signal is applied is (m × (r-1) + p), and a video signal is applied to the (n x (q-1)) th and (n x q) th signal lines.

In the liquid crystal display device of the present invention, even if the distance between the pixel electrode and the pixel electrode is 2 µm or more, the charge ratio is high, and the pixel is driven only by the scan line switching unit and the signal line switching unit formed of n channels. Therefore, the liquid crystal display device of the present invention used in the liquid crystal projector can be produced in a large amount in the process of making polycrystalline by irradiating the laser to amorphous silicon, the production cost is low, and the image quality is excellent.

Claims (6)

The common electrode layer including the glass substrate 85, the common electrode 84 formed on the glass substrate 85, and the alignment layer 95 formed on the common electrode 84, and the pixel electrode 82 on the TFT glass substrate 80. ), A TFT electrode 50, a light blocking film 81, 85, and a pixel electrode layer on which insulating films 90, 91, 92, 93, and 94 are stacked, and a liquid crystal layer 83 formed between the common electrode layer and the pixel electrode layer. In the liquid crystal display device, The material of the thin film transistor substrate 80 is glass, the channel of the thin film transistor (TFT) unit 50 is polysilicon, and the refractive index is different between the pixel electrode 82 and the liquid crystal layer. A liquid crystal display device characterized in that a layered reflective film (97) is coated. The liquid crystal display device according to claim 1, wherein the channel (56) of the thin film transistor portion (50) is made of polycrystalline by irradiating an amorphous silicon film with a laser. The liquid crystal display device according to claim 1, wherein a distance between adjacent pixel electrodes is 2 mu m or more. A common electrode layer including a glass substrate 85, a common electrode 84 formed on the glass substrate 85, and an alignment film 95 formed on the common electrode 84, and a pixel electrode 82 on a TFT glass substrate (80). ), The TFT portion 50, the light blocking films 81, 85, and the pixel electrode layer on which the insulating films 90, 91, 92, 93, and 94 are stacked, and the liquid crystal layer 83 and the TFT formed between the common electrode layer and the pixel electrode layer. A liquid crystal display device comprising a plurality of layers of a reflective film 97 having a refractive index between the pixel electrode 82 and the liquid crystal layer 83, wherein the channel of the portion 50 is polysilicon, and the liquid crystal. A scan line switching unit 300 connected to the scan line of the display device; And a signal line switching unit (200) connected to the signal line of the liquid crystal display device. 5. The thin film polysilicon liquid crystal display driving device according to claim 4, wherein the scan line switching unit (300) comprises a selection period applying unit (310) and a non-selection period applying unit (320). 6. The thin film polycrystalline silicon liquid crystal display device of claim 5, wherein the non-selection period applying unit 320 of the scan line and the selection period applying unit 310 of the scan line are provided separately from the liquid crystal display device. Device.