JPH04344618A - Transistor for driving liquid crystal - Google Patents

Abstract

PURPOSE:To reduce a leak current which flows in an OFF state by forming a thin film transistor(TFT) which drives each picture element by connecting two 1st LDD transistor(TR) and 2nd LDD TR in series. CONSTITUTION:The 1st LDD(Lightly Doped Drain) TR 1 is provided with a gate electrode 6 on a P type area 3 across a gate insulating film 4 and also an N type high-density area across an LDD area 6 to form a source-drain electrode 9. The 2nd LDD TR 2 is provided with a gate electrode 10 on a P type area 8 across a gate insulating film 7 and an N type high-density area across an LDD area 11 to forming a source-drain electrode 12. The P type areas 3 and 8 of the TRs 1 and 2 are connected in series across an N type low-density drain area 13. Therefore, voltages applied to the sources - drains of the TRs 1 and 2 can be reduced in the OFF state.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transistor for driving a liquid crystal, and is particularly suitable for use in reducing leakage current when turned off.

0002

2. Description of the Related Art As is well known, the simple matrix driving method, the active matrix driving method, and the like are known as driving methods for liquid crystal displays. When these driving methods are compared, the active matrix method is generally said to be superior in terms of image quality and responsiveness. The above active matrix method is a driving method that adds a nonlinear element to the pixel, and the nonlinear element may be a diode, a varistor, or an M with a large on/off ratio.
OS type transistors and the like are used. The above MOS
A type transistor is generally called a thin film transistor (TFT) because it is fabricated on a transparent substrate. Among these elements, thin film transistors (TFTs) are currently being actively researched and developed as full-color LCDs.

FIG. 4 shows an equivalent circuit of an active matrix liquid crystal display using thin film transistors TFT. As is clear from FIG. 4, in the active matrix liquid crystal display, the gate bus line 2
0 and the source bus line 21 constitute each pixel 22. A thin film transistor 23 is used to write and hold a signal sent through the pixel electrode 24 in each pixel 22.

The thin film transistor 23 used for this purpose is required to have the ability to conduct current when it is on, and conversely, to prevent current from flowing when it is off. Therefore, since polysilicon TFTs have excellent driving ability, they are used as transistors that can meet these requirements. Furthermore, the polysilicon TFT has the advantage that peripheral circuits can be mounted on-chip, and pixel transistors can be made smaller.

However, it is considered difficult to suppress leakage current in polysilicon TFTs when they are off. Therefore, the following three methods have been conventionally used to solve the problem of suppressing leakage current during off-time. That is, one of them is called a double gate structure, in which two transistors are connected in series to reduce the drain electric field by half, thereby suppressing tunnel current. In addition, a so-called ultra-thin film method is sometimes used in which leakage current is suppressed by reducing the leakage volume by reducing the thickness of the polysilicon film in the channel. In addition, other technologies such as LDD
(Lightly Doped Drain) transistors may also be used.

[0006] As shown in the cross-sectional view of FIG. 5, the LDD transistor has a structure in which a low concentration drain region N is provided to suppress the electric field so that a tunnel current in the junction is not generated. However, if this is done, the depletion layer will expand, so care must be taken regarding the current generated within it. Among these technologies, the LDD structure is suitable for use as a liquid crystal driving transistor because the leakage current is not so affected by the drain electric field, and research and development is being actively carried out. By the way, an LDD is usually provided only on the drain side, so it is called this way. However, in the case of a liquid crystal driving transistor, current must flow from either side, so there is no distinction as to which side is the source and which is the drain. Therefore, in the following description, it is assumed that there are low concentration regions on both sides unless otherwise specified.

[0007]

A characteristic of a transistor having an LDD structure is that leakage current is not significantly affected by the drain electric field. Therefore, in the case of an LDD transistor, since no tunnel current flows, there is no problem of abnormally large leakage occurring. However, in this case, a depletion layer corresponding to the drain voltage expands to the LDD portion, so a current is generated within the depletion layer. For this reason, there is an inconvenience that leakage is larger when displaying black than when displaying halftones.

FIG. 6 shows a typical electrical characteristic diagram of an LDD transistor. As is clear from FIG. 6, current ID flows in at a gate voltage of +Vg. Also, -V at the gate
By applying a voltage of g, the current ID is prevented from flowing. Although it is better for the current Ioff to be small at this time, it has been difficult to reduce it using conventionally known techniques. In view of the above-mentioned problems, it is an object of the present invention to reduce the leakage current flowing through the thin film transistor when it is off.

[0009]

[Means for Solving the Problems] A liquid crystal driving transistor of the present invention is a liquid crystal driving thin film transistor used to drive each pixel of an active matrix liquid crystal display. , LD with low concentration drain region
It is constructed by connecting two D transistors in series. Another feature of the present invention is that the two LDD transistors are connected using only the same semiconductor layer as the lightly doped drain region of the LDD transistor.

0010

[Operation] By connecting two first LDD transistors and two second LDD transistors in series, a voltage is applied between the sources and drains of these first and second transistors when they are off. This reduces the leakage current that flows when the device is off.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a sectional view of a liquid crystal driving transistor showing an embodiment of the present invention. As is clear from FIG. 1, the liquid crystal driving transistor of this embodiment is connected to the first LDD.
It has a configuration in which a transistor 1 and two second LDD transistors 2 are connected in series.

[0012] Furthermore, in this embodiment, two LDDs
When connecting transistors 1 and 2 in series, each LDD
Efforts have been made to shorten the distance between transistors 1 and 2. That is, the first LDD transistor 1 has a gate electrode 5 on the first P-type region 3 via the gate insulating film 4.
A first P type region 3 is provided between the first P type region 3 and the first P type region 3.
An N-type high concentration region is provided across the LDD region 6 of the first
It is constructed by forming one source/drain electrode 9 of the transistor.

Further, the second LDD transistor 2 has a gate electrode 10 provided on the second P-type region 8 via the gate insulating film 7, and a second P-type region 8 between the second P-type region 8 and the second P-type region 8. It is constructed by providing an N-type high concentration region with an LDD region 11 in between, and forming one of the source/drain electrodes 12. Then, the first P-type region 3
and the second P-type region 8 through the N-type low concentration drain region 13, thereby connecting the first LDD transistor 1 and the second LDD transistor 2 in series. Note that each part is insulated with an insulating film made of SiO2.

In the liquid crystal driving transistor of this embodiment constructed in this manner, a source bus line 21 is connected to one source/drain electrode 9 of the first transistor, and a source bus line 21 is connected to one source/drain electrode 9 of the second transistor. - The pixel electrode 24 is connected to the drain electrode 12. Further, a gate bus line 20 is connected to each gate electrode 5, 10, respectively. Therefore, these first and second LDD transistors 1 and 2 operate as one transistor, and can be represented as shown in the configuration diagram of FIG. 2. The equivalent circuit is shown in the circuit diagram of FIG. 3, and the voltage VD is applied to one source of the first transistor.
The drain electrode 9 and one source of the second transistor
A voltage is applied between the drain electrode 12 and the drain electrode 12 .

Therefore, when the potential at the connection point of each transistor is VX, the voltages applied to each transistor 1 and 2 are VX and (VD - VX). Therefore, if the resistance values of each transistor at the time of current Ioff are R1 and R2, VX = (R1 /R1 +R2)・VD
...(1), and the voltage applied to each transistor can be significantly reduced. According to experiments, it was possible to reduce the voltage applied to each transistor 1 and 2 by 1/2 to 1/3.

Note that in the above embodiment, the first LD
When connecting the D transistor 1 and the second LDD transistor 2 in series, the other source/drain electrode of the first transistor and the other source/drain electrode of the second transistor are connected by the low concentration drain region 13. These two transistors are connected in series. Therefore, the connection distance between each transistor can be extremely shortened, and even if two LDD transistors are connected, the required area can be prevented from increasing. However, without making such a connection,
The objects and effects of the present invention can be satisfactorily achieved by simply connecting two LDD transistors in series.

[0017]

Effects of the Invention As described above, according to the invention of claim 1, the first LDD transistor and the second LDD transistor
By connecting two transistors in series to form a thin film transistor that drives each pixel, it is possible to significantly reduce the voltage applied between the source and drain of each thin film transistor when it is off. . Therefore, it is possible to reduce the leakage current that flows when the display is off, and it is possible to improve the contrast and image quality of the liquid crystal display. Further, according to the second aspect of the invention, since the connection distance between each transistor can be shortened, the required area can be prevented from increasing even if two LDD transistors are connected.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a liquid crystal driving transistor showing an embodiment of the present invention.

FIG. 2 is a configuration diagram of a liquid crystal driving transistor.

FIG. 3 is an equivalent circuit diagram of a liquid crystal driving transistor.

FIG. 4 is an equivalent circuit diagram of a liquid crystal display.

FIG. 5 is a cross-sectional view of an LDD transistor.

FIG. 6 is an electrical characteristic diagram of an LDD transistor.

[Explanation of symbols]

1 First LDD transistor 2 Second LDD transistor 3 First P-type region 6 First LDD region 8 Second P-type region 9 One source/drain electrode of the first transistor 11 Second LDD region 12 One source/drain electrode of the second transistor 13 Low concentration drain region

Claims (2)

[Claims] 1. In a liquid crystal driving thin film transistor used to drive each pixel of an active matrix liquid crystal display, the thin film transistor for driving each pixel is formed by connecting two LDD transistors having low concentration drain regions in series. A liquid crystal driving transistor characterized in that it is configured by individually connecting each other. 2. The liquid crystal driving transistor according to claim 1, wherein the two LDD transistors are connected using only the same semiconductor layer as the low concentration drain region of the LDD transistor.