JPH0588644A - Active matrix liquid crystal display device - Google Patents

Abstract

PURPOSE:To provide the active matrix liquid crystal display device which reduces a leak current from a liquid crystal cell and suppresses a voltage drop across the liquid crystal cell. CONSTITUTION:This active matrix type liquid crystal display device is constituted by connecting liquid crystal display cells, each equipped with liquid crystal 16 connected between a signal line 13 and a common electrode 17, two polysilicon TFTs 14 and 15, and auxiliary electrostatic capacitances 18 and 19 connected across one polysilicon TFT 15 between the two polysilicon TFTs 14 and 15, in a matrix.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix liquid crystal display device having a reduced leak current from a liquid crystal cell.

[0002]

2. Description of the Related Art Conventionally, an active matrix liquid crystal display device is constructed by connecting liquid crystal display cells including liquid crystal and TFTs (thin film transistors) in a matrix form to scanning lines and signal lines.

FIG. 4 shows a liquid crystal display cell of a conventional active matrix liquid crystal display device, in which a signal line 2 is connected to a common electrode 6 via a TFT 3 which is a switching element and a liquid crystal 5 in series. The gate of the TFT 3 is connected to the scanning line 1, and the connection point between the TFT 3 and the liquid crystal 5 is grounded via the auxiliary capacitance 4. That is, the scanning line 1 is selected and T
When the FT 3 is on, the potential of the signal line 2 is written to the liquid crystal 5 and the auxiliary capacitance 4 connected in parallel through the TFT 3. On the other hand, scanning line 1 is not selected and TFT
When 3 is off, the signal line 2 from the liquid crystal 5 through the TFT 3
The current flowing through is the leak current I _L 1. In this case, T
If polysilicon TFT is used for FT3, leakage current I _L 1
Is very large, about 100 pA.

In the active matrix liquid crystal display device, the electrostatic capacity of the liquid crystal 5 including the auxiliary electrostatic capacity 4 is C, and the leak current of the TFT 3 composed of a polysilicon TFT is I _L.
Assuming 1, the voltage drop Δ of the liquid crystal cell 5 during the period t
V1 is ΔV1 = I _L 1 · t / C (1)

Can be approximated by C = 0.6 in the current process
pF, t = 16.7 × 10 ⁻³ sec, I _L 1 = 10
It is 0 pA, and by substituting them into the equation (1), it can be approximated as a voltage drop ΔV1 = 2.8V. When the applied voltage of the liquid crystal cell 5 is 10V, the voltage of the liquid crystal cell 5 is 10V-2.8.
Since V = 7.2V, gradation display cannot be performed. Therefore, at present, the polysilicon TFT cannot be used in the active matrix liquid crystal display device.

[0006]

As described above, the conventional active matrix liquid crystal display device has a drawback that the leak current from the liquid crystal cell is large and the voltage drop of the liquid crystal cell is large.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an active matrix liquid crystal display device in which a leak current from a liquid crystal cell is reduced and a voltage drop of the liquid crystal cell is suppressed.

[0008]

To solve the above problems, the present invention provides a liquid crystal and a plurality of switching elements connected in series between a signal line and a common electrode, and one of the plurality of switching elements. A liquid crystal display cell having auxiliary capacitances connected to both ends of one switching element is connected in a matrix.

[0009]

By the above means, the auxiliary capacitance is connected to both ends of the switching element connected in series with the liquid crystal to reduce the potential difference across the switching element, thereby reducing the leak current flowing from the liquid crystal through the switching element. Then, the voltage drop of the liquid crystal cell is suppressed.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 2 is a schematic view of an active matrix liquid crystal display device according to an embodiment of the present invention, which shows a liquid crystal and TF.
The liquid crystal display cell 11 including T includes the scanning line 12 and the signal line 13.
And are connected in a matrix form.

FIG. 1 shows an example of a liquid crystal display cell 11 of the active matrix liquid crystal display device of FIG. 2, in which a signal line 13 is a polysilicon TFT 1 which is two switching elements.
4, 15 and the liquid crystal 16 are connected in series to the common electrode 17. The gates of the TFTs 14 and 15 are connected to the scanning line 12, and the connection point between the TFT 15 and the liquid crystal 16 and the connection point between the TFT 14 and the TFT 15 are grounded via the corresponding auxiliary capacitances 18 and 19, respectively. That is, when the scanning line 12 is selected and the TFTs 14 and 15 are turned on, the potential of the signal line 13 is set to the TFTs 14 and 1.
5, the liquid crystal 16 and the auxiliary capacitance 19 connected in parallel are written through 5, and the auxiliary capacitance 18 is written through the TFT 14. On the other hand, when the scanning line 12 is not selected and the TFTs 14 and 15 are off, the current flowing from the liquid crystal 16 to the signal line 13 through the TFTs 14 and 15 is the leak current I _L.

FIG. 3B shows the drain voltage dependence of the leak current in the TFT shown in FIG. That is, as the drain voltage V _D is raised, the leak current I _L 1 (drain current I _D ) rises sharply. Also, FIG.
(B) also shows that the leak current I _L 1 can be kept small when the voltage applied to the source / drain of the TFT is sufficiently small.

Thus, in the liquid crystal display cell 11 of FIG. 1, the potential of the scanning line 12 is switched from selected to unselected.
When the FTs 14 and 15 are switched from on to off, the auxiliary capacitance 18 and the auxiliary capacitance 19 have the same potential, so that the source and drain of the TFT 15 have the same potential. When the potential of the signal line 13 changes in this state, the change amount V _S is applied to the source / drain of the TFT 14. Then, a leakage current I _L 4 corresponding to the magnitude of the change amount V _S flows through the TFT 14 and the auxiliary capacitance 1
The voltage across 8 changes by ΔV1. This voltage change amount ΔV1 becomes the voltage applied to the source / drain of the TFT 15. According to the approximate calculation method of the equation (1), ΔV1 = 2.
It can be approximated to 8V. That is, with respect to the voltage 10 V applied to the source / drain of the conventional TFT 3, the TF of this embodiment is
The voltage applied to the source / drain of T15 is 3 V or less, which can be reduced to 1/3 or less. TFT15
When the voltage applied to the source and drain of is about 3V,
From FIG. 3B, the leak current I _L flowing in the TFT 15 is almost 0 pA, which means that it can be significantly reduced, and as a result, the potential change across the liquid crystal 16 is almost 0 V, and the applied voltage of the liquid crystal 16 is 10 V. Can be held, and good gradation display can be performed. Therefore,
Polysilicon TF for active matrix liquid crystal display
It becomes possible to use T.

[0015]

As described above, according to the present invention, auxiliary capacitances are connected to both ends of a switching element connected in series with a liquid crystal cell to eliminate the potential difference across the switching element. It is possible to reduce the leak current flowing from the liquid crystal cell through the switching element and suppress the voltage drop of the liquid crystal cell.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of a liquid crystal display cell according to the present invention.

FIG. 2 is a schematic configuration explanatory view showing an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing an example of drain voltage dependency of a leak current in a TFT.

FIG. 4 is a circuit diagram showing a conventional liquid crystal display cell.

[Explanation of symbols]

11 ... Liquid crystal display cell, 12 ... Scan line, 13 ... Signal line, 1
4, 15 ... Polysilicon TFT, 16 ... Liquid crystal, 17 ... Common electrode, 18, 19 ... Auxiliary capacitance.

Claims (2)

[Claims] 1. A liquid crystal and a plurality of switching elements connected in series between a signal line and a common electrode, and auxiliary capacitances respectively connected to both ends of one of the plurality of switching elements. An active matrix liquid crystal display device, comprising liquid crystal display cells comprising the elements connected in a matrix. 2. The active matrix liquid crystal display device according to claim 1, wherein a polysilicon thin film transistor is used as the switching element.