Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-06-15
2003-06-24
Chow, Dennis-Doon (Department: 2675)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S077000, C345S082000
Reexamination Certificate
active
06583775
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an image display apparatus which includes a pixel whose brightness is controlled with a signal, and more particularly to an image display apparatus which includes, for each pixel, a light emitting element for emitting light with brightness which is controlled with current such as an organic electroluminescence (EL) element. More specifically, the present invention relates to an image display apparatus of the active matrix type wherein the amount of current to be supplied to a light emitting element is controlled by an active element such as a field effect transistor of the insulated gate type provided in each pixel.
Generally, in an image display apparatus of the active matrix type, a large number of pixels are arranged in a matrix, and the intensity of light is controlled for each of the pixels in response to brightness information given thereto to display an image. Where liquid crystal is used as an electro-optical substance, the transmission factor of each pixel varies in response to a voltage written in the pixel. Even with an image display apparatus of the active matrix type which employs an organic electroluminescence material as an electro-optical substance, basic operation is similar to that where liquid crystal is employed. However, different from a liquid crystal display apparatus, an organic EL display apparatus is an apparatus of the self light emission type wherein each pixel has a light emitting element. Thus, the organic EL display apparatus is advantageous in that it exhibits a higher degree of visibility than a liquid crystal display apparatus, that it does not require a back light and that it has a higher responding speed. The brightness of each individual light emitting element is controlled with the amount of current. In other words, the organic EL display is significantly different from the liquid crystal display apparatus and so forth in that the light emitting elements are of the current driven type or the current controlled type.
Similarly to the liquid crystal display apparatus, the organic EL display apparatus can possibly use a simple matrix system or an active matrix system as a driving system therefor. Although the former is simple in structure, it is difficult to implement a display apparatus of a large size and a high resolution. Therefore, much effort has been and is directed to development of organic EL display apparatus of the active matrix system. In the organic EL display apparatus of the active matrix system, current to flow to a light emitting element provided in each pixel is controlled by an active element usually in the form of a thin film transistor which is a kind of a field effect transistor of the insulated gate type and may be hereinafter referred to as TFT. An organic EL display apparatus of the active matrix system is disclosed, for example, in Japanese Patent Laid-open No. Hei 8-234683, and an equivalent circuit for one pixel in the organic EL display apparatus is shown in FIG.
10
. Referring to
FIG. 10
, the pixel PXL shown includes a light emitting element OLED, a first thin film transistor TFT
1
, a second thin film transistor TFT
2
, and a holding capacitor Cs. The light emitting element OLED is an organic electroluminescence (EL) element. Since an organic EL element in most cases has a rectification property, it is often called OLED (organic light emitting diode) and, in
FIG. 10
, the mark of a diode is used for the light emitting element OLED. However, the light emitting element is not limited to an OLED, but may be any element only if the brightness thereof is controlled with the amount of current to flow therethrough. It is not always required for an OLED to have a rectification property. In the pixel shown in
FIG. 10
, a reference potential (ground potential) is applied to the source S of the second thin film transistor TFT
2
, and the anode A (positive electrode) of the light emitting element OLED is connected to a power supply potential Vdd while the cathode K (negative electrode) is connected to the drain D of the second thin film transistor TFT
2
. Meanwhile, the gate G of the first thin film transistor TFT
1
is connected to a scanning line X and the source S of the first thin film transistor TFT
1
is connected to a data line Y. The drain D of the first thin film transistor TFT
1
is connected to the holding capacitor Cs and the gate G of the second thin film transistor TFT
2
.
In order to cause the pixel PXL to operate, the scanning line X is placed into a selected state first, and then a data potential Vdata representative of brightness information is applied to the data line Y. Consequently, the first thin film transistor TFT
1
is rendered conducting, and the holding capacitor Cs is charged or discharged and the gate potential of the second thin film transistor TFT
2
becomes equal to the data potential Vdata. Then, if the scanning line X is placed into a non-selected state, then the first thin film transistor TFT
1
is turned off, and the second thin film transistor TFT
2
is electrically disconnected from the data line Y. However, the gate potential of the second thin film transistor TFT
2
is held stably by the holding capacitor Cs. The current flowing to the light emitting element OLED through the second thin film transistor TFT
2
exhibits a value which depends upon a gate-source voltage Vgs of the second thin film transistor TFT
2
, and the light emitting element OLED continues to emit light with a brightness value corresponding to the amount of current supplied from the second thin film transistor TFT
2
.
In the present specification, the operation of selecting a scanning line X to transmit a potential of a data line Y to the inside of a pixel is hereinafter referred to as “write”. Where the current flowing between the drain and the source of the second thin film transistor TFT
2
is represented by Ids, this is driving current flowing to the light emitting element OLED. If it is assumed that the second thin film transistor TFT
2
operates in a saturation region, then the current Ids is represented by the following expression:
Ids
=
(
1
/
2
)
·
μ
·
Cox
·
(
W
/
L
)
·
(
Vgs
-
Vth
)
2
=
(
1
/
2
)
·
μ
·
Cox
·
(
W
/
L
)
·
(
Vdata
-
Vth
)
2
(
1
)
where Cox is a gate capacitance per unit area and is given by the following expression:
Cox=&egr;0
·&egr;r/d
(2)
In the expressions (1) and (2) above, Vth is a threshold voltage for the second thin film transistor TFT
2
, &mgr; is the mobility of carriers, W is the channel width, L is the channel length, &egr; 0 is the dielectric constant of vacuum, &egr; r is the dielectric constant of the gate insulating film, and d is the thickness of the gate insulating film.
According to the expression (1), the current Ids can be controlled with the data potential Vdata to be written into the pixel PXL, and as a result, the brightness of the light emitting element OLED can be controlled. Here, the reason why the second thin film transistor TFT
2
operates in a saturation region is such as follows. In particular, the reason is that, since, in a saturation region, the current Ids is controlled only with the gate-source voltage Vgs but does not rely upon the drain-source voltage Vds, even if the drain-source voltage Vds is fluctuated by a dispersion in characteristic of the light emitting element OLED, a predetermined amount of current Ids can be flowed to the light emitting element OLED.
As described hereinabove, with the circuit construction of the pixel PXL shown in
FIG. 10
, if writing of the data potential Vdata is performed once, then the light emitting element OLED continues to emit light with a fixed brightness value for a period of one scanning cycle (one frame) until it is rewritten. If a large number of such pixels PXL are arranged in a matrix as shown in
FIG. 11
, then an image display apparatus of the active matrix type can be constructed. As seen from
FIG. 11
, a conventional image display apparatu
Sekiya Mitsunobu
Yumoto Akira
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