Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1999-04-28
2003-02-11
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S007000, C345S076000, C345S077000, C345S080000, C345S090000, C345S092000, C345S214000, C315S169300
Reexamination Certificate
active
06518941
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display device equipped with a display element and, more particularly, to a display device equipped with an element that emits light by means of a thin film transistor and current (hereinafter referred to as a “current luminescent display device”.
2. Description of the Related Art
A thin film transistor organic electroluminescent device (hereinafter referred to as “TFT-OELD”) may be cited as a highly promising future current luminescent display device that realizes a larger size, higher definition, a wider viewing angle, and reduced power consumption.
A method for driving a typical conventional TFT-OELD will be described.
FIG. 5
shows an equivalent circuit of the conventional TFT-OELD. Only one pixel is shown in the drawing although there are actually many pixels in a plurality of rows and a plurality of columns.
A pulse is output from a shift register
101
, and an analog signal of an analog signal supply line
1022
is transmitted to a source line
1042
via a transmission switch
1032
. For a gate line
109
that has been selected this time, the analog signal is transmitted to a retention capacitor
1062
via a switching transistor
1052
. The conductance of a current transistor
1072
is controlled in accordance with the analog signal, and an organic EL element
1082
emits light of an intensity level based on the analog signal.
FIG. 6
illustrates the conventional TFT-OELD driving method.
A pulse SR
0
of a shift register of a zero-th column causes an analog signal A to be transmitted to a potential S
0
of a source line of a zero-th column. Further, a pulse SR
1
of a shift register of a first column causes the analog signal A to be transmitted to a potential S
1
of a source line of the first column. First, while a pulse G
0
of a gate line of the zero-th row is being applied, the potential S
0
of the source line of the zero-th column is transmitted to a potential C
00
of a retention capacitor of the zero-th row and the zero-th column, whereas the potential S
1
of the source line of the first column is transmitted to a potential C
01
of a retention capacitor of the zero-th row and the first column. Then, while a pulse G
1
of a gate line of the first row is being applied, the potential S
0
of the source line of the zero-th column is transmitted to a potential C
10
of a retention capacitor in the first row and the zero-th column, whereas the potential S
1
of the source line of the first column is transmitted to a potential C
11
of a retention capacitor in the first row and the first column. Each organic EL element
1082
(
FIG. 5
) emits light of a predetermined intensity level in accordance with the potential of each retention capacitor
1062
(FIG.
5
), i.e., the corresponding analog signal A.
An area gray scale method is known as one of the driving methods of a liquid crystal display device. In general, a liquid crystal display device has a problem of a limited viewing angle range due to a marked change in the transmissivity or the reversal of gray scale in a direction of a viewing angle that deviates from the direction of the normal line with respect to a display surface. The foregoing area gray scale method is intended to solve the problem, and it is adapted to represent a gray scale in terms of an area ratio of full transmission to no transmission. This realizes a wider viewing angle range of a liquid crystal display device.
According to the conventional TFT-OELD driving method mentioned above, the analog signals are used to control the conductance of the current transistor
1072
so as to control the luminous intensity of the organic EL element
1082
. In other words, to obtain a half tone, the conductance of the current transistor
1072
must be set to be equal to the conductance of the organic EL element
1082
, and the voltage applied to the organic EL element
1082
must be controlled by dividing the voltages of the current transistor
1072
and the organic EL element
1082
. In such a case, however, there has been a problem in that, if nonuniformity in the conductance of the current transistor
1072
should be produced within a panel or between panels, then the nonuniform conductance will be visually recognized in the form of nonuniform luminous intensity of the organic EL element
1082
.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to reduce the nonuniformity in the luminous intensity of a luminescent element (an organic EL element in particular) caused by the nonuniformity in the conductance of transistors in a current luminescent display device, particularly in a TFT-OELD, thereby to improve image quality.
A display device in accordance with the present invention has the following configuration.
The display device has a plurality of scanning lines, a plurality of signal lines, and a pixel formed in a matrix pattern by the scanning lines and the signal lines, a plurality of thin film transistors and a plurality of luminescent elements being formed in the pixel;
wherein the thin film transistors and the luminescent elements are respectively connected in series, and the luminous intensities of the respective luminescent elements are different.
This permits the gray scale method to be implemented, in which each of the luminescent elements having the different luminous intensities is controlled to be placed in either a completely ON state or a completely OFF state. With this arrangement, the nonuniformity in the luminous intensity of the luminescent elements caused by the nonuniformity in the conductance of the thin film transistors can be reduced.
In the present invention, the turning ON/OFF of the luminescent elements are preferably controlled by digital signals. This makes it possible to control each of a plurality of luminescent elements having a different luminous intensity in each pixel so as to place it in either the completely ON state or the completely OFF state.
In the present invention, the luminous intensities of the luminescent elements are preferably the geometric progressions of a common ratio of 2. This will provide each pixel with a DA converter, making it possible to obtain the luminous intensity characteristics based on digital signals.
In the present invention, it is preferable that the ON resistance of the thin film transistors is lower than the ON resistance of the luminescent elements, while the OFF resistance of the thin film transistors is higher than the OFF resistance of the luminescent elements. With this arrangement, the ON state and the OFF state of the luminescent elements can be switched by switching the ON state and the OFF state of the thin film transistors. More preferably, the ON resistance of the thin film transistors is so low that it may be ignored, as compared with the ON resistance of the luminescent elements. At this time, the current passed through the luminescent elements is determined only by the ON resistance of the luminescent elements, so that it is independent of some increase or decrease in the ON resistance of the thin film transistors. This suppresses the nonuniformity in the luminous intensity resulting from the nonuniformity of the conductance of the transistors. Further preferably, the OFF resistance of the thin film transistors is far higher than the OFF resistance of the luminescent elements. Thus, the luminescent elements can be securely placed in the OFF state.
In the present invention, the thin film transistors are preferably the polycrystalline silicon thin film transistors produced at a low temperature process at 600 degrees Celsius or lower. This makes it possible to implement larger areas at low cost and also to achieve such features as high mobility for enabling the drive of the luminescent elements and high reliability.
In the present invention, the luminescent elements are preferably the organic electroluminescent elements produced by an ink-jdt process. With this arrangement, it is possible to pattern an organic electroluminescent element, which achieves outstanding
Kovalick Vincent E.
Oliff & Berridge PLC.
Seiko Epson Corporation
Shalwala Bipin
LandOfFree
Display device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Display device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Display device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3127475