Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-07-26
2003-10-14
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S036000, C345S045000, C345S072000, C345S092000, C315S169100, C315S169300, C315S169400
Reexamination Certificate
active
06633270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a display device including an active device, and more particularly to an active matrix type display device including a spontaneous light-emitting device such as an organic electroluminescence (EL).
2. Description of the Related Art
A portable communication terminal such as a cellular phone has been widely used recently. As a display unit of such a portable communication terminal, a liquid crystal display device is widely used.
A liquid crystal display device including a back light unit is accompanied with a problem of much power consumption for enhancing a brightness in a display screen. To solve this problem, a display device including an organic electroluminescence (hereinafter, referred to simply as “organic EL display device”) attracts an attention as a display device suitable to a portable communication terminal, as having been suggested in Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.
Hereinbelow is set forth a summary of Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.
As a display device including a spontaneous light-emitting device which emits a light when a current runs therethrough, there are known a plasma display (PDP) device and an electroluminescence (EL) device. An electroluminescence device is grouped into an inorganic one and an organic one with respect to a material of which an electroluminescence device is composed, and is further grouped into a simple matrix type device and an active matrix type device with respect to a structure.
FIG. 1
is a block diagram of a simple matrix type organic EL display device.
As illustrated in
FIG. 1
, the simple matrix type organic EL display device includes a data line driver circuit
55
to which a plurality of data lines
53
are electrically connected, a scanning line driver circuit
56
to which a plurality of scanning lines
54
are electrically connected, and a plurality of pixels arranged in a matrix.
Each of the pixels is comprised of an electroluminescence device
51
, a capacitor
52
electrically connected between an anode and a cathode of the electroluminescence device
51
, one of the data lines
53
to which the anode of the electroluminescence device
51
is electrically connected, and one of the scanning lines
54
to which the cathode of the electroluminescence device
51
is electrically connected.
The data line driver circuit
55
activates one of the data lines
53
and the scanning line driver circuit
56
activates one of the scanning lines
54
to thereby supply the electroluminescence device
51
electrically connected to the thus activated data and scanning lines
53
and
54
, with a current from the activated data line
53
towards the activated scanning line
54
. As a result, the electroluminescence device
51
emits a light with a brightness determined in accordance with the current running through the electroluminescence device
51
.
Since a simple matrix type organic EL display device has a relatively simple structure as mentioned above, it can be fabricated with low costs. However, it is difficult for a simple matrix type organic EL display device to increase the number of pixels for accomplishing a higher density in pixels.
Since a scanning line is selected one by one, and then, a light-emitting diode in an associated pixel is made to emit a light in a simple matrix type organic EL display device, a period of time during which a light-emitting diode in a pixel emits a light is equal to A/B wherein A indicates a frame period and B indicates the number of scanning lines. In order to keep a brightness constant in such a limited period of time, it would be necessary to instantaneously flow a much current through a pixel.
If the number of pixels is increased, the data line
53
would have an increased wire length. The data lines
53
are generally composed of a transparent material such as ITO (Indium Tin Oxide), and hence, has a high wire resistivity. As a result, as the data lines
53
have an increased wire length, the data lines
53
would have an increased wire resistance.
Thus, there occurs a significant voltage drop in the data lines
53
, because the data lines
53
have an increased wire resistance, and further because a much current runs through the data lines
53
.
Such a significant voltage drop results in that a voltage on the data line
53
located farther away from the data line driver circuit
55
becomes smaller than a voltage on the data line
53
located closer to the data line driver circuit
55
. This causes that a smaller current runs through the electroluminescence device
51
electrically connected to the data line
53
located farther away from the data line driver circuit
55
.
That is, since a smaller current runs through the electroluminescence device
51
electrically connected to the data line
53
located farther away from the data line driver circuit
55
, because of an increased wire resistance of the data lines
53
, the electroluminescence device
51
would emit a light in a smaller amount, resulting in non-uniformity in a brightness in a display screen. Specifically, a pixel located farther away from the data line driver circuit
55
would have a smaller brightness.
FIG. 2
is a block diagram of a conventional active matrix type organic electroluminescence display device.
As illustrated in
FIG. 2
, the conventional active matrix type organic EL display device includes a data line driver circuit
68
to which a plurality of data lines
65
are electrically connected, a scanning line driver circuit
69
to which a plurality of scanning lines
66
are electrically connected, a bias voltage source
610
, a common bias voltage line
611
through which a bias voltage is applied from the bias voltage source
610
, a plurality of bias voltage lines
67
electrically connected to the bias voltage line
611
, and a plurality of pixels arranged in a matrix.
Each of the pixels is comprised of an electroluminescence device
61
, a first thin film transistor (TFT)
62
electrically connected between an anode of the electroluminescence device
61
and one of the bias voltage lines
67
, a second thin film transistor (TFT)
63
electrically connected between one of the data lines
65
and a gate of the first thin film transistor
62
, and a capacitor
64
electrically connected between a gate of the first thin film transistor
62
and one of the bias voltage lines
67
.
When the scanning line driver circuit
69
activates one of the scanning lines
66
, the second thin film transistor
63
electrically connected to the thus activated scanning line
66
is turned on, and hence, a current runs to the capacitor
64
through the data line
65
and the second thin film transistor
63
from the data line driver circuit
68
, resulting in that the capacitor
64
is electrically charged.
Thus, a gate voltage of the first thin film transistor
62
becomes high. When the gate voltage of the first thin film transistor
62
becomes higher than a threshold voltage, the first thin film transistor
62
is turned on, resulting in that a current is supplied to the electroluminescence device
61
through the common bias voltage line
611
and the bias voltage line
67
from the bias voltage source
610
. Thus, the electroluminescence device
610
emits a light at a brightness in accordance with the current supplied thereto.
As is obvious in view of the above, the active matrix type organic EL display device and is characterized in that even if the number of scanning lines were increased, it is ensured to have a period of time during which a light is emitted, equal to a frame period of time, differently from the simple matrix type organic EL display device.
Herein, an active matrix type liquid crystal display device is compared to the above-mentioned active matrix type organic EL display device.
In an active matrix type liquid crystal display device, a transmissivity, which corresponds to a brightness in an active matrix type organic EL display device, is in proportion to a voltage to be applied to
Hjerpe Richard
Lesperance Jean
NEC Electronics Corporation
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