Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
1999-02-26
2001-02-27
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C313S506000
Reexamination Certificate
active
06194837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an active matrix display device in which driving of luminescent elements such as LEDs (light emitting diodes) or EL (electroluminescent) elements which emit light when a driving current is passed through an organic semiconductor film is controlled by thin film transistors (hereinafter referred to as TFTs). More specifically, the present invention relates to a technique of optimizing the layout to improve the display performance.
2. Description of Related Art
Active matrix display devices using current-controlled luminescent elements such as EL elements or LEDs have been proposed. Any types of luminescent elements used in display devices of this type have the capability of emitting light. Therefore, backlight is not required in display devices of this type unlike the liquid crystal display devices.
FIG. 13
is a block diagram illustrating an example of such an active matrix display device in which carrier injection type organic thin film EL elements are employed. The display device
1
A shown in this figure includes various elements formed on a transparent substrate, such as a plurality of scanning lines “gate”, a plurality of data lines “sig” extending in a direction crossing the direction in which the plurality of scanning lines “gate” extend, a plurality of common power supply lines “com” extending in a direction parallel to the data lines “sig”, and pixel regions located at respective intersections of the data lines “sig” and the scanning lines “gate”. To drive the data lines “sig”, there is provided a data line driving circuit
3
including a shift register, level shifters, video lines, and analog switches. Similarly, to drive the scanning lines, there is provided a scanning line driving circuit
4
including a shift register and level shifters. Each pixel region
7
includes a first TFT
20
having a gate electrode to which a scanning signal is supplied via a scanning line, a holding capacitor “cap” for holding an image signal supplied from a data line “sig” through the first TFT
20
, a second TFT
30
having a gate electrode to which the image signal held by the holding capacitor “cap” is supplied, and a luminescent element
40
into which a driving current flows when the luminescent element
40
is electrically connected to a common power supply line “com” via the second TFT
30
.
In each pixel region, as shown in FIGS.
14
(A) and
14
(B), the first TFT
20
and the second TFT
30
are formed using two respective island-shaped semiconductor films wherein one of the source/drain regions of the second TFT
30
is electrically connected to an interconnecting electrode
35
via a contact hole formed in a first interlayer insulating film
51
and the interconnecting electrode
35
is electrically connected to a pixel electrode
41
. At upper layers above the pixel electrodes
41
, there are provided a hole injection layer
42
, an organic semiconductor film
43
, and an opposite electrode “op”, which are formed in a multilayer structure. The opposite electrode “op” extends across the data lines “sig” and other lines over a plurality of pixel regions
7
.
The other one of the source/drain regions of the second TFT
30
is electrically connected to the common power supply line “com” via a contact hole. On the other hand, in the first TFT
20
, one of the source/drain regions is electrically connected to a potential sustaining electrode “st” which in turn is electrically connected to an extension
310
of the gate electrode
31
. A semiconductor film
400
, which is doped with an impurity so that it exhibits conductivity, is disposed below the extension
310
such that the semiconductor film
400
and the extension
310
face each other via a gate insulating film
50
. As a result, a holding capacitor “cap” is formed with the extension
310
, the gate insulating film
50
, and the semiconductor film
400
. The semiconductor film
400
is electrically connected to the common power supply line “com” via a contact hole formed in the first interlayer insulating film
51
. The holding capacitor “cap” holds the image signal supplied from the data line “sig” via the first TFT
20
so that the gate electrode
31
of the second TFT
30
is maintained at a potential corresponding to the image signal even after the first TFT
20
is turned off. As a result, the driving current keeps flowing from the common power supply line “com” into the luminescent element
40
and thus the luminescent element
40
keeps emitting light.
However, in the display device described above, unlike liquid crystal display devices, the opposite electrode “op” opposing the pixel electrodes
41
is formed on the same transparent substrate
10
on which the pixel electrodes
41
are formed, such that the opposite electrode “op” extends over the entire surface of the transparent substrate
10
or over the plurality of pixel regions
7
, and thus there is only a second insulating film
52
between the opposite electrode “op” and the data lines “sig”. As a result, the data lines “sig” have a large parasitic capacitance which causes the data lines “sig” to have a large load. Similarly, a large parasitic capacitance is present between the opposite electrode and interconnection layers included in the data line driving circuit
3
or the scanning line driving circuit
4
, because the opposite electrode “op” extends over the data line driving circuit
3
and the scanning line driving circuit
4
. As a result, the data line driving circuit
3
also has a problem of a large load caused by the large parasitic capacitance.
SUMMARY OF THE INVENTION
The inventor of the present invention has developed a technique of forming an organic semiconductor film in a desired area by emitting a liquid material from an ink-jet head. The inventor has also developed a technique of defining an area where an organic semiconductor film is to be formed by surrounding the area by a bank layer so that the organic semiconductor film can be formed precisely in the defined area by means of the ink-jet technique without producing a part protruding outward from the defined area. Herein, the inventor presents a technique of solving the above-described problems using the above techniques.
That is, it is an object of the present invention to provide a display device including organic semiconductor films formed on a substrate, in particular areas defined by a bank layer thereby preventing data lines and driving circuits from having parasitic capacitance.
According to an aspect of the present invention, to achieve the above object, there is provided a display device comprising elements formed on a substrate, the elements including: a plurality of scanning lines; a plurality of data lines extending in a direction crossing the direction in which the scanning lines extend; a plurality of common power supply lines extending in a direction parallel to the data lines; and pixel regions formed in the shape of a matrix defined by the data lines and the scanning lines, each pixel region including: a first TFT having a gate electrode to which a scanning signal is supplied via one of the scanning lines; a holding capacitor for holding an image signal supplied from a corresponding data line via the first TFT; a second TFT having a gate electrode to which the image signal held by the holding capacitor is supplied; and a luminescent element including an organic semiconductor film formed between a pixel electrode provided in each pixel region and an opposite electrode extending across the data lines such that the opposite electrode faces the plurality of pixel electrodes, the luminescent element being adapted to emit light when the organic semiconductor film is driven by a driving current which flows between the pixel electrode and the opposite electrode when the pixel electrode is electrically connected to a corresponding common power supply line via the second thin film transistor, wherein light emitting areas of the organic semiconductor film are surrounded by a bank layer made up of an in
Oliff & Berridg,e PLC
Seiko Epson Corporation
Vu David
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