Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-10-08
2004-01-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S309000, C313S495000
Reexamination Certificate
active
06674244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroluminescence (EL) display device comprising a thin film transistor (TFT) and an EL element in which the TFT is employed as a switching element.
2. Description of the Related Art
FIG. 1
is a plan view showing a display pixel portion of an EL display device of a related art, and
FIGS. 2A and 2B
are cross sectional views illustrating the EL display device taken along the lines A—A and B—B in
FIG. 1
, respectively.
Referring to
FIG. 1
, a display pixel is formed in a region surrounded by gate signal lines
51
and drain signal lines
52
. A first TFT
30
is provided near an intersection of these signal lines, and has a source
13
s
serving as a capacitor electrode
55
which forms a capacitor with a storage capacitor electrode line
54
, and connected to a gate
41
of the second TFT
40
. The second TFT
40
has a source
43
s
connected to an anode
61
of an organic EL element
60
, and a drain
43
d
connected to a power source line
53
for driving the organic EL element.
The storage capacitor electrode line
54
is disposed near the TFTs inparallel to the gate signal line
51
. The line
54
is composed of chromium or the like and stores electric charges to form a capacitor with the capacitor electrode
55
connected to the source
13
s
of the TFT with a gate insulating film
12
interposed therebetween. The storage capacitor is provided to retain a voltage applied to the gate electrode
41
of the second TFT
40
.
Thus, display pixels each having the organic EL element
60
and the TFTs
30
and
40
are arranged in a matrix on a substrate
10
, thereby forming an organic EL display device.
Referring to
FIGS. 2A and 2B
, the organic EL display device is composed of the TFTs and the organic EL element formed in succession on a substrate
10
, such as a substrate formed of glass, synthetic resin, or the like, an electrically conductive substrate, and a semiconductor substrate. When a conductive or semiconductor substrate is used for the substrate
10
, an insulating film of SiO
2
, SiN, or the like is first formed on the substrate
10
before the TFTs
30
and
40
and the organic EL display element
60
are provided.
The first TFT
30
for switching operation will next be described.
As shown in
FIG. 2A
, on the insulating substrate
10
of quartz glass, non-alkaline glass, or the like, the gate signal line
51
formed of refractory metal, such as chromium (Cr) and molybdenum (Mo), and serving as a gate electrode
11
, and the drain signal line
52
formed of aluminum (Al) are provided, and the power source line
53
formed of Al and serving as a driving power source for the organic EL element is disposed.
Thereafter, a gate insulating film
12
and an active layer
13
of a polysilicon (p-Si) film are formed in this order. The active layer
13
is of a so-called LDD (lightly doped drain) structure, and the source
13
s
and a drain
13
d
are provided on the outer sides thereof.
An interlayer insulating film
15
composed of an SiO
2
film, an SiN film, and an SiO
2
film formed in this order is provided over the entire surface, covering the gate insulating film
12
, the active layer
13
, and a stopper insulating film
14
. A drain electrode
16
is formed by filling a metal such as Al in a contact hole provided corresponding to the drain
13
d
. Further, a planarization insulating film
17
is formed of organic resin or the like over the entire surface for planarization.
The second TFT
40
for driving the organic EL element
60
will next be described.
As shown in
FIG. 2B
, the gate electrode
41
is formed of refractory metal, such as Cr and Mo, on the insulating substrate
10
formed of quartz glass, non-alkaline glass, or the like.
The gate insulating film
12
and an active layer
43
formed of a p-Si film are provided in succession.
The active layer
43
includes an intrinsic, or substantially intrinsic, channel
43
c
provided over the gate electrode
41
, and the source
43
s
and the drain
43
d
provided on respective sides of the channel
43
c
by ion doping.
The interlayer insulating film
15
composed of an SiO
2
film, an SiN film, and an SiO
2
film formed in this order is next provided over the entire surface to cover the gate insulating film
12
and the active layer
43
. The power source line
53
connected to a driving power source
50
is formed by filling a metal such as Al in a contact hole provided corresponding to the drain
43
d
. The planarization insulating film
17
of organic resin or the like is formed over the entire surface for the purpose of planarization. A contact hole is formed in the planarization insulating film
17
and the interlayer insulating film
15
at a position corresponding to the source
43
s
. A transparent electrode formed of ITO, i.e. the anode
61
of the organic EL element, is formed on the planarization insulating film
17
so as to contact the source
13
s
through the contact hole.
The organic EL element
60
includes the anode
61
formed of a transparent electrode of ITO or the like, an emissive element layer
62
of an organic compound, and a cathode
63
of a magnesium-indium alloy, formed in this order. The cathode
63
is provided over the entire surface of the substrate
10
which forms the organic EL display device, i.e. over the entire plane of the FIG.
1
.
In the organic EL element, holes and electrons injected from the anode and cathode, respectively, are recombined in the emissive layer to excite organic molecules forming the emissive layer, thereby producing excitons. Light is released from the emissive layer during the process in which the excitons deactivate, and this release of light from the transparent anode through the transparent insulating substrate results in the emission of light.
With such a configuration, after the anode
61
is formed, an insulating film
64
is formed in the peripheral region of the anode
61
(the region excluding the area surrounded by broken lines) to prevent a short circuit between the cathode
63
and the anode
61
, which would otherwise be generated from a crack in the emissive layer resulting from a difference in level created by the thickness of the anode
61
. The emissive element layer
62
and the cathode
63
are next formed. Light emitted from the emissive element layer
62
exits after being transmitted through the insulating substrate
10
.
It should be noted that while light emitted from the emissive element layer
62
advances radially outward from the emissive layer
62
, not all components of the light emitted from the emissive layer
62
toward the substrate
10
reach the insulating substrate
10
, and some light is reflected by, for example, the surface of the planarization insulating film, or attenuated in the film.
This results from a difference in materials between the interlayer insulating film and the planarization insulating film, especially from a difference in refractive index between these insulating films. The light may be partly reflected at the interface between respective insulating films, or even if transmitted through the film, the light may be attenuated as it is repeatedly reflected within the insulating film depending on the angle incident to the insulating film.
FIG. 3
shows how light radiated from the emissive layer travels toward the substrate.
Referring to
FIG. 3
, the emitted light incident on the SiN film advances at an angle &agr;
1
from a normal line C to the SiN film, and is then reflected at the interface between the SiN film (having a refractive index n
1
=2.0) and the SiO
2
film (having a refractive index n
2
=1.46). Assuming that the reflected ray forms an angle &agr;
2
with the normal line C, an equation n
1
×Sin &agr;
1
=n
2
×Sin &agr;
2
holds true from Snell's law, and &agr;
1
=Sin
−1
(n
1
2
)≈47°. As the value n
1
is larger than the value n
2
in this example, light is totally reflected at this interface when the angle &agr;
1
exceeds 47°.
Th
Cantor & Colburn LLP
Sanyo Electric Co,. Ltd.
Vu Jimmy T.
Wong Don
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