Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-09-27
2002-09-24
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S498000, C345S092000
Reexamination Certificate
active
06456013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor and a display device, and particularly to an electroluminescence display device using an electroluminescence element and a thin film transistor.
2. Description of the Related Art
In recent years, display devices using electroluminescence (hereinafter referred to as “EL”) elements have gained attention as display devices that may replace CRTs and LCDs. Progress is being made in research and development of a display device using thin film transistors (hereinafter referred to as “TFTs”) as switching elements for driving the EL elements.
FIG. 1
is a plan view showing an area around a display pixel in a conventional EL display device.
FIG. 2A
shows a cross-sectional view taken along line A—A of
FIG. 1
, while
FIG. 2B
shows a cross-sectional view taken along line B—B of FIG.
1
.
FIG. 5B
illustrates a state in which a laser is irradiated on a conventional TFT.
FIG. 6
shows the energy distribution in the short-axis direction of a linear shape laser.
As shown in
FIG. 1
, formed overlapping one another are a plurality of gate signal lines
51
a
,
51
b
extending in the row direction (horizontal direction in the figure), and a plurality of data signal lines
52
a
,
52
b
and power source lines
53
a
,
53
b
extending in the column direction (vertical direction in the figure). An area surrounded by the two types of signal lines (
51
a
,
51
b
,
52
a
,
52
b
) constitutes one display pixel region
110
. Arranged in each display pixel region
110
are an EL display element
60
, switching TFT
30
, storage capacitor, and EL element driving TFT
40
.
A display pixel region
110
formed at the intersecting portion between the gate signal line
51
a
and the data signal line
52
a
is referred to as an example in the following description. The EL display element
60
, switching TFT
30
, storage capacitor, and EL element driving TFT
40
of the display pixel region
110
are described according to
FIGS. 1
,
2
A, and
2
B.
The switching TFT
30
includes gate electrodes
11
connected to the gate signal line
51
a
and supplied with a gate signal, a drain electrode
16
connected to the data signal line (drive signal line)
52
a
and supplied with a data signal (drive signal), and a source electrode
13
s
connected to a gate electrode
41
of the EL element driving TFT
40
. The TFT
30
is formed by disposing on an insulating substrate
10
a polysilicon film (hereinafter referred to as “p-Si film”)
13
serving as an active layer, and then sequentially forming a gate insulating film
12
and gate electrodes
11
. The gate electrodes are shaped as two perpendicular protrusions from the gate signal line
51
a
, constituting a what is known as a double gate structure.
A storage capacitor electrode line
54
is disposed in parallel to the gate signal line
51
a.
Charges are stored between the storage capacitor electrode line
54
and a capacitor electrode
55
formed in an underlying layer beneath the gate insulating film
12
, thereby creating a capacitor. The capacitor electrode
55
is formed by extending a portion of the source
13
s
, and is provided for retaining a voltage applied to the gate electrode
41
of the EL element driving TFT
40
.
The EL element driving TFT
40
includes a gate electrodes
41
connected to the source electrode
13
s
of the switching TFT
30
, a source electrode
43
s
connected to the anode
61
of the EL element
60
, and a drain electrode
43
d
connected to the power source line
53
b
supplying power to the EL element
60
.
The EL element
60
comprises an anode
61
connected to the source electrode
43
s
, a cathode which is a common electrode, and an emissive element layer
66
disposed between the anode
61
and the cathode
67
.
When a gate signal from the gate signal line
51
a
is applied to the gate electrodes
11
, the switching TFT
30
is turned on. In response, a data signal is supplied from the data signal line
52
a
to the gate electrode
41
of the EL element driving TFT
40
. The potential of the gate electrode
41
thereby becomes equal to the potential of the data signal line
52
a
. As a result, a current corresponding to the value of the voltage supplied to the gate electrode
41
is supplied to the EL element
60
from the power source line
53
b
connected to a power source, causing light emission from the EL element
60
.
The EL element
60
is formed by first providing the anode
61
, which is made of a transparent electrode composed of a material such as ITO (indium tin oxide). The emissive element layer
66
is then superimposed. The emissive element layer
66
comprises a first hole-transport layer
62
composed of MTDATA (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), a second hole-transport layer
63
composed of TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), an emissive layer
64
composed of Bebq
2
(bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivatives, and an electron transport layer
65
composed of Bebq
2
. Subsequently, the cathode
67
is formed by laminating lithium fluoride (LiF) and aluminum (Al), or by using a magnesium-indium alloy. These layers constituting the EL element
60
are laminated in the described order.
In the EL element, holes injected from the anode and electrons injected from the cathode recombine in the emissive layer. As a result, organic molecules constituting the emissive layer are excited, generating excitons. Through the process in which these excitons undergo radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode via the transparent insulating substrate, finally resulting in light emission.
A p-Si film is employed as the active layer in the TFTs
30
,
40
. The p-Si film is formed by depositing an amorphous silicon film (hereinafter referred to as “a-Si film”) on the substrate
10
by CVD or other methods, and polycrystallizing the a-Si film by irradiating a linear shape laser. Subsequently, the gate electrodes
11
are disposed thereon after the gate insulating film
12
is deposited.
Laser irradiation is performed, as shown in
FIG. 5B
, by repeating spot irradiation of a linear shape laser from one end of the substrate to the other, so as to scan the substrate. In the figure, after irradiating a laser in the area indicated by a dotted line, the laser is shifted towards the right by a predetermined distance to irradiate the next spot indicated by one dot chain line. This irradiation process is continually repeated from one end of the substrate to the other. The laser irradiation is performed by orienting the long-axis direction of the laser orthogonally to the gate signal line as shown in FIG.
1
.
As shown in
FIG. 6
, the energy distribution of the linear shape laser in its short-axis direction is gradually reduced towards both peripheral portions compared to the center portion. In other words, the intensity of the laser light is not uniform. There may be cases when, as shown in
FIG. 5B
, a periphery portion of the footprint of the low energy laser overlaps a junction portion (indicated by dotted circle B in the figure) between the channel
13
c
and the source
13
s
, the portion in which the active layer is overlapped by the subsequently formed gate
11
. In such cases, as crystallization cannot be fully performed in that region of p-si film, the grain size in that region will be smaller than in regions irradiated with higher laser energy. While theoretically it is possible to irradiate the region irradiated with a low energy laser a second time with a laser having a high energy so as to increase the grain sizes, such a process is not practical because the resulting grain sizes are not identical to those of other regions. A TFT characteristic obtained by irradiating laser only once on an a-Si film differs from a TFT characteristic obtained by irradiating laser for a second time on
Komiya Naoaki
Sano Keiichi
Cantor & Colburn LLP
Sanyo Electric Co,. Ltd.
Tran Thuy Vinh
Wong Don
LandOfFree
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