Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1999-02-02
2002-02-19
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S066000, C257S088000, C349S043000
Reexamination Certificate
active
06348702
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image display system, in particular, relates to a high quality image display system which is used in an organic EL (Electro Luminescence) display panel.
In recent years, an image display panel using an organic EL (Electro Luminescence) element has been developed. When an organic EL panel which has a large number of organic EL elements is activated by using an active matrix circuit, a pixel of each EL is coupled with an FET (Field Effect Transistor) which is implemented by a TFT (Thin Film Transistor) for controlling supply current to each pixel. Therefore, each pixel is associated with a first TFT which biases an organic EL element to allow current to flow in said organic EL element, and a second TFT transistor for switching said first TFT transistor.
FIG. 9
shows a circuit diagram of a prior active matrix type organic EL display panel. The organic EL display panel comprises first signal lines in X-direction
301
-
1
,
301
-
2
, et al, second signal lines in Y-direction
302
-
1
,
302
-
2
, et al, power supply (Vdd) lines
303
-
1
,
303
-
2
, et al, switching transistors (TFT)
304
-
1
,
304
-
2
, et al, bias transistors (TFT)
305
-
1
,
305
-
2
, et al for controlling bias current in an EL element, organic EL elements
306
-
1
,
306
-
2
, et al, capacitors
307
-
1
,
307
-
2
, et al, a drive circuit in X-direction
308
, and a drive circuit in Y-direction
309
.
A pixel is selected by an X-direction signal line
301
, and a Y-direction signal line
302
, and the switching TFT transistor
304
associated with the selected pixel is turned ON so that the capacitor
307
associated with the selected pixel holds image data. Therefore, the bias TFT transistor
305
is turned ON, then, the organic EL element
306
is supplied with bias current associated with image data from a power supply line
303
so that the organic EL element
306
emits light.
When a signal line
301
-
1
in X-direction receives a signal associated with image data, and a signal line
302
-
1
receives a scanning signal in Y-direction, a switching TFT transistor
304
-
1
which is selected by said signal lines
301
-
1
and
302
-
1
is turned ON, then, a bias TFT transistor
305
-
1
is turned ON by a signal associated with image data, so that the selected EL element
306
-
1
is supplied bias current, and said EL element
306
-
1
emits light.
Therefore, each pixel of active matrix type EL display panel comprises a thin film type EL element, a bias TFT transistor for controlling light emit of said EL element, a signal hold capacitor coupled with a gate electrode of said bias TFT transistor, and a switching TFT transistor for writing data in said capacitor. The light intensity of each EL element depends upon current flowing in a bias TFT transistor, and said current depends upon voltage charged in a signal hold capacitor. This is described in (A66-in 201 pi Electro luminescent Display T. P. Brody, F. C. Luo, et al., IEEE Trans. Electron Devices, Vol. ED-22, No.9, Sep. 1975, pages 739-749).
The capacitance of a signal hold capacitor must be small so that a switching TFT transistor can charge the capacitor in a short selected time, and simultaneously, must be large so that the voltage across the capacitor does not decrease by the leak current through switching transistors which are not selected thereby for keeping excellent image quality until the next selection of a pixel.
By the way, an active matrix type display panel is requested having the size larger than 4 inches because of visibility when no optical enlarge system is used. A silicon single crystal substrate is impossible to implement that size of panel, because of high cost, since each sheet of single crystal Si substrate can provide very small number of panels under the current producing technology.
Therefore, it is preferable that an active matrix type display panel is comprised of a thin film transistor (TFT) by a non-single crystal Si semiconductor produced on a plane substrate, such as a glass substrate.
As for a non-single crystal semiconductor element on a plane substrate, an amorphous silicon layer has been used because large area of panel is easily obtained. However, a TFT transistor produced in an amorphous silicon layer has the disadvantage that a picture quality is deteriorated, since the threshold voltage of the transistor drifts when current in one direction is flown in a transistor for a long time. Further, as the mobility of a transistor which is produced in an amorphous silicon layer is small, the current with high speed response must be small. Further, as a P-channel is difficult, even a small size of CMOS circuit is impossible in an amorphous silicon layer.
Accordingly, it is preferable to use a poly-crystalline silicon semiconductor layer for an active matrix type organic EL display panel, since relatively large area of panel is possible, with high reliability and high mobility, and CMOS circuit is possible.
A TFT transistor produced in a prior poly-crystalline silicon layer has the disadvantage that trap level density depends upon a number of crystal boundaries in a channel, and that change of the trap level density affects the characteristics of the transistor. Therefore, when the channel length, and/or the channel width of a transistor is close to crystal grain diameter, the variation of a number of grain boundary in a channel is large. This causes the increase of variation of trap level density in a channel, and increase of variation of characteristics of a TFT transistor. The increase of the variation of the characteristics of a TFT transistor deteriorates of picture quality of a display panel.
Another disadvantage of a prior EL display panel concerns that light is obtained through a transparent electrode on a substrate. The reason of that is described in Japanese patent laid open publication 234683/1996, paragraph [0011] that (1) a non-transparent metal such as MgAg alloy which has work function less than 4 eV must be used in an anode electrode of an organic EL element, (2) the patterning of that metal is difficult, and (3) an organic thin film for a light emit layer and/or a transportation layer of a carrier is damaged and layers are separated in a photo-resist process using organic solvent.
Because of the above reasons, the separation of pixels in an organic display panel is carried out by patterning a transparent cathode electrode which is produced before an organic layer is produced, so that light is obtained through a substrate. Further, a non-transparent element, including a TFT transistor, a capacitor, and/or a lead line, prevents light, therefore, those elements are produced outside of an organic EL element.
FIG. 10A
shows a pixel of an active matrix type organic EL panel, comprising a switching TFT transistor
304
, a bias TFT transistor
305
, an organic EL element
306
, and a signal hold capacitor
307
.
The light intensity emitted by a pixel depends upon area (&agr;) of a pixel. It is assumed that a pixel of a passive type organic EL element 406 has the area (&agr;) as shown in FIG.
10
B. In an active matrix type organic EL element, the area for light emission is the area (&bgr;) which is the area of an organic EL element 306, and (&bgr;)<(&agr;).
When there is no switching transistor 304, no bias transistor 305, and no capacitor
307
,
FIG. 10A
coincides with FIG.
10
B.
It is assumed that the light intensity for each unit area in
FIG. 10B
is A, and the light intensity for each unit area by an organic EL element
306
in
FIG. 10A
is B.
When the total intensity of a pixel in
FIG. 10A
is equal to that in
FIG. 10B
, the following relations are obtained.
(&bgr;)B=(&agr;)A
B=(&agr;/&bgr;)A
An active matrix type organic EL element must have higher light intensity for each unit area of an organic EL element
306
if the same light intensity as that of the passive EL element
406
is desired, because of the presence of a switching TFT transistor
304
, a bias TFT transistor
305
and a signal hold capacitor
307
.
Arai Michio
Takayama Ichirou
Arent Fox Kintner & Plotkin & Kahn, PLLC
Jackson, Jr. Jerome
TDK Corporation
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