Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-10-19
2003-07-01
Shankar, Vijay (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S076000, C345S092000, C345S084000
Reexamination Certificate
active
06587086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an EL (electro-luminescence) display (an electro-optical device) formed by preparing an EL element on a substrate. More particularly, the invention relates to an EL display using a semiconductor element (an element using a semiconductor thin film). Furthermore, the present invention relates to an electronic equipment in which the EL display is used in a display portion thereof.
2. Description of the Related Art
In recent years, technology for forming a TFT on a substrate has been largely improved, and an application development of the TFT to an active matrix type display device has been carried out. In particular, the TFT using a polysilicon film has a higher electric field effect mobility than the TFT using a conventional amorphous silicon film, and therefore, the TFT may be operated at a high speed. Thus, the pixel control which has been conducted at a driver circuit outside of the substrate may be conducted at the driver circuit which is formed on the same substrate as the pixel.
Such an active matrix type display device can, by preparing various circuits and elements on the same substrate, obtain various advantages such as a decrease in the manufacturing cost, a decrease in the size of the display device, an increase in the yield, and a decrease in the throughput.
Further, research on the active matrix type EL display having an EL element as a self-light-emitting device is becoming more and more active. The EL display is referred to as an organic EL display (OELD) or an organic light-emitting diode (OLED).
The EL display is a self-light-emitting type unlike a liquid crystal display device. The EL element is constituted in such a manner that an EL layer is sandwiched between a pair of electrodes. However, the EL layer normally has a lamination structure. Typically, the lamination structure of a “positive hole transport layer/a luminous layer/an electron transport layer” proposed by Tang et al. of the Eastman Kodak Company can be cited. This structure has a very high light-emitting efficiency, and this structure is adopted in almost all the EL displays which are currently subjected to research and development.
In addition, the structure may be such that on the pixel electrode, a positive hole injection layer/a positive hole transport layer/a luminous layer/ an electron transport layer, or a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer/an electron injection layer may be laminated in order. Phosphorescent dye or the like may be doped into the luminous layer.
In this specification, all the layers provided between the pixel electrode and an opposite electrode are generally referred to as EL layers. Consequently, the positive hole injection layer, the positive hole transport layer, the luminous layer, the electron transport layer, the electron injection layer and the like are all included in the EL layers.
Then, a predetermined voltage is applied to the EL layer having the above structure from the pair of the electrodes, so that a recombination of carriers is generated in the luminous layer and light is emitted. Incidentally, in this specification, the fact that the EL element is emitted is described as the fact that the EL element is driven. Furthermore, in this specification, the light-emitting element formed of the anode, the EL layer and the cathode is referred to as an EL element.
An analog type driver method (analog drive) can be given as a method of driving an EL display. An analog drive EL display is explained using
FIGS. 18 and 19
.
The structure of a pixel portion of the analog drive EL display is shown in
FIG. 18. Y
gate signal lines (G
1
to Gy) for inputting gate signals are connected to gate electrodes of switching TFTs
1801
of pixels. One of a source region and a drain region of the switching TFT
1801
of each pixel is connected to x source signal lines (also referred to as data signal lines) (S
1
to Sx) for inputting analog video signals, and the other is connected to the gate electrode of an EL driver TFT
1804
of each pixel and to a capacitor
1808
of each pixel.
The source region and the drain region of the EL driver TFT
1804
incorporated in each of the pixels is connected to the power source supply lines (V
1
to Vx) while the other is connected to the EL element
1806
. The potential of the power source supply lines (V
1
to Vx) is referred to as the potential of the power source. Note that, the power source supply lines (V
1
to Vx) is connected to a capacitor
1808
incorporated in each of the pixels.
The EL element
1806
comprises an anode, a cathode and an EL layer provided between the anode and the cathode. In the case where the anode is connected to the source region or the drain region of the EL driver TFT
1804
, namely, in the case where the anode is the pixel electrode, the cathode which is the opposite electrode is held at a constant potential. On the contrary, in the case where the cathode is connected to the source region or the drain region of the EL driver TFT
1804
, that is, in the case the cathode is the pixel electrode, the anode, which is an opposite electrode is held at a constant potential.
The opposite electrodes are normally maintained at a constant electric potential, and in the present specification, the electric potential of the opposite electrodes is referred to as a steady-state electric potential. Note that an power source for imparting the steady-state electric potential to the opposite electrodes is referred to as a steady-state power source. The electric potential difference between the steady-state electric potential of the opposite electrodes and the power source electric potential of the pixel electrodes is an EL driver voltage, and the EL driver voltage is applied to the EL layers.
A timing chart for a case of driving the EL display by the analog method is shown in
FIG. 19. A
period during which one gate signal line is selected is referred to as one line period (L). Further, a period until selection of all the gate signal lines (G
1
to Gy) is completed corresponds to one frame period (F). There are y gate signal lines for the case of the EL display of
FIG. 18
, and therefore y line periods (L
1
to Ly) are formed during one frame period.
Note that
60
or more frame periods are formed during one second in the EL display drive. In other words,
60
or more images are displayed during one second. If the number of images displayed in one second becomes less than
60
, then problems such as image flicker start to become visually conspicuous.
The number of line periods during one frame period increases as the number of gradations increases, and the driver circuit must operate at a high frequency.
First, electric power source supply lines (V
1
to Vx) are maintained in an off-power source electric potential. Note that the off-power source electric potential in an analog driver method is in a range in which the EL elements do not emit light and is the same strength as the steady-state electric potential. Note also that the difference between the off-power source electric potential and the steady-state electric potential is referred to as an off EL driver voltage. Ideally, it is preferable that the off EL driver voltage be 0 V, but it is acceptable provided that it is such that EL elements
1806
do not emit light.
A gate signal is input to the gate signal line G
1
in the first line period (L
1
). An analog video signal is then input to the source signal lines (S
1
to Sx) in order. A switching TFT (
1
,
1
) is therefore in an On state (on), and consequently the analog video signal input to the source signal line S
1
is input to the gate electrode of an EL driver TFT (
1
,
1
), through the switching TFT (
1
,
1
).
The electric potential of the power source supply line V
1
then changes from the off-power source electric potential to a saturation power source electric potential. Note that, throughout this specification, saturation power source electric potential refe
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Shankar Vijay
Shapiro Leonid
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