Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-05-07
2004-03-30
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S045000, C315S169100, C315S169400
Reexamination Certificate
active
06714178
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic display (electro optical device) formed by fabricating an EL (electro luminescence) element on a substrate. Particularly, the present invention relates to a display device using a semiconductor element (an element employing a semiconductor thin film), and furthermore to electronic equipment using the EL display device as a display portion.
2. Description of the Related Art
In recent years, remarkable progress has been made in a technique for forming thin film transistors (hereinafter referred to as TFTs) on a substrate, and developing the application of TFTs to an active matrix display device is proceeding. TFTs using a polycrystalline semiconductor film such as poly-silicon film, in particular, have a higher electric field effect mobility (also referred to as mobility) than that of conventional TFTs using an amorphous semiconductor film such as an amorphous silicon film, and hence a high speed operation may be made. Thus, control of pixels, which in the past has been controlled by a driver circuit external to a substrate, can now be made by driver circuits formed on the same substrate as the pixels.
Various merits such as reduction of manufacturing cost, miniaturization of a display device, and increase of yield and reduction of throughput can be obtained from such an active matrix display device using a polycrystalline semiconductor film by forming various circuits and elements on the same substrate.
A research on active matrix EL display devices having an EL element as a self-luminous element is being actively carried out. The EL display device is also referred to as an organic EL display (OLED) or an organic light emitting diode (OLED).
The EL element has a structure composed of a pair of electrodes (anode and cathode) and an EL layer, which is usually a laminate structure, sandwiched therebetween. The laminate structure (hole transporting layer, light-emitting layer, electron transporting layer) proposed by Tang, et al. from Eastman Kodak Company can be cited as a typical laminate structure of the EL layer. This laminate structure has an extremely high luminescence efficiency, and therefore at present, most of the EL display devices in which research and development are proceeding adopt this laminate structure of the EL layer.
In addition to the above laminate structure, a structure in which the layers are laminated on the anode in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, and an electron transporting layer or in the order of a hole injection layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injection layer may be formed. The light-emitting layer may be doped with a fluorescent pigment or the like.
The EL layer is a generic term in the present specification indicating all the layers formed between the cathode and anode. Therefore, the above-mentioned hole injection layer, the hole transporting layer, the light-emitting layer, the electron transporting layer, the electron injection layer, etc. are all included in the EL layer.
If a predetermined voltage from the pair of electrodes is applied to the EL layer having the above structure, a re-coupling of carriers in the light-emitting layer occurs to thereby emit light. It is to be noted that throughout the present specification, the emission of light by the EL element is called a drive by the EL element. In addition, a luminescent element formed of the anode, the EL layer, and the cathode is called the EL element in the present specification.
It is to be noted that an EL element as used herein includes one utilizing light emission from singlet excited state (fluorescence) and one utilizing light emission from triplet excited state (phosphorescence).
A driving method of the analog system (analog drive) can be cited as a driving method of the EL display device. An explanation regarding the analog drive of the EL display device will be made with references to
FIGS. 18 and 19
.
FIG. 18
is a diagram showing the structure of a pixel portion
1800
in the EL display device having the analog drive. A gate signal line (G
1
to Gy) for inputting a selected signal from a gate signal line driver circuit is connected to a gate electrode of a switching TFT
1801
of the respective pixels. As to a source region and a drain region of the switching TFT
1801
of the respective pixels, one is connected to a source signal line (also called data signal line) (S
1
to Sx) for inputting an analog video signal whereas the other is connected to a gate electrode of a driver TFT
1804
and a capacitor
1808
of each of the pixels, respectively.
A source region and a drain region of the driver TFT
1804
of each of the pixels is connected to power supply lines (V
1
to Vx), and a drain region thereof is connected to an EL element
1806
, respectively. An electric potential of the power supply lines (V
1
to Vx) is called a power supply potential. Each of the power supply lines (V
1
to Vx) is connected to the capacitor
1808
of the respective pixels.
The EL element
1806
is composed of an anode, a cathode, and an EL layer sandwiched therebetween. When the anode of the EL element
1806
is connected to either the source region or the drain region of the EL driver TFT
1804
, the anode and the cathode of the EL element
1806
become a pixel electrode and an opposing electrode, respectively. Alternatively, if the cathode of the EL element
1806
is connected to either the source region or the drain region of the EL driver TFT
1804
, then the anode of the EL element
1806
becomes the opposing electrode whereas the cathode thereof becomes the pixel electrode.
It is to be noted that the electric potential of an opposing electrode is herein referred to as opposing potential. It is to be noted that a power supply for giving opposing potential to an opposing electrode is herein referred to as an opposing power supply. The difference between the electric potential of a pixel electrode and the electric potential of an opposing electrode is the EL driving voltage, which is applied to the EL layer.
FIG. 19
is a timing chart illustrating the EL display shown in
FIG. 18
when it is being driven by the analog system. A period from the selection of one gate signal line to the selection of a next different gate signal line is called a 1 line period (L). In addition, a period from the display of one image to the display of the next image corresponds to a 1 frame period (F). In the case of the EL display device of
FIG. 18
, there are y number of the gate signal lines and thus a y number of line periods (L
1
to Ly) are provided in 1 frame period.
Because the number of line periods in 1 frame period increases as resolution becomes higher, driver circuits must be driven at a high frequency.
First of all, the power supply lines (V
1
to Vx) are held at a constant power supply potential, and the opposing electric potential that is the electric potential of the opposing electrode is also held at a constant electric potential. There is a difference in electric potential between the opposing electric potential and the power supply potential to a degree that the EL element can emit light.
A selected signal from the gate signal line driver circuit is inputted to the gate signal line G
1
in the first line period (L
1
). An analog video signal is then sequentially inputted to source signal lines (S
1
to Sx). All the switching TFTs connected to the gate signal line G
1
are turned on to thereby input the analog video signal that is inputted to the source signal lines to the gate electrode of the driver TFT through the switching TFT.
The amount of electric current through the channel forming region of a TFT for driving is controlled by its gate voltage.
Here, description is made with regard to, by way of example, a case where the source regions of the TFTs for driving are connected to the power supply lines and the drain regions of the TFTs for driving are connected to the
Kimura Hajime
Koyama Jun
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Hjerpe Richard
Nguyen Kimnhung
Semiconductor Energy Laboratory Co,. Ltd.
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