Active solid-state devices (e.g. – transistors – solid-state diode – Gate arrays – With particular signal path connections
Reexamination Certificate
2001-04-23
2004-09-14
Coleman, W. David (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Gate arrays
With particular signal path connections
C257S296000
Reexamination Certificate
active
06791129
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic display formed by fabricating an EL (electro luminescence) on a substrate, in particular, to an EL display using a semiconductor element (an element which uses a semiconductor thin film). Further, the present invention relates to a light emitting device using an EL display in a display portion.
2. Description of the Related Art
Recently, a technique for forming TFTs on a substrate has been remarkably developed, and its application to an active matrix electronic display has been in proceeding. In particular, TFTs using a polysilicon film can operate at high speed, because such TFTs have a higher field effect mobility than TFTs using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally conducted by a driver circuit provided outside a substrate, can be performed by a driver circuit provided on the same substrate on which the pixels are provided.
Such an active matrix electronic display includes various circuits and elements formed on the same substrate. With this structure, the active matrix electronic display provides various advantages such as reduced manufacturing cost, reduced size of an electronic display, an increased yield, and an increased throughput.
Furthermore, an active matrix EL display including an EL element as a self-light emitting element has been actively studied. The EL display is also called Organic EL Display (OELD) or Organic Light Emitting Diode (OLED).
In contrast with the liquid crystal display device, the EL display has a type of self-light emitting. The EL element has such a structure that a layer containing an organic compound (hereinafter, referred to as an EL layer) is sandwiched between a pair of electrodes (anode and cathode). The EL layer generates luminescence by applying an electric field across the pair of electrodes and has normally a laminate structure. As a typical example of the laminate structures, a laminate structure “hole transporting layer/light emitting layer/electron transporting layer” proposed by Tang et al. of Eastman Kodak Company is cited. This structure has an extremely high light emitting efficiency. For this advantage, most light emitting devices, which are currently under study and development, employ this structure.
Furthermore, the light emitting device may have such a laminate structure that a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer, or a hole injecting layer/a hole transporting layer/a light emitting layer/an electron transporting layer/an electron injecting Layer are deposited on an anode in this order. Moreover, the light emitting layer may be doped with a fluorescent pigment or the like.
All layers formed between a cathode and an anode are referred to generically as EL layers within this specification. The above stated hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer, and the like are therefore all contained within the EL layer. There is emission of light when returning to a base state from a triplet excitation state (phosphorescence), and when returning to a base state from a singlet excitation state (fluorescence), in luminescence of organic compounds, and a light emitting device of the present invention may use any one of the above light emitting types, or may also use both types of light emission.
A predetermined voltage is then applied to the EL layer with the above structure by a pair of electrodes, recombination of carriers thus occurs in the light emitting layer, and light is emitted. Note that the emission of light by the EL element is referred to as driving the EL element throughout this specification. Further, a light emitting element formed by an anode, an EL layer, and a cathode is referred to as an EL element throughout this specification.
As a method of driving an EL display, an analog driving method (analog drive) can be given. The analog drive of an EL display is described with reference to
FIGS. 25 and 26
.
FIG. 25
shows a structure of a pixel portion
1800
of an EL display that is driven in an analog manner. Gate signal lines (G
1
through Gy) to which gate signals from a gate signal line driver circuit are input are connected to gate electrodes of switching TFTs
1801
included in respective pixels. Source regions or drain regions of the switching TFTs
1801
included in the respective pixels is connected to source signal lines (also referred to as data signal lines) S
1
to Sx to which an analog video signal is input, whereas the others are connected gate electrodes of EL driver TFTs
1804
and capacitors
1808
included in the respective pixels.
Source region of the EL driver TFTs
1804
included in pixels are connected to power source supply lines Vd
1
through Vx, whereas drain regions of the EL driver TFTs
1804
are connected to EL elements
1806
. An electric potential of the power source supply lines V
1
through Vx is referred to as an power source electric potential. The power source supply lines V
1
through Vx are connected to the capacitors
1808
included in the respective pixels.
The EL element
1806
includes an anode, a cathode and an EL layer sandwiched between the anode and the cathode. If the anode of the EL element
1806
is connected to 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. On the other hand, if the cathode of the EL element
1806
is connected to the drain region of the EL driver TFT
1804
, the anode and the cathode of the EL element
1806
become an opposing electrode and a pixel electrode, respectively.
Note that the electric potential of the opposing electrode is referred to as an opposing electric potential in this specification. Note also that an power source for imparting the opposing electric potential to the opposing electrode is referred to as an opposing electric power supply. The electric potential difference between the electric potential of the pixel electrode and the electric potential of the opposing electrode is an EL driver voltage, and the EL driver voltage is applied to the EL layer.
FIG. 26
shows a timing chart in the case where the EL display shown in
FIG. 25
is driven in an analog manner. The period from the selection of one gate signal line to the selection of the next gate signal line is called one line period (L). The period from starting display of one image to starting display the next image corresponds to one frame period (F). In the case of the EL display shown in
FIG. 25
, since there are y gate signal lines, y line periods (L
1
to Ly) are provided within one frame period.
With the enhancement in resolution, the number of line periods within one frame period increases. As a result, the driver circuit must be driven at a high frequency.
An power source electric potential at the power source supply lines V
1
through Vx is held constant, and an opposing electric potential at the opposing electrodes is also held constant. There is a potential difference between the opposing electric potential and the power source electric potential to such a degree that a EL element emits light.
The gate signal line G
1
is selected in the first line period L
1
by a gate signal input to the gate signal line G
1
from the gate signal line driver circuit.
Note that, in this specification, the term “a gate signal line is selected” refers to a state in which all of the thin film transistors which have gate electrodes connected to the gate signal line are in an ON state.
An analog video signal is then input to the source signal lines S
1
to Sx in order. All of the switching TFTs
1801
connected to the gate signal line G
1
are in an ON state, and therefore the analog video signals input to the source signal lines S
1
to Sx are input to gate electrodes of the EL driver TFTs
1804
through the switching TFTs
1801
.
The amount of current flowing through a
Coleman W. David
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
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