Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-06-12
2003-03-04
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000, C345S077000, C257S057000
Reexamination Certificate
active
06528951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electronic display device fabricated by forming EL (electroluminescence) elements on a substrate, specifically, to an EL display device using a semiconductor element (an element formed from a semiconductor thin film). The invention also relates to electronic equipment employing the EL display device as a display unit.
The EL element herein includes both an element that utilizes light emission from a singlet exciton (fluorescence) and an element that utilizes light emission from a triplet exciton (phosphorescence).
2. Description of the Related Art
Development of EL display devices having an EL element as a self-luminous element is flourishing in recent years. The EL display devices are also called organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
The EL display devices are self-luminous unlike liquid crystal display devices. The EL element is structured such that an EL layer is sandwiched between a pair of electrodes (an anode and a cathode). The EL layer usually has a laminate structure. Typical example thereof is a laminate structure consisting of a hole transportation layer, a light emitting layer and an electron transportation layer which has been proposed by Tang, et al. of Eastman Kodak Company. This structure is very high in light emission efficiency, and is employed by almost all of EL display devices currently under development.
Other examples of the structure of the EL layer include a laminate structure consisting of an anode, a hole injection layer, a hole transportation layer, a light emitting layer and an electron transportation layer which are layered in this order, and a laminate structure consisting of an anode, a hole injection layer, a hole transportation layer, a light emitting layer, an electron transportation layer and an electron injection layer which are layered in this order. The light emitting layer may be doped with a fluorescent pigment or the like.
In this specification, all layers that are formed between an anode and a cathode are collectively called an EL layer. Therefore the EL layer includes all of the above hole injection layer, hole transportation layer, light emitting layer, electron transportation layer and electron injection layer.
A pair of electrodes (a cathode and an anode) applies a given voltage to the EL layer structured as above, whereby carrier recombination takes place in the light emitting layer to cause the layer to emit light. The voltage applied between two electrodes (an anode and a cathode) of an EL element is herein referred to as EL driving voltage. An EL element emitting light is herein expressed as an EL element being driven. A light emitting element composed of an anode, an EL layer and a cathode herein will be referred to as EL element.
FIG. 4
is a block diagram showing a multi-gray scale EL display device. The display device shown here is of the type that obtains gray scale by inputting a digital signal into a source signal line driving circuit and uses a digital gray scale method. Particularly the case of using time division gray scale method for varying the luminance by controlling the period of time during which a pixel emits light will be described.
The EL display device of
FIG. 4
has a pixel portion
101
and a source signal line driving circuit
102
and a gate signal line driving circuit
103
which are arranged in the periphery of the pixel portion
101
. The pixel portion and the driving circuits are composed of thin film transistors (hereinafter referred to as TFTs) formed on a substrate. An external switch
116
for controlling the EL driving voltage is connected to the pixel portion
101
.
The source signal line driving circuit
102
includes, basically, a shift register
102
a
, a latch (A)
102
b
and a latch (B)
102
c
. The shift register
102
a
receives input of a clock signal (CLK) and a start pulse (SP). The latch (A)
102
b
receives input of digital data signals (denoted by VD in
FIG. 4
) whereas the latch (B)
102
c
receives input of latch signals (denoted by S_LAT in FIG.
4
).
The digital data signals VD to be inputted to the pixel portion
101
are generated in a time division gray scale data signal generating circuit
114
. This circuit converts video signals that are analog signals or digital signals containing image information into the digital data signals VD for time division gray scale. The circuit
114
also generates a timing pulse or the like that is necessary for time division gray scale display.
Typically, the time division gray scale data signal generating circuit
114
includes means for dividing one frame period into a plurality of sub-frame periods in accordance with n bit gray scale (n is an integer of 2 or greater), means for selecting either a writing period or a display period in each of the plural sub-frame periods, and means for setting the length of the display period.
The pixel portion
101
is structured generally as shown in FIG.
5
. In
FIG. 5
, the pixel portion
101
is provided with gate signal lines (G
1
to Gy) to which a selecting signal is inputted and source signal lines (also called data signal lines) (S
1
to Sx) to which a digital data signal is inputted. The digital data signal refers to a digital video signal.
The pixel portion also has power supply lines (V
1
to Vx) parallel to the source signal lines (S
1
to Sx). The electric potential of the power supply lines (V
1
to Vx) is called a power supply electric potential. Wirings (Vb
1
to Vby) are provided in parallel with the gate signal lines (G
1
to Gy). The wirings (Vb
1
to Vby) are connected to the external switch
116
.
A plurality of pixels
104
are arranged in matrix in the pixel portion
101
. One of the pixels
104
is enlarged and shown in FIG.
6
. In
FIG. 6
, reference symbol
1701
denotes a TFT functioning as a switching element (hereinafter referred to as switching TFT).
1702
denotes a TFT functioning as an element for controlling a current supplied to an EL element
1703
(current controlling element) (The TFT will be called a driving TFT). Designated by
1704
is a capacitor storage.
The switching TFT
1701
has a gate electrode connected to a gate signal line
1705
that is one of the gate signal lines (G
1
to Gy) to which a gate signal is inputted. The switching TFT
1701
has a source region and a drain region one of which is connected to a source signal line
1706
and the other of which is connected to a gate electrode of the driving TFT
1702
and to the capacitor storage
1704
. The source signal line
1706
is one of the source signal lines (S
1
to Sx) to which a digital data signal is inputted.
The driving TFT
1702
has a source region and a drain region one of which is connected to a power supply line
1707
and the other of which is connected to the EL element
1703
. The power supply line
1707
is one of the power supply lines (V
1
to Vx). The capacitor storage
1704
is connected to the power supply line
1707
that is one of the power supply lines (V
1
to Vx).
The EL element
1703
is composed of an anode, a cathode, and an EL layer interposed between the anode and the cathode. When the anode is connected to the source region or the drain region of the driving TFT
1702
, the anode serves as a pixel electrode whereas the cathode serves as an opposite electrode. On the other hand, when the cathode is connected to the source region or the drain region of the driving TFT
1702
, the cathode serves as the pixel electrode whereas the anode serves as the opposite electrode. The electric potential of the opposite electrode is herein called an opposite electric potential. The difference in electric potential between the opposite electrode and the pixel electrode generates the EL driving voltage, which is applied to the EL layer.
The opposite electrode of the EL element
1703
is connected to the external switch
116
through one of the wirings (Vb
1
to Vby). (See
FIG. 5.
)
Next, driving the multi-gray scale EL display device in accordance with the time
Hosoki Kazue
Koyama Jun
Yamazaki Shunpei
Fish & Richardson P.C.
Lee Wilson
Semiconductor Energy Laboratory Co,. Ltd.
Wong Don
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