Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-07-22
2004-02-17
Nguyen, Hoang (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S076000, C345S082000
Reexamination Certificate
active
06693388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display in which each pixel is provided with a light emitting element, the luminance of which is controlled by a current such as an organic electroluminescent (EL) element. More specifically, the present invention relates to an active matrix display for supplying a current to a light emitting element by an active element such as an insulated-gate field-effect transistor provided inside each pixel.
2. Related Background Art
In recent years, displays using an organic EL element have been developed. As a method of driving the element, there are a simple matrix system and an active matrix system. Since the former is simple in its structure but has difficulty in realizing a large and high definition display, many active matrix type displays have been developed.
If a large number of organic EL elements are used and driven by an active matrix circuit, an insulted-gate field-effect transistor, a so-called thin film transistor (hereinafter referred to as TFT), for controlling supply of a driving current for driving a light emitting element, is connected to each pixel. A light emitting operation of the organic EL element is controlled by controlling this TFT.
Background Example 1
FIG. 9
shows an equivalent circuit for one pixel disclosed in U.S. Pat. No. 5,684,365.
A pixel circuit provided in a pixel is constructed by an organic EL element OLED, a thin film transistor (TFT)
1
, a thin film transistor (TFT)
2
and a capacitor C. Since an organic EL element generally has a rectification characteristic, it is sometimes called an organic light emitting diode (OLED). In the figure, a symbol of a diode is used. However, a light emitting element is not always limited to the OLED, but also may be any light emitting element as long as its luminance is controlled by a current flowing to the element. In addition, the rectification characteristic is not always required. In
FIG. 9
, a source and a drain of the p-type TFT
2
are connected to a power supply potential Vdd and an anode of the organic EL element OLED, respectively, and a cathode of the organic EL element OLED is connected to a ground potential. On the other hand, a gate, a source and a drain of the p-type TFT
1
are connected to a scanning line Scan, a data line Data, and one end of the capacitor C, and a gate of the TFT
2
, respectively, and the other end of the capacitor C is connected to the power supply potential Vdd.
First, when the TFT
1
is turned ON by the scanning line Scan to apply a data potential Vw representing luminance information to the data line Data in order to operate the pixel, the capacitor C is charged or discharged, whereby a gate potential of the TFT
2
becomes equal to the data potential Vw. When the TFT
1
is turned OFF by the scanning line Scan, the gate potential of the TFT
2
is held by the capacitor C, and a driving current corresponding to a gate to source voltage Vgs of the TFT
2
is supplied to the organic EL element OLED. Thus, the organic EL element OLED continues to emit light at a luminance corresponding to an amount of the current.
Background Example 2
FIG. 10
shows an equivalent circuit for one pixel disclosed in JP 2001-56667 A.
A pixel circuit provided in a pixel is constructed by an organic EL element OLED, a TFT
1
for converting a signal current to a voltage or supplying a current to the organic EL element OLED, a TFT
2
for controlling an operating state of the TFT
1
, a TFT
3
and a TFT
4
for selecting a state in which a signal current is taken in or a state in which a driving current is supplied to the organic EL element OLED, and a capacitor C for holding a voltage.
In
FIG. 10
, a source and a gate of the TFT
1
are connected to a power supply potential Vdd, and a source of the TFT
2
and one end of the capacitor C, respectively. The other end of the capacitor C is connected to the power supply potential Vdd. A drain of the TFT
1
is connected to a drain of the TFT
2
, a drain of the TFT
3
and a drain of the TFT
4
. A source of the TFT
4
is connected to an anode of the organic EL element OLED, and a cathode of the organic EL element OLED is connected to a ground potential. A source of the TFT
3
is connected to a data signal line Data, and all gates of the TFT
2
, TFT
3
and TFT
4
are connected to a scanning line Scan.
First, when the TFT
2
and the TFT
3
are turned ON and the TFT
4
is turned OFF by the scanning line Scan in order to operate the pixel, a signal current Iw is taken in the TFT
1
, a gate to source voltage Vgs required for flowing the signal current Iw is generated in the TFT
1
, and the voltage Vgs is held in the capacitor C. When the TFT
2
and the TFT
3
are turned OFF and the TFT
4
is turned ON by the scanning line Scan, the TFT
1
continues to flow a driving current to the organic EL element OLED based on the voltage held in the capacitor C. Thus, the organic EL element OLED continues to emit light at a luminance corresponding to an amount of the current.
Background Example 3
FIG. 11
shows an equivalent circuit for one pixel disclosed in JP 2001-147659 A (EP A2 1102234).
A pixel circuit provided in a pixel is constructed by a TFT
1
for converting a signal current to a voltage, a TFT
2
for controlling a driving current flowing to a light emitting element, a TFT
3
for taking in a current which connects or disconnects the pixel circuit and a data line by a scanning line ScanA, a transistor for switching TFT
4
that shorts between a gate and a drain of the TFT
1
while luminance information is written by a scanning line ScanB, a capacitor C for holding a gate to a source voltage of the TFT
1
even after the luminance information is written, and an organic EL element OLED.
In
FIG. 11
, sources of the TFT
1
and the TFT
2
are connected to a power supply potential Vdd, and a gate of the TFT
1
is connected to a gate of the TFT
2
, one end of the capacitor C and a drain of the TFT
4
. The other end of the capacitor C is connected to the power supply potential Vdd. A drain of the TFT
2
is connected to an anode of an organic EL element OLED, and a cathode of the organic EL element OLED is connected to a ground potential. A drain of the TFT
1
is connected to a source of the TFT
4
and a drain of the TFT
3
. A source of the TFT
3
is connected to a data signal line Data. A gate of the TFT
3
is connected to a scanning line ScanA, and a gate of the TFT
4
is connected to a scanning line ScanB.
First, when the TFT
3
and the TFT
4
are turned ON by the scanning lines ScanA and ScanB in order to operate the pixel, the TFT
1
and the TFT
2
come to have a current mirror structure. A signal current Iw is taken in the TFT
1
, the TFT
2
flows a current to the organic EL element OLED in accordance with a current mirror ratio, and a voltage generated in the gate of the TFT
1
is held in the capacitor C. When the TFT
3
and the TFT
4
are turned OFF by the scanning lines ScanA and ScanB, the current mirror structure of the TFT
1
and the TFT
2
is released. The TFT
2
continues flowing a current to the organic EL element OLED in accordance with the voltage held in the capacitor C. The light emitting element continues to emit light at a luminance corresponding to an amount of the current.
In an active matrix display, thin film transistors functioning as active elements are generally formed on a single glass substrate simultaneously using amorphous silicon or polysilicon. However, the TFTs that are formed using amorphous silicon or polysilicon are known to have large variation of their characteristics, because the TFTs have worse crystallinity and worse controllability of a transmission mechanism compared with monocrystal (single crystal) silicon.
Therefore, it is not rare that, even in the TFTs formed on the same substrate, their threshold voltages Vth vary by several hundred mV or, in some cases, 1V or more for each pixel. In this case, for example, since the Vth varies depending on a pixel even if the same signal potenti
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hoang
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