Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-09-18
2004-08-10
Lee, Wilson (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S207000
Reexamination Certificate
active
06774578
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self light emitting device, and more particularly, to an active matrix self light emitting device. Among such devices, in particular, the present invention relates to an active matrix self light emitting device using self light emitting elements, such as organic electroluminescence (EL) elements, in a pixel portion.
2. Description of the Related Art
The spread of self light emitting devices in which a semiconductor thin film is formed on an insulator such as a glass substrate, in particular to active matrix self light emitting devices using thin film transistors (hereafter referred to as TFTs), has been remarkable recently. Active matrix self light emitting devices have from several hundred thousand to several million TFTs formed in a matrix shape in a pixel portion, and display of an image is performed by controlling the electric charge of each of the pixels.
In addition, techniques relating to polysilicon TFTs used for simultaneously forming driver circuits using TFTs formed in the periphery of the pixel portion, in addition to pixel TFTs structuring the pixels, have been developed recently, and these contribute greatly to the miniaturization of devices, and also to reducing the electric power consumption of the devices. Self light emitting devices have thus become indispensable devices in display portions of mobile devices having a remarkably wide range of applications in recent years.
Further, self light emitting devices which apply self light emitting materials such as organic EL materials as flat displays in substitute for LCDs (liquid crystal displays) are under the spotlight, and are being enthusiastically researched.
A schematic diagram of a normal self light emitting device is shown in FIG.
15
A. The use of an organic EL element (hereafter referred to simply as an EL element) as one example of a self light emitting element is explained in this specification. A pixel portion
1504
is arranged in the center of an insulating substrate (such as glass, for example)
1501
. In addition to source signal lines and gate signal lines, electric current supply lines
1505
for supplying electric current to EL elements are arranged in the pixel portion
1504
. A source signal line driver circuit
1502
for controlling the source signal lines is arranged on the top side of the pixel portion
1504
, and gate signal line driver circuits
1503
are placed on the left and the right of the pixel portion
1504
in order to control the gate signal lines. Note that although the gate signal line driver circuits
1503
are arranged on both the left and right sides of the pixel portion in
FIG. 15A
, they may also both be placed on the same side. However, from the perspectives of drive efficiency and reliability, it is preferable to arrange the gate signal lines on both sides. Input of signals from the outside into the source signal line driver circuit
1502
and the gate signal line driver circuits
1503
is performed via a flexible printed circuit (FPC)
1506
.
An expanded view of a portion surrounded by a dotted line frame
1500
within
FIG. 15A
is shown in FIG.
15
B. The pixel portion has pixels arranged in a matrix shape, as shown in the figure. A portion additionally surrounded by a dotted line frame
1510
within
FIG. 15B
is one pixel, and the pixel has a source signal line
1511
, a gate signal line
1512
, an electric current supply line
1513
, a switching TFT
1514
, an EL driver TFT
1515
, a storage capacitor
1516
, and an EL element
1517
.
Operation of active matrix self light emitting devices is explained next while referring to the same FIG.
15
B. First, a voltage is applied to the gate electrode of the switching TFT
1514
when the gate signal line
1512
is selected, and the switching TFT
1514
is placed in a conductive state. The signal (voltage signal) of the source signal line
1511
is stored as an electric charge in the storage capacitor
1516
by doing so. A voltage V
GS
between a gate and a source of the EL driver TFT
1515
is determined by the electric charge accumulated in the storage capacitor
1516
, and an electric current corresponding to the voltage of the storage capacitor
1516
flows in the EL driver TFT
1515
and in the EL element
1517
. The EL element
1517
turns on as a result.
The brightness of the EL element
1517
, equal to the amount of electric current flowing in the EL element
1517
, can be controlled in accordance with V
GS
of the EL driver TFT
1515
. V
GS
is the voltage of the storage capacitor
1516
, and that is the signal (voltage) input to the source signal line
1511
. In other words, the brightness of the EL element
1517
is controlled by controlling the signal (voltage) input to the source signal line
1511
. Finally, the gate signal line
1512
is placed in an unselected state, the gate of the switching TFT
1514
is closed, and the switching TFT
1514
is placed in an unselected state. The electric charge which has accumulated in the storage capacitor
1516
is maintained at this point. V
GS
of the EL driver TFT
1515
is therefore maintained as is, and the amount of electric current corresponding to V
GS
continues to flow in the EL element
1517
via the EL driver TFT
1515
.
Information regarding EL element drive is reported upon in papers such as the following: Current Status and Future of Light Emitting Polymer Display Driven by Poly-Si TFT, SID99 Digest, p. 372; High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver, ASIA DISPLAY 98, p. 217; and 3.8 Green OLED with Low Temperature Poly-Si TFT, Euro Display 99 Late News, p. 27.
A method of gray scale display in the EL element
1517
is discussed next. An analog gray scale method for controlling the brightness of the EL elements
1517
by the voltage V
GS
between the gate and the source of the EL driver TFT
1515
has a disadvantage in that it is weak with respect to dispersion in the electric current characteristics of the EL driver TFTs
1515
. That is, if the electric current characteristics of the EL driver TFTs
1515
differ, then the value of the electric current flowing in the EL driver TFTs
1515
and the EL elements
1517
changes even if the same gate voltages are applied. As a result, the brightnesses of the EL elements
1517
, namely the gray scales, also change.
A method referred to as a digital gray scale method has therefore been proposed in order to reduce the influence of dispersion in the characteristics of the EL driver TFTs
1515
and obtain a uniform screen picture. This method is a method for controlling the gray scale by two states, a state in which the absolute value |V
GS
| between a gate and a source of the EL driver TFT
1515
is below the turn on start voltage (in which almost no electric current flows), and a state in which the absolute value |V
GS
| is greater than the brightness saturation voltage (in which an electric current close to the maximum flows). In this case, the value of the electric current becomes close to I
MAX
even if there are dispersions in the electric current characteristics of the EL driver TFTs
1515
, provided that the absolute values |V
GS
| of the EL driver TFTs
1515
are sufficiently larger than the brightness saturation voltage. The influence of EL driver TFT dispersions can therefore be made extremely small. The gray scales are thus controlled by two states, an ON state (bright state due to maximum electric current flow) and an OFF state (dark state due to no electric current flow). This method is therefore referred to as a digital gray scale method.
However, only two gray scales can be displayed with the digital gray scale method. A plurality of techniques which can achieve multiple gray scales, in which another method is combined with the digital gray scale method, have been proposed.
A time gray scale method is one method which can be used to achieve multiple gray scales. The time gray scale metho
A Minh Dieu
Fish & Richardson PC
Lee Wilson
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