Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-05-29
2004-02-03
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C345S076000
Reexamination Certificate
active
06686699
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2001-161890 filed May 30, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix type display apparatus having an active device in each pixel and controlling display in the pixel unit by means of the active device, and a driving method thereof, and particularly to an active matrix type display apparatus using an electrooptic device that varies brightness according to a current flowing therein, an active matrix type organic EL display apparatus using an organic-material electroluminescence (hereinafter described as organic EL (electroluminescence)) device as the electrooptic device, and driving methods thereof.
A liquid crystal display using a liquid crystal cell as a display device of a pixel, for example, has a large number of pixels arranged in a matrix manner, and controls light intensity in each pixel according to information of an image to be displayed, thereby effecting driving for image display. The same display driving is effected by an organic EL display using an organic EL device as a display device of a pixel and the like.
Since the organic EL display is a so-called self-luminous type display using a light emitting device as a display device of a pixel, however, the organic EL display has advantages such as higher visibility of images, no need for a backlight, and a higher response speed as compared with the liquid crystal display. Moreover, brightness of each light emitting device is controlled by the value of a current flowing therein. That is, the organic EL display differs greatly from the liquid crystal display or the like of a voltage-controlled type, in that the organic EL device is of a current-controlled type.
As with the liquid crystal display, the organic EL display uses a passive matrix method and an active matrix method as its driving method. Although the former has a simple construction, however, the former has problems such as difficulty in realizing a large high-definition display. Thus, the active matrix method has recently been actively developed which controls a current flowing through a light emitting device within a pixel by means of an active device, for example an insulated gate field-effect transistor (typically a thin film transistor; TFT) also disposed within the pixel.
FIG. 1
shows a conventional example of a pixel circuit (circuit of a unit pixel) in an active matrix type organic EL display (for more detailed description, see U.S. Pat. No. 5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683).
As is clear from
FIG. 1
, the pixel circuit according to the conventional example includes: an organic EL device
101
having an anode connected to a positive power supply Vdd; a TFT
102
having a drain connected to a cathode of the organic EL device
101
and a source connected to a ground (hereinafter described as “grounded”); a capacitor
103
connected between a gate of the TFT
102
and the ground; and a TFT
104
having a drain connected to the gate of the TFT
102
, a source connected to a data line
106
, and a gate connected to a scanning line
105
.
Since the organic EL device has a rectifying property in many cases, the organic EL device may be referred to as an OLED (Organic Light Emitting Diode). Therefore, in FIG.
1
and other figures, a symbol of a diode is used to denote the organic EL device as the OLED. In the following description, however, a rectifying property is not necessarily required of the OLED.
The operation of the thus formed pixel circuit is as follows. First, when potential of the scanning line
105
is brought to a selected state (high level in this case) and a writing potential Vw is applied to the data line
106
, the TFT
104
conducts, the capacitor
103
is charged or discharged, and thus a gate potential of the TFT
102
becomes the writing potential Vw. Next, when the potential of the scanning line
105
is brought to a non-selected state (low level in this case), the TFT
102
is electrically disconnected from the scanning line
105
, while the gate potential of the TFT
102
is stably retained by the capacitor
103
.
A current flowing through the TFT
102
and the OLED
101
assumes a value corresponding to a gate-to-source voltage Vgs of the TFT
102
, and the OLED
101
continues emitting light at a brightness corresponding to the value of the current. The operation of selecting the scanning line
105
and transmitting to the inside of the pixel brightness data supplied to the data line
106
will hereinafter be referred to as “writing.” As described above, once the pixel circuit shown in
FIG. 1
writes the potential Vw, the OLED
101
continues emitting light at a fixed brightness until next writing.
An active matrix type display apparatus (organic EL display) can be formed by arranging a large number of such pixel circuits (which may hereinafter be described simply as pixels)
111
in a matrix manner as shown in
FIG. 2
, and repeating writing from a voltage driving type data line driving circuit (voltage driver)
114
through data lines
115
-
1
to
115
-
m
while selecting scanning lines
112
-
1
to
112
-
n
sequentially by a scanning line driving circuit
113
. A pixel arrangement of m columns and n rows is shown in this case. Of course, in this case, the number of data lines is m and the number of scanning lines is n.
Each light emitting device in a passive matrix type display apparatus emits light only at an instant when the light emitting device is selected, whereas a light emitting device in an active matrix type display apparatus continues emitting light even after completion of writing. Thus, the active matrix type display apparatus is advantageous especially for use as a large high-definition display in that the active matrix type display apparatus can decrease peak brightness and peak current of the light emitting device as compared with the passive matrix type display apparatus.
In an active matrix type organic EL display, a TFT (thin film field-effect transistor) formed on a glass substrate is generally used as an active device. It is known, however, that amorphous silicon and polysilicon used to form the TFT have inferior crystallinity and inferior controllability of the conducting mechanism to single-crystal silicon, and thus the formed TFT has great variations in characteristics.
When a polysilicon TFT is formed on a relatively large glass substrate, in particular, the polysilicon TFT is generally crystallized by a laser annealing method after formation of an amorphous silicon film, in order to avoid problems such as thermal deformation of the glass substrate. However, it is difficult to irradiate the large glass substrate with uniform laser energy, and thus the crystallized state of the polysilicon is varied depending on a location within the substrate. As a result, the threshold value Vth of even TFTs formed on the same substrate can be varied from pixel to pixel by a few hundred mV, or 1 V or more in some cases.
In that case, even when the same potential Vw is written to different pixels, for example, the threshold value Vth of the TFTs varies from pixel to pixel. This results in great variation from pixel to pixel in the current Ids flowing through the OLED (organic EL device), and hence deviation of the current Ids from a desired value. Therefore high picture quality cannot be expected of the display. This is true for not only variation in the threshold value Vth but also variation in carrier mobility &mgr; and the like.
In order to remedy such a problem, the present inventor has proposed a current writing type pixel circuit shown in
FIG. 3
as an example (see International Publication Number WO01/06484).
As is clear from
FIG. 3
, the current writing type pixel circuit includes: an OLED
121
having an anode connected to a positive power supply Vdd; an N-channel TFT
122
having a drain connected to a cathode of the OLED
121
and a source grounded; a c
A Minh D
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
Wong Don
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