Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-09-20
2004-08-17
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S076000, C345S092000
Reexamination Certificate
active
06777888
ABSTRACT:
This application is a continuation of International Application No. PCT/JP02/02470, filed Mar. 15, 2002, which claims the benefit of Japanese Patent Application No. 080505/2001, filed Mar. 21, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a drive circuit to be used in an active matrix type light-emitting element array for driving and controlling an array of emission type elements such as organic and inorganic electroluminescent (to be referred to as “EL” hereinafter”) elements or light-emitting diodes (to be referred to as “LED” hereinafter) and also to an active matrix type display panel realized by using such a drive circuit.
2. Related Background Art
Display devices adapted to display characters and images by means of a dot matrix formed by arranging light-emitting elements such as organic or inorganic EL elements or LEDs are currently popularly being used in television sets, mobile terminals and other applications.
Particularly, display devices comprising emission type elements are attracting attention because, unlike display devices utilizing liquid crystal, they have a number of advantages including that they do not require a backlight for illumination and provide a wide view angle. Above all, display devices referred to as active matrix type devices that are realized by combining transistors and light-emitting elements and adapted to be operated in a drive mode referred to as static drive have been drawing attention because they provide remarkable advantages including high brightness, high contrast and high definition if compared with display devices that operate on a time division drive basis in a simple matrix drove mode.
FIG. 8
of the accompanying drawings is quoted from Preliminary Papers “Eurodisplay ‘90” for Autumn Convention 1990, pp. 216-219, published by Society for Information Display. It illustrates a known display circuit of the type under consideration. More specifically, it shows a light-emitting element drive circuit of an active matrix type display device comprising EL elements as light-emitting elements.
As seen from
FIG. 8
, when the scan line
36
that is connected to the gate of transistor
35
of the drive circuit is selected and activated, the transistor
35
becomes ON and a signal is written in capacitor
38
from the data line
37
connected to the transistor
35
. The capacitor
38
determines the voltage between the gate and the source of transistor
41
. When the scan line
36
is no longer selected and the transistor
35
becomes OFF, the voltage between the opposite ends of the capacitor
38
is held unchanged until the scan line
36
is selected in the next cycle and the transistor
41
is held ON during that period.
As the transistor
41
becomes ON, an electric current flows from power supply electrode
39
to common electrode
42
by way of EL element
40
and the drain/source of the transistor
41
to drive the organic EL element
40
to emit light.
Generally speaking, for the display terminal of a computer, the monitor screen of a personal computer or the display screen of a television set to display a moving image, it is desirable that each pixel can change the brightness so as to display gradation. As far as organic EL elements are concerned, known systems that have hitherto been used to provide displayed images with gradation include the analog gradation system, the area gradation system and the time gradation system.
The analog gradation system is designed to control the brightness of emitted light of an organic EL element as a function of the quantity of the electric current flowing through the organic EL element. If a thin film transistor (to be referred to as “TFT” hereinafter) is used as switching element for supplying the electric current, a control signal is applied as gate voltage according to a video signal so as to control the conductance of the switching element by using a rising region (to be referred to as “saturated region” here for the sake of convenience) of the source current characteristic (Vg-Is characteristic) relative to the gate voltage.
Then, it is necessary to make the gamma (&ggr;) characteristic of the video signal change according to the brightness—voltage characteristic of the organic EL element.
Currently available TFTs include those of the amorphous silicon (a-Si) type and those of the polysilicon (polycrystalline silicon) type (p-Si), of which polycrystalline silicon TFTs are in the mainstream because they show a high mobility and can be downsized in addition to that the process of manufacturing polycrystalline silicon TFTs can be conducted at low temperature due to the recent advancement of laser processing technology. However, generally, polycrystalline silicon TFTs are apt to be affected by the crystal grain boundaries thereof and their electric characteristics can vary remarkably particularly in the saturated region. In other words, even if a uniform video signal voltage is applied to the pixels of the display device, an uneven image can be displayed.
Furthermore, most TFTs are currently being used as switching elements. More specifically, they are adapted to be used in a linearly operating region where the drain current changes proportionally relative to the source voltage when a gate voltage that is considerably higher than the threshold voltage of the transistor is applied so that they are not significantly affected by the varying electric characteristics in the saturated region. However, if polysilicon TFTs are operated in the saturated region in order to adopt the analog gradation system, the display performance of the display device can become unstable as the operation of the TFTs are affected by the varying electric characteristics.
When, for instance, the organic EL element
40
is driven by the TFT circuit to display analog gradation in
FIG. 8
, the voltage applied between the gate and the source of the transistor
41
is slightly higher than the threshold voltage (Vth) of the transistor.
FIG. 9
is a graph illustrating the Vg-Is characteristics of different transistors. The transistors are adapted to utilize the part of the characteristic curve where the source current rises as the gate voltage increases (or the saturated region). However, if the gate voltage—source current characteristic (Vg-Is characteristic) varies as shown in
FIG. 9
(or the threshold voltage Vth of the transistor varies), the electric current that flows through the transistor
41
can also vary as indicated by IA (intersection of the curve of a solid line and VA) and IB (intersection of the curve of a broken line and VA) even if a constant gate voltage VA is applied to the gate electrode of the transistor
41
in FIG.
8
. Additionally, the brightness of light emitted when a constant voltage is applied may vary depending on the manufacturing process that can involve problems such as film thickness distribution of an organic layer. Such variances are particularly significant when brightness is related to providing gradation. Referring to
FIG. 8
again, the part surrounded by dotted lines
43
indicates a region that is apt to produce such variances. Then, organic EL elements
40
that are supposed to show a same level of brightness when a same voltage is applied can actually show different levels of brightness. Such variances in brightness can degrade the quality of the displayed image.
On the other hand, the area gradation system is proposed in AM-LCD2000, AM3-1. It is a system of dividing each pixel into a plurality of sub-pixels so that each sub-pixel can be turned ON and OFF and gradation may be defined by the total area of the pixels that are ON.
With this mode of utilizing organic EL elements, TFTs are used as switching elements so that a gate voltage that is much higher than the threshold voltage is applied to exploit a region of the characteristic curve where the drain voltage is proportional to the source voltage (or the linear region) in order to avoid variances in the TFT characteristic and stabilize the light-emitting characteristic. However, this gradation m
Alemu Ephrem
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Wong Don
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