Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-06-21
2004-08-10
Clinger, James (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169100, C345S076000
Reexamination Certificate
active
06774574
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device obtained by fabricating semiconductor elements (elements formed by using a thin semiconductor film) on a substrate and to an electronic device having the EL display device as a display unit.
2. Prior Art
Technology has been greatly advanced in recent years for forming TFTs on a substrate, and attempts have been made to develop an active matrix-type display device. In particular, TFTs using a polysilicon film exhibit a higher electric-field mobility (also called mobility) than that of the conventional TFTs using an amorphous silicon film, and make it possible to accomplish a high-speed operation. This makes it possible to control the pixels, which has heretofore been done by a drive circuit outside the substrate, by using a drive circuit formed on the same substrate as the pixels.
The active matrix-type display device is drawing attention owing it its advantages such as a decrease in the cost of production, a decrease in the size of the display device, an increase in the yield and a decrease in the throughput, as a result of fabricating various circuits and elements on the same substrate.
The active matrix-type EL display devices have heretofore been employing pixels of a structure that is generally as shown in FIG.
3
. In
FIG. 3
, reference numeral
301
denotes a TFT (hereinafter referred to as switching TFT) that works as a switching element,
302
denotes a TFT (hereinafter referred to as current control TFT) working as an element (current control element) for controlling the current supplied to an EL element
303
, and
304
denotes a capacitor (holding capacity). The switching TFT
301
is connected to a gate wiring
305
and to a source wiring (data wiring)
306
. The drain of the current control TFT
302
is connected to the EL element
303
, and the source thereof is connected to a current feeder line
307
.
When the gate wiring
305
is selected, the gate of the switching TFT
301
is opened, a data signal of the source wiring
306
is accumulated in the capacitor
304
, and the gate of the current control TFT
302
is opened. After the gate of the switching TFT
301
is closed, the gate of the current control TFT
302
remains opened due to the electric charge accumulated in the capacitor
304
and, during this period, the EL element
303
emits light. The amount of light emitted by the EL element
303
varies depending on the amount of electric current that flows.
Here, the amount of current supplied to the EL element
303
is controlled by a gate voltage of the current control TFT
302
as shown in FIG.
4
.
FIG.
4
(A) is a graph illustrating transistor characteristics of the current control TFT, wherein a curve
401
represents Id-Vg characteristics (or an Id-Vg curve),Id represents a drain current and Vg represents a gate voltage. From this graph, it is possible to know the amount of current that flows relative to any gate voltage.
Usually, the EL element is driven by utilizing the Id-Vg characteristics over a region surrounded by a dotted line
402
. FIG.
4
(B) is a view illustrating the region surrounded by
402
on an enlarged scale.
In FIG.
4
(B), the hatched region is called sub-threshold region. In practice, this region has a gate voltage close to or lower than the threshold voltage (Vth) and where the drain current exponentially varies depending upon a change in the gate voltage. In this region, the current is controlled based on the gate voltage.
The data signal input to the pixel as the switching TFT
301
is opened is, first, accumulated in the capacitor
304
and directly serves as a gate voltage for the current control TFT
302
. Here, the drain current is determined for the gate voltage at a ratio of 1 to 1 in compliance with the Id-Vg characteristics shown in FIG.
4
(A). That is, a predetermined current flows through the EL element
303
depending on the data signal, and the EL element
303
emits light in an amount corresponding to the amount of current.
Thus, the amount of light emitted by the EL element is controlled by the data signal, and the gradation display is accomplished by controlling the amount of light that is emitted. This system is a so-called analog gradation; i.e., the gradation display is accomplished relying on a change in the amplitude of the signal.
However, the analog gradation system has a defect that it is very susceptible to dispersion in the characteristics of the TFTs. For example, considered below is a case where a switching TFT that would exhibit the same gradation has ID-Vg characteristics different from those of a switching TFT of the neighboring pixel (generally shifted toward the positive side or the negative side).
In this case, the drain currents flowing into the switching TFTs vary depending upon the degree of dispersion, and different gate voltages are applied to the current control TFTs of the pixels. That is different currents flow into the EL elements and, as a result, light is emitted in different amounts making it no longer possible to accomplish the same gradation display.
Further, even when the same gate voltage is applied to the current control TFTs of the pixels, the same drain current cannot be produced if there is a dispersion in the Id-Vg characteristics of the current control TFTs. As will be obvious from FIG.
4
(A), further, since use is made of the region where the drain current exponentially changes relative to the change in the gate voltage, even a slight difference in the Id-Vg characteristics results in a large change in the amount of current despite an equal gate voltage is applied. Then, the amount of light emitted by the EL elements greatly varies among the neighboring pixels.
In practice, the situation becomes more severe due to synergistic effect of dispersion of both the switching TFTs and the current control TFTs. Thus, the analog gradation system is very susceptible to the dispersion in the characteristics of the TFTs, hindering the attempt for realizing the multi-color active matrix EL display device.
SUMMARY OF THE INVENTION
The present invention was accomplished in view of the above-mentioned problems, and provides an active matrix-type EL display device capable of producing a vivid multi-gradation color display. The invention further provides an electronic device of high performance using the active matrix-type EL display device as a display unit.
The present applicant has discovered the fact that the problems of the analog gradation system stem from the dispersion in the characteristics of the current control TFTs that control the current flowing into the EL elements and from the dispersion in the on-resistance of the current control TFTs. Here, the on-resistance is a value obtained by dividing the drain voltage of the TFT by the drain current flowing at that moment.
That is, the on-resistance varies among the current control TFTs and, hence, different currents (drain currents) flow even under the same condition, making it difficult to obtain a desired gradation.
According to this invention, therefore, a resistor (R) is connected in series between the drain of the current control TFT and the EL element to control the amount of current supplied from the current control TFT to the EL element. For this purpose, it is necessary to provide a resistor having a resistance very larger than the on-resistance of the current control TFT. The resistance may be selected over a range of from 1 k&OHgr; to 50 M&OHgr; (preferably, from 10 k&OHgr; to 10 M&OHgr; and, more preferably, from 50 k&OHgr; to 1 M&OHgr;).
In carrying out the invention, further, the amount of current flowing into the EL element is determined by the resistance of the resistor (R), and the supplied current becomes constant at all times. That is, the invention does not use the analog gradation system that produces the gradation display by controlling the current value that is done by the prior art. The invention therefore uses the gradation display of the time-division system (hereinafter refe
Clinger James
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Tran Chuc
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