Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-11-17
2004-01-20
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S503000
Reexamination Certificate
active
06680577
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film) made on a substrate, and to an electronic apparatus having the EL display device as a display (display portion).
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been greatly advancing in recent years, and development of applications to an active matrix type display device have been progressing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT which uses a conventional amorphous silicon film, and high speed operation is therefore possible.
Shown in
FIG. 3
is a general pixel structure of an active matrix type EL display device. Reference numeral
301
in
FIG. 3
denotes a TFT which functions as a switching element (hereafter referred to as a switching TFT), reference numeral
302
denotes a TFT which functions as an element (hereafter referred to as an electric current control element) for controlling electric current provided to an EL element
303
, and
304
denotes a capacitor (storage capacitor). The switching TFT
301
is connected to a gate wiring
305
and to a source wiring (data wiring)
306
. A drain of the electric current control TFT
302
is connected to the EL element
303
, and a source of the electric current control TFT
302
is connected to an electric current supply wiring
307
.
A gate of the switching TFT
301
opens when the gate wiring
305
is selected, a data signal of the source wiring
306
is stored in the capacitor
304
, and a gate of the electric current control TFT
302
opens. After the gate of the switching TFT
301
closes, the gate of the electric current control TFT
302
remains open in accordance with the electric charges stored in the capacitor
304
, and the EL element
303
emits light during that period. The amount of light emitted by the EL element
303
is changed by the amount of electric current.
In other words, the amount of electric current flowing in the electric current control TFT
302
is controlled by the data signal input from the source wiring
306
in an analog drive gradation display, and the amount of light emitted by the EL element thereby changes.
FIG. 4A
is a graph showing the transistor characteristics of the electric current control TFT
302
, and reference numeral
401
denotes an Id-Vg characteristic (also referred to as an Id-Vg curve). Id is a drain current, and Vg is a gate voltage here. The amount of electric current flowing with respect to an arbitrary gate voltage can be found with this graph.
A region of the Id-Vg characteristic shown by a dotted line
402
is normally used in driving the EL elements. An enlargement of the region enclosed by the region
402
is shown in FIG.
4
B.
The shaded region in
FIG. 4B
is referred to as a subthreshold region. In practice, this indicates a region having a gate voltage in the neighborhood of the threshold voltage (Vth) or below, and the drain current changes exponentially with respect to changes in the gate voltage within this region. Electric current control is performed in accordance with the gate voltage by using this region.
The data signal input to the pixel when the switching TFT
301
in
FIG. 3
is open is first stored in the capacitor
304
, and then the signal becomes the gate voltage of the electric current control TFT
302
, as is. The drain current is determined at this time by a one to one correspondence with respect to the gate voltage, in accordance with the Id-Vg characteristic shown in FIG.
4
A. Namely, a predetermined electric current flows in the EL element
303
in correspondence with the data signal, and the EL element
303
emits light with the amount of light corresponding to the amount of current flow.
The amount of light emitted by the EL element is thus controlled by the input signal, and gradation display is performed by controlling the amount of light emitted. This method is referred to as analog gradation, and gradation display is performed by changing the amplitude of the signal.
However, the above analog gradation method has a disadvantage of being extremely weak with respect to dispersions in the TFT characteristics. For example, suppose that the Id-Vg characteristic is a switching TFT and differs from that of a switching TFT of an adjacent pixel displaying the same gradation (a case of an overall positive of negative shift).
In this case the drain current of each switching TFT differs on the order of the dispersion, and the gate voltages applied to the current control TFTs of each pixel therefore also differ. In other words, the electric current flowing differs for each of the EL elements, and as a result, the amount of light emitted also differs, and the same gradation display cannot be performed.
Further, even supposing that equal gate voltages are applied to the electric current control TFTs of each pixel, the same drain current cannot be output if there are variations in the Id-Vg characteristics of the electric current control TFTs. In addition, even if equal gate voltages are applied, the amount of electric current output differs greatly if even small deviations exist in the Id-Vg characteristics when using a region in which the drain current changes exponentially with respect to changes in the gate voltage, as is clear from FIG.
4
A. The amount of light emitted by adjacent pixels will differ greatly as a result.
In practice, there is a multiplier effect between dispersions in both the switching TFTs and the electric current control TFTs, and this makes achieving the conditions more difficult. Thus the analog gradation method is extremely sensitive with respect to variations in the TFT characteristics, and this becomes an obstacle to multiple colorization of a conventional active matrix EL display device.
SUMMARY OF THE INVENTION
In consideration of the above problems, an object of the present invention is to provide an active matrix type EL display device capable of sharp, multi-gradation color display. In addition, an object of the present invention is to provide a high performance electrical apparatus furnished as a display portion of this type of active matrix EL display device.
The applicant of the present invention considers that in order to make a pixel structure which is not readily influenced by dispersions in TFT characteristics, a digital driver gradation method, in which an electric current control TFT is used as a simple electric current supply switching element, is better than a conventional analog driver gradation method of controlling the amount of light emitted by an EL element in accordance with electric current control.
It is considered that a time division method of gradation display (hereafter referred to as time division gradation) will be performed by a digital driver in the active matrix type EL display device.
In addition, a panel display can be made higher speed by dividing video lines and inputting a plurality of data at one time when inputting a video signal into a source driver circuit. Note that the video signal referred to here is a data signal input into the source driver circuit throughout this specification.
FIGS. 5A
to
5
F show the overall driver timing of the write-in period and the display period when performing time division gradation display. A case of performing 64 gradation display in accordance with a 6 bit digital driver method is explained here. Note that the write-in period is the time required for a signal to be written into all pixels structuring one frame, and that the display period is the period in which the pixels perform display of the write-in signal.
An EL driver power supply is cut (all pixels turn off) during the write-in period, and the EL elements within the pixels are in a state of no applied voltage. Further, the EL driver power supply is input during the display period, placing the EL elements within the pixels in a state of having an appl
Inukai Kazutaka
Koyama Jun
A Minh D
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Wong Don
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