Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-05-18
2003-04-01
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C438S029000, C345S092000
Reexamination Certificate
active
06542137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display apparatus in which a current light-emitting element is driven with a thin-film transistor (current-drive thin-film transistor display apparatus).
2. Description of the Related Art
A number of thin-film transistor display apparatuses of various types for realizing lightness in weight, smallness in size, high image qualities and high resolution have been used. Thin-film transistor display apparatuses hitherto developed, as represented by thin-film transistor liquid crystal displays, are mainly for transmission of signal voltages or transfer of minute charges. However, it is anticipated that an element capable of current driving and having a memory function will become indispensable to self-light-emitting type panels, such as EL (electroluminescence) displays, heat-developing panels and the like which are expected to be developed in future.
FIGS.
10
(
a
) and
10
(
b
) are an equivalent circuit diagram and a potential relationship diagram, respectively, of a current-drive thin film transistor display apparatus, in which an organic fluorescent material is used as a light emitting material.
In FIG.
10
(
a
), symbol
121
represents a scanning line; symbol
122
, a signal line, symbol
123
, a common current supply line; symbol
131
, a switching thin-film transistor; symbol
132
, a current thin-film transistor; symbol
151
, a holding capacitor; symbol
152
, a pixel electrode; symbol
164
, an organic fluorescent material; and symbol
165
, an opposed electrode. In FIG.
10
(
b
), line
421
represents a scanning potential; line
422
, a signal potential; line
423
, a common potential; line
451
, a held potential; line
452
, a pixel potential; and line
465
, a counter potential.
The switching thin-film transistor
131
is a transistor for controlling conduction between the signal line
122
and holding capacitor
151
by a potential on the scanning line
122
. That is, signal potential
422
is transmitted to held potential
451
by scanning potential
421
. With respect to a displaying pixel, signal potential
422
becomes high and held potential
451
becomes high. With respect to a non-displaying pixel, signal potential
422
becomes low and held potential
451
becomes low.
On the other hand, the current thin-film transistor
132
is a transistor for controlling conduction between the common current supply line
123
and the pixel electrode
152
by the potential on the holding capacitor
151
. That is, common potential
423
is transmitted to pixel potential
452
by held potential
451
. With respect to a displaying pixel, conduction is effected between the common current supply line
123
and the pixel electrode
152
. With respect to a non-displaying pixel, the common current supply line
123
and the pixel electrode
152
are shut off from each other.
Consequently, a current is caused to flow between the pixel electrode
152
and the opposed electrode
165
with respect to a displaying pixel, thereby causing the organic fluorescent material
164
to emit light. With respect to a non-displaying pixel, no current flows and emission of light is not caused.
Thus, the current-drive thin film transistor display apparatus has the switching thin film transistor
131
and the current thin-film transistor
132
, each of which is a field effect type transistor manufactured by an ordinary semiconductor manufacturing process. As such, for the two thin-film transistors for conventional current-drive thin film transistor display apparatuses, thin-film transistors of the same structure have been used because the manufacturing cost can be reduced if the two transistors are made in accordance with the same specifications.
Actually, even if the structures of the two thin-film transistors are the same, the current-drive thin-film transistor display apparatus has no serious defect relating to it. However, if a high-quality product is aimed at, it is preferable to construct the above-described two thin-film transistors in such a manner that importance is attached to making the characteristics of the two transistors different from each other.
That is, with respect to switching thin-film transistor
131
, a reduction in off-current is needed for the purpose of enabling the charge to be retained more reliably in the holding capacitor
151
. In contrast, with respect to current thin-film transistor
132
, an increase in on-current is needed for the purpose of increasing the luminance of emission of light from the organic fluorescent material
164
.
However, no technical idea of positively making the characteristics of the above-described two thin-film transistors different from each other in a current-drive thin-film transistor display apparatus has been conceived.
SUMMARY OF THE INVENTION
The present invention has been achieved based on such a knowledge, and an object of the present invention is to provide a current-drive thin-film transistor display apparatus in which a reduction in the off-current of switching thin-film transistor
131
and an increase in the on-current of current thin-film transistor
132
are achieved simultaneously.
To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, the switching thin-film transistor is designed in such a manner that importance is attached to reducing the off-current, and the current thin-film transistor is designed in such a manner that importance is attached to increasing the on-current.
According to another aspect of the invention, each of the switching thin-film transistor and the current thin-film transistor is constructed according to required performance of the transistor in such a manner that importance is attached to reducing the off-current or increasing the on-current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably.
To achieve the above-described object, according to another aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a low concentration impurity region is formed between a channel region and a high concentration impurity region of the switching thin-film transistor, and a channel region and a high concentration impurity region of the current thin-film transistor are directly connected to each other.
That is, in the invention set forth in claim 2, the switching thin-film transistor is formed as a transistor of Lightly Doped Drain (LDD) structure while the current thin-film transistor is formed as a transistor of self-alignment structure.
According to another aspect of the invention, the switching thin-film transistor is designed so as to reduce the off-current while the current thin-film transistor is designed so as to increase the on-current, so
Kiguchi Hiroshi
Kimura Mutsumi
Hjerpe Richard
Nguyen Kevin M.
Oliff & Berridg,e PLC
Seiko Epson Corporation
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