U.S. patent application number 17/704109 was filed with the patent office on 2022-07-07 for shift register and display device and driving method thereof.
The applicant listed for this patent is Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Jun KOYAMA.
Application Number | 20220216242 17/704109 |
Document ID | / |
Family ID | |
Filed Date | 2022-07-07 |
United States Patent
Application |
20220216242 |
Kind Code |
A1 |
KOYAMA; Jun |
July 7, 2022 |
SHIFT REGISTER AND DISPLAY DEVICE AND DRIVING METHOD THEREOF
Abstract
The power consumption of a shift register or a display device
including the shift register is reduced. A clock signal is supplied
to a shift register by a plurality of wirings, not by one wiring.
Any one of the plurality of wirings supplies a clock signal in only
part of the operation period of the shift register, not during the
whole operation period of the shift register. Therefore, the
capacity load caused with the supply of clock signals can be
reduced, leading to reduction in power consumption of the shift
register.
Inventors: |
KOYAMA; Jun; (Sagamihara,
JP) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Semiconductor Energy Laboratory Co., Ltd. |
Atsugi-shi |
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JP |
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Appl. No.: |
17/704109 |
Filed: |
March 25, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15806418 |
Nov 8, 2017 |
11296120 |
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17704109 |
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14010843 |
Aug 27, 2013 |
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15806418 |
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12897360 |
Oct 4, 2010 |
8526567 |
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14010843 |
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International
Class: |
H01L 27/12 20060101
H01L027/12; G09G 3/3266 20060101 G09G003/3266; G09G 3/36 20060101
G09G003/36; G11C 19/28 20060101 G11C019/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 9, 2009 |
JP |
2009-234845 |
Claims
1. (canceled)
2. A display device, comprising: a scanning line drive circuit
comprising a plural stage circuits, wherein one of the plural stage
circuits comprises first to sixth transistor, wherein one of a
source and a drain of the first transistor is electrically
connected to a first wiring, and another of the source and the
drain of the first transistor is electrically connected to a second
wiring, wherein one of a source and a drain of the second
transistor is electrically connected to the first wiring, and
another of the source and the drain of the second transistor is
electrically connected to a third wiring, wherein one of a source
and a drain of the third transistor is electrically connected to a
gate of the first transistor, and another of the source and the
drain of the third transistor is electrically connected to a fourth
wiring, and a gate of the third transistor is electrically
connected to a fifth wiring, wherein one of a source and the drain
of the fourth transistor is electrically connected to the gate of
the first transistor, and another of the source and the drain of
the fourth transistor is electrically connected to the third
wiring, and the gate of the fourth transistor is electrically
connected to a gate of the second transistor, wherein one of a
source and a drain of the fifth transistor is electrically
connected to the gate of the second transistor, wherein one of a
source and a drain of the sixth transistor is electrically
connected to the gate of the second transistor, another of the
source and the drain of the sixth transistor is electrically
connected to the third wiring, and a gate of the sixth transistor
is electrically connected to the fifth wiring, wherein the one of
the plural stage circuits is configured to output a scanning signal
to the first wiring, wherein each of the first to sixth transistors
is a bottom gate type transistor, wherein each of the first to
sixth transistors comprises a channel forming region in an oxide
semiconductor layer, a first silicon nitride layer below the oxide
semiconductor layer, an oxide insulating layer over the oxide
semiconductor layer, a second silicon nitride layer over the oxide
insulating layer, wherein, in a plan view, the oxide insulating
layer is in contact with the oxide semiconductor layer in a region
overlapping the channel forming region, and wherein, in the plan
view, the oxide semiconductor layer is surrounded by a region where
the first silicon nitride layer and the second silicon nitride
layer overlap, and wherein the second silicon nitride layer does
not have a region in contact with the oxide semiconductor
layer.
3. A display device, comprising: a scanning line drive circuit
comprising a plural stage circuits, wherein one of the plural stage
circuits comprises first to sixth transistor, wherein one of a
source and a drain of the first transistor is electrically
connected to a first wiring, and another of the source and the
drain of the first transistor is electrically connected to a second
wiring, wherein one of a source and a drain of the second
transistor is electrically connected to the first wiring, and
another of the source and the drain of the second transistor is
electrically connected to a third wiring, wherein one of a source
and a drain of the third transistor is electrically connected to a
gate of the first transistor, and another of the source and the
drain of the third transistor is electrically connected to a fourth
wiring, and a gate of the third transistor is electrically
connected to a fifth wiring, wherein one of a source and the drain
of the fourth transistor is electrically connected to the gate of
the first transistor, and another of the source and the drain of
the fourth transistor is electrically connected to the third
wiring, and the gate of the fourth transistor is electrically
connected to a gate of the second transistor, wherein one of a
source and a drain of the fifth transistor is electrically
connected to the gate of the second transistor, wherein one of a
source and a drain of the sixth transistor is electrically
connected to the gate of the second transistor, another of the
source and the drain of the sixth transistor is electrically
connected to the third wiring, and a gate of the sixth transistor
is electrically connected to the fifth wiring, wherein the one of
the plural stage circuits is configured to output a scanning signal
to the first wiring, wherein each of the first to sixth transistors
is a bottom gate type transistor, wherein each of the first to
sixth transistors comprises a channel forming region in an oxide
semiconductor layer, a first silicon nitride layer below the oxide
semiconductor layer, an oxide insulating layer over the oxide
semiconductor layer, a second silicon nitride layer over the oxide
insulating layer, wherein the oxide semiconductor layer comprises
In, Ga, and Zn, wherein, in a plan view, the oxide insulating layer
is in contact with the oxide semiconductor layer in a region
overlapping the channel forming region, and wherein, in the plan
view, the oxide semiconductor layer is surrounded by a region where
the first silicon nitride layer and the second silicon nitride
layer overlap, and wherein the second silicon nitride layer does
not have a region in contact with the oxide semiconductor layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 15/806,418, filed Nov. 8, 2017, now allowed, which is a
continuation of U.S. application Ser. No. 14/010,843, filed Aug.
27, 2013, now abandoned, which is a continuation of U.S.
application Ser. No. 12/897,360, filed Oct. 4, 2010, now U.S. Pat.
No. 8,526,567, which claims the benefit of a foreign priority
application filed in Japan as Serial No. 2009-234845 on Oct. 9,
2009, all of which are incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to a shift register and a
display device including the shift register.
BACKGROUND ART
[0003] A thin film transistor (TFT) formed over a flat plate such
as a glass substrate, which is typically used in a liquid crystal
display device, is generally formed using a semiconductor material
such as amorphous silicon or polycrystalline silicon. Although a
TFT which is formed using amorphous silicon has low field-effect
mobility, such a TFT can be formed over a larger glass substrate.
On the other hand, a TFT formed using polycrystalline silicon has
high field-effect mobility, but such a TFT needs a crystallization
step such as laser annealing and is not always suitable for being
formed over a larger glass substrate.
[0004] In view of the foregoing, a TFT formed using an oxide
semiconductor as a semiconductor material has attracted attention.
For example, Patent Documents 1 and 2 each disclose a technique in
which a TFT is formed using zinc oxide or an In--Ga--Zn--O-based
oxide semiconductor as a semiconductor material and used for a
switching element in an image display device.
[0005] A TFT in which a channel formation region is formed in an
oxide semiconductor can have higher electric-field mobility than a
TFT formed using amorphous silicon. Further, an oxide semiconductor
film can be formed at a temperature of 300.degree. C. or lower by a
sputtering method or the like, and a manufacturing process of the
TFT formed using an oxide semiconductor is simpler than that of the
TFT formed using polycrystalline silicon.
[0006] TFTs which are formed using such an oxide semiconductor are
expected to be applied to switching elements included in a pixel
portion and a driver circuit of a display device such as a liquid
crystal display, an electroluminescent display, and electronic
paper. For example, Non-Patent Document 1 discloses a technique by
which a pixel portion and a driver circuit of a display device
include TFTs formed using the above oxide semiconductor.
[0007] Note that the TFTs which are formed using an oxide
semiconductor are all n-channel transistors. Therefore, in the case
where a driver circuit is formed using a TFT formed using an oxide
semiconductor, the driver circuit includes only n-channel TFTs
(hereinafter also referred to as unipolar TFTs).
[Patent Document 1] Japanese Published Patent Application No.
2007-123861
[Patent Document 2] Japanese Published Patent Application No.
2007-096055
[Non-Patent Document 1] T. Osada, et al., SID 09 DIGEST, pp.
184-187 (2009)
DISCLOSURE OF INVENTION
[0008] A driver circuit includes a shift register, a buffer, and
the like. For example, in the case where the shift register
includes only unipolar TFTs, there is a problem in that a signal
decreases or increases by the same amount as the threshold voltage
of the TFT. Therefore, bootstrap operation is often performed in a
portion where such a problem occurs. Specifically, the bootstrap
operation is often performed in an analog switch by which a signal
line or a scan line in a display device is driven.
[0009] Further, in the case where the load of a driver circuit
utilizing bootstrap is heavy, a gate width of a TFT included in the
driver circuit needs to be made larger. In accordance with that,
parasitic capacitance caused in the TFT becomes larger. In
particular, parasitic capacitance becomes larger in a TFT in which
a conductive layer serving as a gate terminal and a conductive
layer serving as a source or drain terminal need to be overlapped
with each other with a gate insulating layer provided therebetween
(e.g., a so-called inverted staggered TFT). As a result, there is a
problem in that the power consumption of a clock signal inputted to
the driver circuit is made large by the parasitic capacitance.
[0010] In view of the above problems, an object of one embodiment
of the present invention is to reduce the power consumption of a
shift register or a display device including the shift
register.
[0011] The above problems can be solved by dividing a clock signal
line included in a shift register into a plurality of pulse signal
lines. In other words, a plurality of flip-flops included in the
shift register is not electrically connected to one clock signal
line; and a plurality of pulse signal lines is provided and part of
the plurality of flip-flops is electrically connected to one of the
plurality of pulse signal lines. The pulse signal line supplies a
clock signal in part of an operation period of the shift register,
not during the whole operation period of the shift register.
Therefore, the capacity load caused with the supply of clock
signals to the shift register can be reduced, leading to reduction
in power consumption of the shift register.
[0012] One embodiment of the present invention is a shift register
including a first pulse signal line serving as a wiring for
supplying a clock signal that periodically alternates between a
high power supply potential and a low power supply potential during
a first period; a second pulse signal line serving as a wiring for
supplying a clock signal during a second period; a third pulse
signal line serving as a wiring for supplying an inverted clock
signal that is an inversion of the clock signal during a third
period; a fourth pulse signal line serving as a wiring for
supplying an inverted clock signal during a fourth period; a first
flip-flop which is electrically connected to the first pulse signal
line and outputs the high power supply potential in the first
period; a second flip-flop which is electrically connected to the
second pulse signal line and outputs the high power supply
potential in the second period; a third flip-flop which is
electrically connected to the first flip-flop and the third pulse
signal line and outputs the high power supply potential in the
third period; and a fourth flip-flop which is electrically
connected to the second flip-flop and the fourth pulse signal line
and outputs the high power supply potential in the fourth period.
The third period overlaps with the first period, and the fourth
period overlaps with the second period.
[0013] Another embodiment of the present invention is a shift
register with the above-mentioned structure, in which the first
pulse signal line serves as a wiring for supplying the low power
supply potential in a period except the first period; the second
pulse signal line serves as a wiring for supplying the low power
supply potential in a period except the second period; the third
pulse signal line serves as a wiring for supplying the low power
supply potential in a period except the third period; and the
fourth pulse signal line serves as a wiring for supplying the low
power supply potential in a period except the fourth period.
[0014] Another embodiment of the present invention is a shift
register with any one of the above-mentioned structures, in which a
flip-flop includes a transistor whose channel formation region is
formed using an oxide semiconductor.
[0015] Another embodiment of the present invention is a shift
register with any one of the above-mentioned structures, in which a
pulse signal line is electrically connected to a reference clock
signal line or an inverted reference signal line via a transistor
which is kept on in a period when the pulse signal line supplies a
clock signal or an inverted clock signal.
[0016] Another embodiment of the present invention is a shift
register with any one of the above-mentioned structures, in which a
pulse signal line is electrically connected to a wiring for
supplying a low power supply potential via a transistor which is
kept on in the period when the pulse signal line does not supply a
clock signal or an inverted clock signal.
[0017] Another embodiment of the present invention is a display
device which includes a shift register with any one of the
above-mentioned structures.
[0018] In a shift register which is one embodiment of the present
invention, a clock signal is supplied by a plurality of wirings,
not by one wiring. Any one of the plurality of wirings supplies a
clock signal in only part of the operation period of the shift
register, not during the whole operation period of the shift
register. Therefore, the capacity load caused with the supply of
clock signals can be reduced, leading to reduction in power
consumption of the shift register.
BRIEF DESCRIPTION OF DRAWINGS
[0019] In the accompanying drawings:
[0020] FIGS. 1A and 1B are a diagram illustrating a configuration
example and a timing chart, respectively, of a shift register
described in Embodiment 1;
[0021] FIGS. 2A and 2B are a diagram illustrating a configuration
example and a timing chart, respectively, of a flip-flop described
in Embodiment 1;
[0022] FIGS. 3A and 3B are a diagram illustrating a configuration
example and a timing chart, respectively, of a pulse signal line
described in Embodiment 1;
[0023] FIGS. 4A and 4B are a diagram illustrating a configuration
example and a timing chart, respectively, of a pulse signal line
described in Embodiment 1;
[0024] FIGS. 5A to 5C are diagrams illustrating modification
examples of the shift register described in Embodiment 1;
[0025] FIGS. 6A and 6B are diagrams and FIG. 6C is a timing chart
illustrating modification examples of the shift register described
in Embodiment 1;
[0026] FIGS. 7A and 7B are a diagram and a timing chart,
respectively, illustrating a modification example of the flip-flop
described in Embodiment 1;
[0027] FIGS. 8A to 8C are a top diagram and cross-sectional
diagrams, respectively, illustrating a transistor described in
Embodiment 2;
[0028] FIGS. 9A and 9B are a top diagram and a cross-sectional
diagram, respectively, of a transistor described in Embodiment
2;
[0029] FIGS. 10A and 10B are a top diagram and a cross-sectional
diagram, respectively, of a plurality of transistors described in
Embodiment 2;
[0030] FIGS. 11A to 11D are cross-sectional diagrams illustrating a
process for manufacturing a transistor described in Embodiment
2;
[0031] FIGS. 12A and 12B are a top diagram and a cross-sectional
diagram, respectively, of a plurality of transistors described in
Embodiment 3;
[0032] FIGS. 13A and 13B are a top diagram and a cross-sectional
diagram, respectively, of a plurality of transistors described in
Embodiment 4;
[0033] FIGS. 14A to 14C are a block diagram, a block diagram of a
scan line driver circuit, and a block diagram of a signal line
driver circuit, respectively, of a display device described in
Embodiment 5;
[0034] FIGS. 15A to 15C are a circuit diagram, a top diagram, and a
cross-sectional diagram, respectively, of a pixel of a liquid
crystal display device described in Embodiment 6;
[0035] FIG. 16A is a circuit diagram and FIGS. 16B to 16D are
cross-sectional diagrams, of a pixel of a light-emitting display
device described in Embodiment 7;
[0036] FIGS. 17A and 17B are a top diagram and a cross-sectional
diagram, respectively, of a light-emitting display device described
in Embodiment 7;
[0037] FIGS. 18A to 18C are a circuit diagram, a top diagram, and a
cross-sectional diagram, respectively, of a pixel of electronic
paper described in Embodiment 7;
[0038] FIG. 19 is a diagram illustrating an example of an
electronic book described in Embodiment 8;
[0039] FIGS. 20A and 20B are diagrams each illustrating an example
of an electronic appliance described in Embodiment 9;
[0040] FIGS. 21A and 21B are diagrams each illustrating an example
of an electronic appliance described in Embodiment 9; and
[0041] FIGS. 22A and 22B are diagrams each illustrating an example
of an electronic appliance described in Embodiment 9.
BEST MODE FOR CARRYING OUT THE INVENTION
[0042] Hereinafter, embodiments of the present invention will be
described in detail with reference to the accompanying drawings.
Note that the present invention is not limited to the description
below, and it is easily understood by those skilled in the art that
a variety of changes and modifications in modes and details thereof
can be made without departing from the spirit and scope of the
present invention. Therefore, the present invention should not be
interpreted as being limited to the description of the embodiments
below.
[0043] Note that it is difficult to determine which one of the
terminals of a transistor is a source terminal or a drain terminal
because it changes depending on the structure, operating
conditions, and the like of the transistor. Therefore, in this
document, one of a source terminal and a drain terminal is referred
to as a first terminal and the other thereof is referred to as a
second terminal for distinction.
[0044] Note that the size, the thickness of a layer, or a region of
each structure illustrated in drawings or the like in embodiments
is exaggerated for simplicity in some cases. Therefore, embodiments
of the present invention are not limited to such scales. Further,
in this specification, ordinal numbers such as "first", "second",
and "third" are used in order to avoid confusion among components,
and the terms do not limit the components numerically.
Embodiment 1
[0045] In this embodiment, examples of a structure and operation of
a shift register are described with reference to FIGS. 1A and 1B,
FIGS. 2A and 2B, FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5A to 5C,
FIGS. 6A to 6C, and FIGS. 7A and 7B. Specifically, a shift register
which includes a pulse signal line and a flip-flop is described.
The pulse signal line serves as a wiring for supplying a clock
signal in part of the operation period of the shift register and
serves as a wiring for supplying a low power supply potential in
the other periods. The flip-flop is electrically connected to the
pulse signal line.
Example of Structure of Shift Register
[0046] A shift register in this embodiment includes first to sixth
pulse signal lines and first to tenth flip-flops.
[0047] The first pulse signal line (PS1) is electrically connected
to the first flip-flop (FF1) and the third flip-flop (FF3). The
second pulse signal line (PS2) is electrically connected to the
fifth flip-flop (FF5) and the seventh flip-flop (FF7). The third
pulse signal line (PS3) is electrically connected to the ninth
flip-flop (FF9). The fourth pulse signal line (PS4) is electrically
connected to the second flip-flop (FF2) and the fourth flip-flop
(FF4). The fifth pulse signal line (PS5) is electrically connected
to the sixth flip-flop (FF6) and the eighth flip-flop (FF8). The
sixth pulse signal line (PS6) is electrically connected to the
tenth flip-flop (FF10) (see FIG. 1A).
[0048] An output terminal of each flip-flop is electrically
connected to an input terminal of the subsequent flip-flop. Note
that an input terminal of the first flip-flop (FF1) is electrically
connected to a wiring for supplying a start pulse (SP).
[0049] In a first period (t1), the first pulse signal line (PS1)
serves as a wiring for supplying a clock signal that periodically
alternates between a high power supply potential and a low power
supply potential. In a second period (t2), the second pulse signal
line (PS2) serves as a wiring for supplying a clock signal. In a
third period (t3), the third pulse signal line (PS3) serves as a
wiring for supplying a clock signal. In a fourth period (t4), the
fourth pulse signal line (PS4) serves as a wiring for supplying an
inverted clock signal that is the inversion of a clock signal. In a
fifth period (t5), the fifth pulse signal line (PS5) serves as a
wiring for supplying an inverted clock signal. In a sixth period
(t6), the sixth pulse signal line (PS6) serves as a wiring for
supplying an inverted clock signal (see FIG. 1B).
Example of Operation of Shift Register
[0050] The operation of the shift register in this embodiment is
described below.
[0051] A high power supply potential signal is inputted as a start
pulse (SP) to the input terminal of the first flip-flop (FF1). The
first flip-flop (FF1) operates on the basis of the inputted signal,
and outputs a high power supply potential signal as an output
signal (FF1out) of the first flip-flop with a half clock cycle
delay.
[0052] The output signal (FF1out) is inputted to an input terminal
of the second flip-flop (FF2). Like the first flip-flop (FF1), the
second flip-flop (FF2) operates on the basis of the inputted
signal, and outputs a high power supply potential signal as an
output signal (FF2out) of the second flip-flop with a half clock
cycle delay.
[0053] Similarly, as for the other flip-flops, a high power supply
potential signal is inputted to an input terminal of the subsequent
flip-flop, and the flip-flop outputs a high power supply potential
signal with a half clock cycle delay.
Specific Eexample of Flip-Flop
[0054] A specific example of a circuit configuration of the
flip-flop in this embodiment is illustrated in FIG. 2A. Note that
only configurations of the first flip-flop (FF1) and the second
flip-flop (FF2) are illustrated in FIG. 2A for convenience.
[0055] The first flip-flop (FF1) includes transistors 101 to 106.
Note that in this case, the transistors 101 to 106 are n-channel
transistors.
[0056] A gate terminal of the transistor 101 is electrically
connected to the output terminal of the second flip-flop (FF2). A
first terminal of the transistor 101 is electrically connected to a
wiring for supplying a high power supply potential (VDD)
(hereinafter also referred to as a high power supply potential
line).
[0057] A gate terminal of the transistor 102 is electrically
connected to a wiring for supplying a start pulse (SP) (hereinafter
also referred to as a start pulse line). A first terminal of the
transistor 102 is electrically connected to a second terminal of
the transistor 101. A second terminal of the transistor 102 is
electrically connected to a wiring for supplying a low power supply
potential (VSS) (hereinafter also referred to as a low power supply
potential line).
[0058] A gate terminal of the transistor 103 is electrically
connected to the start pulse line. A first terminal of the
transistor 103 is electrically connected to the high power supply
potential line.
[0059] A gate terminal of the transistor 104 is electrically
connected to the second terminal of the transistor 101 and the
first terminal of the transistor 102. A first terminal of the
transistor 104 is electrically connected to a second terminal of
the transistor 103. A second terminal of the transistor 104 is
electrically connected to the low power supply potential line.
[0060] A gate terminal of the transistor 105 is electrically
connected to the second terminal of the transistor 103 and the
first terminal of the transistor 104. A first terminal of the
transistor 105 is electrically connected to the first pulse signal
line (PS1).
[0061] A gate terminal of the transistor 106 is electrically
connected to the second terminal of the transistor 101, the first
terminal of the transistor 102, and the gate terminal of the
transistor 104. A first terminal of the transistor 106 is
electrically connected to a second terminal of the transistor 105.
A second terminal of the transistor 106 is electrically connected
to the low power supply potential line.
[0062] Note that hereinafter, for convenience, a node in which the
second terminal of the transistor 101, the first terminal of the
transistor 102, the gate terminal of the transistor 104, and the
gate terminal of the transistor 106 are electrically connected is
referred to as a node A; and a node in which the second terminal of
the transistor 103, the first terminal of the transistor 104, and
the gate terminal of the transistor 105 are electrically connected
to each other is referred to as a node B.
[0063] In addition to the above configuration, a capacitor may be
provided between the gate terminal and the source terminal of the
transistor 105. The capacitor is provided, so that a bootstrap
operation, which is described below, can be surely performed.
Example of Operation of Flip-Flop
[0064] As an example of operation of the above-mentioned
flip-flops, operation of the first flip-flop (FF1) is described
below with reference to FIG. 2B.
[0065] First, the potential of the start pulse line electrically
connected to the first flip-flop (FF1) is increased to a high level
(hereinafter referred to as an H level). Accordingly, H-level
signals are inputted to the gate terminals of the transistors 102
and 103. Thus, the transistors 102 and 103 are turned on. Then, the
potential of the node A is decreased to a low level (hereinafter
referred to as an L level) and the potential of the node B is
increased to an H level. In accordance with this, the transistor
105 is also turned on. As a result, an L-level potential, which is
the potential of the first pulse signal line (PS1) in this period,
is outputted as the output signal (FF1out) of the first
flip-flop.
[0066] In the subsequent period, the potential of the start pulse
line is decreased to an L level. Thus, the transistors 102 and 103
are turned off. As a result, the potential of the node A and the
potential of the node B go into a floating state. At this time,
there is a potential difference from the L level to the H level
between the source terminal and the gate terminal of the transistor
105, and the potential difference is maintained because the node B
goes into a floating state. That is, the transistor 105 is kept on
regardless of the state of the source terminal. Moreover, the
potential of the first pulse signal line (PS1) is increased to an H
level. Thus, the potential of the node B which is in a floating
state and electrically connected to the gate terminal of the
transistor 105 is further increased by an H-level potential of the
first pulse signal line (PS1) in this period. The operation in
which the potential of the node B is increased by capacitive
coupling of the source terminal and the gate terminal that is
electrically connected to the node B in the floating state, of the
transistor 105 as described above, is called bootstrap.
Accordingly, an H-level potential, which is the potential of the
first pulse signal line (PS1), is outputted as the output signal
(FF1out) of the first flip-flop.
[0067] Note that in this case, the transistor 105 is an n-channel
transistor. Thus, in this period when the potential of the first
pulse signal line (PS1) is set to an H level, a terminal of the
transistor 105 which is electrically connected to the output
terminal of the first flip-flop (FF1) functions as a source
terminal, and a terminal of the transistor 105 which is
electrically connected to the first pulse signal line (PS1)
functions as a drain terminal. A transistor is turned on or turned
off depending on the potential difference between a source terminal
and a gate terminal. Therefore, in the case where an H-level
potential of the first pulse signal line (PS1) is outputted as the
output signal (FF1out) of the first flip-flop via an n-channel
transistor in which bootstrap does not occur, the outputted
potential is reduced from the H-level potential by the threshold
voltage (Vth) of the n-channel transistor. However, since the
bootstrap occurs in the transistor 105, the output signal (FF1out)
of the first flip-flop can be outputted without a reduction in the
potential of the first pulse signal line (PS1).
[0068] An H-level signal which is the output signal of the first
flip-flop (FF1) is inputted to the second flip-flop (FF2). In this
case, the second flip-flop (FF2) has the same structure as that of
the first flip-flop (FF1) except that the fourth pulse signal line
(PS4), instead of the first pulse signal line (PS1), is
electrically connected to the second flip-flop (FF2). Thus, the
above-mentioned description of the first flip-flop (FF1) applies to
detailed description of the circuit operation of the second
flip-flop (FF2). In this period, the second flip-flop (FF2) outputs
an L-level potential which is the potential of the fourth pulse
signal line (PS4) in this period.
[0069] In the subsequent period, the potential of the first pulse
signal (PS1) is decreased to an L level and the potential of the
fourth pulse signal line (PS4) is increased to an H level. As a
result, the output signal (FFlout) of the first flip-flop is
decreased to an L level. An H-level potential which is the
potential of the fourth pulse signal line (PS4) is outputted as the
output signal (FF2out) of the second flip-flop.
[0070] The output signal (FF2out) of the second flip-flop is
inputted to the third flip-flop (not illustrated) and to the gate
terminal of the transistor 101 included in the first flip-flop
(FF1). Thus, the transistor 101 included in the first flip-flop
(FF1) is turned on, and then the potential of the node A is set to
an H level. Accordingly, the transistors 104 and 106 are also
turned on. Since the transistor 104 is turned on, the potential of
the node B is decreased to an L level. That is, the potential of
the gate terminal of the transistor 105 is decreased to an L level.
Therefore, the transistor 105 is turned off. In addition, since the
transistor 106 is turned on, the output signal (FF1out) of the
first flip-flop is changed from the L level of the first pulse
signal line (PS1) via the transistor 105 in this period to an L
level of the low power supply potential (VSS) via the transistor
106. In other words, the output signal (FF1out) of the first
flip-flop is outputted via a different transistor without being
substantially changed.
[0071] In the subsequent period, the potential of the fourth pulse
signal line (PS4) is decreased to an L level. That is, the output
signal (FF2out) of the second flip-flop is decreased to an L level.
Thus, the transistor 101 included in the first flip-flop (FF1) is
turned off. As a result, a node electrically connected to the gate
terminal of the transistor 104 and a node electrically connected to
the gate terminal of the transistor 106 go into a floating state
while such nodes keep the H-level signals. Thus, the transistors
104 and 106 are kept on and the output signal (FF1out) of the first
flip-flop is maintained at an L level. Such a state is continued
until an H-level potential is inputted to the input terminal of the
first flip-flop (FF1) again.
[0072] In the above-mentioned manner, the first flip-flop (FF1)
illustrated in FIG. 2A can output a signal which is inputted with a
half clock cycle delay.
Example of Pulse Signal Line
[0073] The first pulse signal line (PS1) to the sixth pulse signal
line (PS6) each serve as a wiring for supplying a clock signal in
part of the operation period, and the first to sixth pulse signal
lines each serve as a wiring for supplying a low power supply
potential in the other periods. An example of a wiring having such
functions is described with reference to FIGS. 3A and 3B and FIGS.
4A and 4B.
[0074] Each of the first pulse signal line (PS1) to the sixth pulse
signal line (PS6) illustrated in FIG. 3A is electrically connected
to a reference clock signal line (CK) or an inverted reference
clock signal line (CKB) via a source terminal and a drain terminal
of any one of clock signal selection transistors 111, 112, and 113
and inverted clock signal selection transistors 114, 115, and 116.
Note that in this case, the clock signal selection transistors 111,
112, and 113 and the inverted clock signal selection transistors
114, 115, and 116 are n-channel transistors.
[0075] Specifically, a gate terminal of the clock signal selection
transistor 111 is electrically connected to a control terminal a. A
first terminal of the clock signal selection transistor 111 is
electrically connected to the first pulse signal line (PS1). A
second terminal of the clock signal selection transistor 111 is
electrically connected to the reference clock signal line (CK). A
gate terminal of the clock signal selection transistor 112 is
electrically connected to a control terminal b. A first terminal of
the clock signal selection transistor 112 is electrically connected
to the second pulse signal line (PS2). A second terminal of the
clock signal selection transistor 112 is electrically connected to
the reference clock signal line (CK). A gate terminal of the clock
signal selection transistor 113 is electrically connected to a
control terminal c. A first terminal of the clock signal selection
transistor 113 is electrically connected to the third pulse signal
line (PS3). A second terminal of the clock signal selection
transistor 113 is electrically connected to the reference clock
signal line (CK).
[0076] A gate terminal of the inverted clock signal selection
transistor 114 is electrically connected to a control terminal d. A
first terminal of the inverted clock signal selection transistor
114 is electrically connected to the fourth pulse signal line
(PS4). A second terminal of the inverted clock signal selection
transistor 114 is electrically connected to the inverted reference
clock signal line (CKB). A gate terminal of the inverted clock
signal selection transistor 115 is electrically connected to a
control terminal e. A first terminal of the inverted clock signal
selection transistor 115 is electrically connected to the fifth
pulse signal line (PS5). A second terminal of the inverted clock
signal selection transistor 115 is electrically connected to the
inverted reference clock signal line (CKB). A gate terminal of the
inverted clock signal selection transistor 116 is electrically
connected to a control terminal f. A first terminal of the inverted
clock signal selection transistor 116 is electrically connected to
the sixth pulse signal line (PS6). A second terminal of the
inverted clock signal selection transistor 116 is electrically
connected to the inverted reference clock signal line (CKB).
[0077] As illustrated in FIG. 3B, the reference clock signal line
is a wiring for supplying a clock signal that periodically
alternates between a high power supply potential and a low power
supply potential regardless of a period, and the inverted reference
clock signal line is a wiring for supplying an inverted clock
signal that is the inversion of a clock signal, regardless of a
period.
[0078] The potential of the control terminal a is set to an H level
in the first period (t1), and is set to an L level in the other
periods. Thus, in the first period (t1), the first pulse signal
line (PS1) can serve as a wiring for supplying a clock signal. In
other words, the first period (t1) is a period in which the
potential of the control terminal a is set to an H level.
[0079] Similarly, the potentials of the control terminals b to f
are set to an H level in the second period (t2) to the sixth period
(t6), respectively, and are set to an L level in the other periods.
Thus, the second pulse signal line in the second period and the
third pulse signal line in the third period can serve as a wiring
for supplying a clock signal; the fourth pulse signal line in the
fourth period, the fifth pulse signal line in the fifth period, and
the sixth pulse signal line in the sixth period can serve as a
wiring for supplying an inverted clock signal. In other words, the
second period (t2) to the sixth period (t6) are periods in which
the potentials of the respective control terminals b to fare set to
an H level.
[0080] Each of the first pulse signal line (PS1) to the sixth pulse
signal line (PS6) illustrated in FIG. 4A is electrically connected
to a wiring for supplying a low power supply potential (VSS) via a
source terminal and a drain terminal of one of low power supply
potential selection transistors 121 to 126. Note that in this case,
the low power supply potential selection transistors 121 to 126 are
n-channel transistors.
[0081] A gate terminal of the low power supply potential selection
transistor 121 is electrically connected to a control terminal g. A
first terminal of the low power supply potential selection
transistor 121 is electrically connected to the first pulse signal
line (PS1). A second terminal of the low power supply potential
selection transistor 121 is electrically connected to the wiring
for supplying a low power supply potential (VSS). A gate terminal
of the low power supply potential selection transistor 122 is
electrically connected to a control terminal h. A first terminal of
the low power supply potential selection transistor 122 is
electrically connected to the second pulse signal line (PS2). A
second terminal of the low power supply potential selection
transistor 122 is electrically connected to the wiring for
supplying a low power supply potential (VSS). A gate terminal of
the low power supply potential selection transistor 123 is
electrically connected to a control terminal i. A first terminal of
the low power supply potential selection transistor 123 is
electrically connected to the third pulse signal line (PS3). A
second terminal of the low power supply potential selection
transistor 123 is electrically connected to the wiring for
supplying a low power supply potential (VSS). A gate terminal of
the low power supply potential selection transistor 124 is
electrically connected to a control terminal j. A first terminal of
the low power supply potential selection transistor 124 is
electrically connected to the fourth pulse signal line (PS4). A
second terminal of the low power supply potential selection
transistor 124 is electrically connected to the wiring for
supplying a low power supply potential (VSS). A gate terminal of
the low power supply potential selection transistor 125 is
electrically connected to a control terminal k. A first terminal of
the low power supply potential selection transistor 125 is
electrically connected to the fifth pulse signal line (PS5). A
second terminal of the low power supply potential selection
transistor 125 is electrically connected to the wiring for
supplying a low power supply potential (VSS). A gate terminal of
the low power supply potential selection transistor 126 is
electrically connected to a control terminal l. A first terminal of
the low power supply potential selection transistor 126 is
electrically connected to the sixth pulse signal line (PS6). A
second terminal of the low power supply potential selection
transistor 126 is electrically connected to the wiring for
supplying a low power supply potential (VSS).
[0082] The potential of the control terminal g is set to an L level
in the first period (t1) and is set to an H level in the other
periods. Thus, in the periods except the first period (t1), the
first pulse signal line (PS1) can serve as a wiring for supplying a
low power supply potential (VSS).
[0083] Similarly, the potentials of the control terminals h to l
are set to an L level in the second period (t2) to the sixth period
(t6), respectively, and are set to an H level in the other periods.
Thus, the second pulse signal line in the periods except the second
period, the third pulse signal line in the periods except the third
period, the fourth pulse signal line in the periods except the
fourth period, the fifth pulse signal line in the periods except
the fifth period, and the sixth pulse signal line in the periods
except the sixth period can serve as a wiring for supplying a low
power supply potential (VSS).
[0084] In the shift register in this embodiment, clock signals are
supplied by a plurality of wirings, not by one wiring. Further, any
one of the plurality of wirings supplies clock signals in a part of
period, not in the entire operation period of the shift register.
Therefore, the capacity load caused with the supply of clock
signals can be reduced, leading to a reduction in power consumption
of the shift register.
Modification Example
[0085] The above-described shift register is an example of this
embodiment, and a shift register which has a different point from
the above description is included in this embodiment.
[0086] For example, in the above shift register, two flip-flops are
electrically connected to each of the pulse signal lines (see FIG.
1A); however, a larger number of flip-flops may be electrically
connected to each of the pulse signal lines. Specifically, as
illustrated in FIG. 5A, x (x is a natural number of 3 or more)
flip-flops can be electrically connected to each of the pulse
signal lines, for example.
[0087] Further, six pulse signal lines are included in the above
shift register (see FIG. 1A), but a larger number of pulse signal
lines may be included. Specifically, a structure illustrated in
FIG. 5B can be employed, which includes the first pulse signal line
(PS1) to the y-th (y is a natural number of 4 or more) pulse signal
line (PSy) which supply clock signals in part of the operation
period; and the (y+1)th pulse signal line (PSy+1) to the 2y-th
pulse signal line (PS2y) which supply inverted clock signals in
part of the operation period. Two flip-flops are electrically
connected to each of the pulse signal lines in the structure
illustrated in FIG. 5B.
[0088] Furthermore, two flip-flops are electrically connected to
each of the pulse signal lines and six pulse signal lines are
included in the above shift register (see FIG. 1A); however, a
larger number of flip-flops may be electrically connected to each
of the pulse signal lines and a larger number of pulse signal lines
may be included. Specifically, a structure illustrated in FIG. 5C
can be employed: the first pulse signal line (PS1) to the y-th (y
is a natural number of 4 or more) pulse signal line (PSy) which
supply clock signals in part of the operation period are included;
the (y+1)th pulse signal line (PSy+1) to the 2y-th pulse signal
line (PS2y) which supply inverted clock signals in part of the
operation period are included; and x flip-flops are electrically
connected to each of the pulse signal lines, for example.
[0089] In the above shift register, the numbers of flip-flops which
are electrically connected to each of pulse signal lines are the
same (see FIG. 1A and FIGS. 5A to 5C), but the number of flip-flops
which are electrically connected to each of pulse signal lines may
be different depending on a pulse signal line. Specifically, as
illustrated in FIG. 6A, x flip-flops can be electrically connected
to the first pulse signal line (PS1) and the fourth pulse signal
line (PS4), and z flip-flops (z is a natural number of 2 or more
and different from x) can be electrically connected to the second
pulse signal line (PS2) and the fifth pulse signal line (PS5).
[0090] In the above shift register, the number of flip-flops which
are electrically connected to the first pulse signal line (PS1) and
the number of flip-flops which are electrically connected to the
fourth pulse signal line (PS4) are the same (see FIG. 1A, FIG. 5A,
and FIG. 6A), but the number of flip-flops which are electrically
connected to the first pulse signal line (PS1) and the number of
flip-flops which are electrically connected to the fourth pulse
signal line (PS4) may be different from each other. Specifically,
as illustrated in FIG. 6B, x flip-flops can be electrically
connected to the first pulse signal line (PS1) and (x+z) flip-flops
can be electrically connected to the fourth pulse signal line
(PS4).
[0091] In the above shift register, the first period (t1) and the
second period (t2) do not overlap with each other (see FIG. 1B),
but the first period (t1) and the second period (t2) may be overlap
with each other. Specifically, as illustrated in FIG. 6C, a period
(T) in which the first period (t1) and the second period (t2)
overlap with each other can be provided, for example. As
illustrated in FIG. 1B and FIG. 6C, periods need to be provided so
that at least one of plural pulse signal lines serves as a wiring
for supplying a clock signal and at least one of a plurality of
pulse signal lines serves as a wiring for supplying an inverted
clock signal.
[0092] The circuit configuration of a flip-flop illustrated in FIG.
2A is an example, and any circuit configuration may be employed as
long as an inputted signal is outputted with a delay. Specifically,
the circuit illustrated in FIG. 7A can be applied to the flip-flop
in this embodiment.
[0093] The first flip-flop (FF1) illustrated in FIG. 7A includes
transistors 131 to 134. Note that in this case, the transistors 131
to 134 are n-channel transistors.
[0094] A gate terminal and a first terminal of the transistor 131
are electrically connected to the start pulse line.
[0095] A gate terminal of the transistor 132 is electrically
connected to the output terminal of the second flip-flop (FF2), a
first terminal of the transistor 132 is electrically connected to a
second terminal of the transistor 131, and a second terminal of the
transistor 132 is electrically connected to the low power supply
potential line.
[0096] A gate terminal of the transistor 133 is electrically
connected to the second terminal of the transistor 131 and the
first terminal of the transistor 132, and a first terminal of the
transistor 133 is electrically connected to the first pulse signal
line (PS1).
[0097] A gate terminal of the transistor 134 is electrically
connected to the output terminal of the second flip-flop (FF2), a
first terminal of the transistor 134 is electrically connected to a
second terminal of the transistor 133, and a second terminal of the
transistor 134 is electrically connected to the low power supply
potential line.
[0098] Note that hereinafter, for convenience, a node in which the
second terminal of the transistor 131, the first terminal of the
transistor 132, and the gate terminal of the transistor 133 are
electrically connected to each other is referred to as a node
C.
[0099] The operation of the first flip-flop (FF1) illustrated in
FIG. 7A is described below with reference to FIG. 7B.
[0100] First, the potential of the start pulse line electrically
connected to the first flip-flop (FF1) is increased to an H level.
Accordingly, H-level signals are inputted to the gate terminal of
the transistor 131 and the first terminal of the transistor 131,
and the transistor 131 which is diode-connected is turned on. Thus,
the potential of the node C is increased to an H level.
Accordingly, the transistor 133 is also turned on. As a result, an
L-level potential, which is the potential of the first pulse signal
line (PS1) in this period, is outputted as the output signal
(FF1out) of the first flip-flop.
[0101] In the subsequent period, the potential of the start pulse
line is decreased to an L level. Thus, the transistor 131 is turned
off. As a result, the potential of the node C goes into a floating
state. At this time, there is a potential difference from the L
level to the H level between the source terminal and the gate
terminal of the transistor 131, and the potential difference is
maintained because the node C goes into a floating state. That is,
the transistor 131 is kept on regardless of the state of the source
terminal. Moreover, the potential of the first pulse signal line
(PS1) is increased to an H level. Thus, the potential of the node C
which is in a floating state and electrically connected to the gate
terminal of the transistor 133 is further increased by an H-level
potential of the first pulse signal line (PS1) in this period.
Accordingly, an H-level potential, which is the potential of the
first pulse signal line (PS1), is outputted as the output signal
(FF1out) of the first flip-flop.
[0102] An H-level signal which is the output signal of the first
flip-flop (FF1) is inputted to the second flip-flop (FF2). In this
case, the second flip-flop (FF2) has the same structure as that of
the first flip-flop (FF1) except that the fourth pulse signal line
(PS4), instead of the first pulse signal line (PS1), is
electrically connected to the second flip-flop (FF2). Thus, the
above-mentioned description of the first flip-flop (FF1) applies to
detailed description of the circuit operation of the second
flip-flop (FF2). In this period, the second flip-flop (FF2) outputs
an L-level potential which is the potential of the fourth pulse
signal line (PS4) in this period.
[0103] In the subsequent period, the potential of the first pulse
signal (PS1) is decreased to an L level and the potential of the
fourth pulse signal line (PS4) is increased to an H level. As a
result, the output signal (FF1out) of the first flip-flop is
decreased to an L level. An H-level potential which is the
potential of the fourth pulse signal line (PS4) is outputted as the
output signal (FF2out) of the second flip-flop.
[0104] The output signal (FF2out) of the second flip-flop is
inputted to the third flip-flop (not illustrated) and to the gate
terminals of the transistors 132 and 134 included in the first
flip-flop (FF1). Thus, the transistors 132 and 134 included in the
first flip-flop (FF1) are turned on. Accordingly, the potential of
the gate terminal of the transistor 132 (the node C) is set to an L
level, and the output signal (FF1out) of the first flip-flop is
changed from the L level of the first pulse signal (PS1) via the
transistor 133 in this period to an L level of the low power supply
potential (VSS) via the transistor 134.
[0105] In the subsequent period, the potential of the fourth pulse
signal line (PS4) is decreased to an L level. That is, the output
signal (FF2out) of the second flip-flop is decreased to an L level.
Thus, the transistors 132 and 134 included in the first flip-flop
(FF1) are turned off. Such a state is continued until an H-level
potential is inputted to the input terminal of the first flip-flop
(FF1) again.
[0106] In the above-mentioned manner, the first flip-flop (FF1)
illustrated in FIG. 7A can output a signal which is inputted with a
half clock cycle delay. Therefore, the first flip-flop (FF1) can be
applied to the flip-flop in this embodiment.
[0107] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 2
[0108] In this embodiment, an example of a transistor which is
applicable to the shift register described in Embodiment 1 will be
described.
[0109] Examples of structures of the transistors of this embodiment
will be described with reference to FIGS. 8A to 8C. FIGS. 8A to 8C
illustrate examples of the structures of the transistors of this
embodiment. FIG. 8A is a top diagram and FIG. 8B is a
cross-sectional diagram taken along line Z1-Z2 in FIG. 8A.
[0110] The transistor in FIGS. 8A and 8B includes a conductive
layer 211 over a substrate 201, an insulating layer 202 over the
conductive layer 211, an oxide semiconductor layer 213 over the
insulating layer 202, and a conductive layer 215a and a conductive
layer 215b over the oxide semiconductor layer 213.
[0111] Note that in the transistor, the conductive layer 211 serves
as a gate terminal, the insulating layer 202 serves as a gate
insulating layer, one of the conductive layers 215a and 215b serves
as a source terminal, and the other of the conductive layers 215a
and 215b serves as a drain terminal. The oxide semiconductor layer
213 includes a channel formation region. The oxide semiconductor
layer 213 is subjected to dehydration or dehydrogenation treatment
when formed.
[0112] Further, as for the transistor in FIGS. 8A and 8B, the oxide
semiconductor layer 213 is subjected to dehydration or
dehydrogenation treatment, and moreover, an oxide insulating layer
207 is formed in contact with part of the oxide semiconductor layer
213. In the transistor including the oxide semiconductor layer 213
as a channel formation region, in which the oxide insulating layer
207 is formed after the dehydration or dehydrogenation treatment is
performed, the threshold voltage (Vth) shift due to long-term use
and large load hardly occurs and thus reliability is high.
[0113] Note that a nitride insulating layer may be provided over
the oxide insulating layer 207. It is preferable that the nitride
insulating layer be in contact with the insulating layer 202
provided below the oxide insulating layer 207 or an insulating
layer serving as a base, so that impurities such as moisture,
hydrogen ions, and OH.sup.- from the vicinity of side surfaces of
the substrate are prevented from entering. In particular, it is
effective to use a silicon nitride layer for the insulating layer
202 in contact with the oxide insulating layer 207 or the
insulating layer serving as a base. That is, when the silicon
nitride layers are provided so as to surround a lower surface, an
upper surface, and side surfaces of the oxide semiconductor layer
213, reliability of the transistor is improved.
[0114] Further, a planarization insulating layer may be provided
over the oxide insulating layer 207 (over the nitride insulating
layer in the case where the nitride insulating layer is
provided).
[0115] Alternatively, as illustrated in FIG. 8C, the transistor may
have a structure in which an oxide conductive layer 214a and an
oxide conductive layer 214b are provided over parts of the oxide
semiconductor layer 213, the conductive layer 215a is provided so
as to be in contact with the oxide conductive layer 214a, and the
conductive layer 215b is provided so as to be in contact with the
oxide conductive layer 214b.
[0116] The oxide conductive layer 214a and the oxide conductive
layer 214b have conductivity higher than that of the oxide
semiconductor layer 213 and serves as a source region (also
referred to as a low-resistance source region) and a drain region
(also referred to as a low-resistance drain region) of the
transistor 251.
[0117] As a material of the oxide conductive film used for forming
the oxide conductive layer 214a and the oxide conductive layer
214b, a conductive material having a light-transmitting property
with respect to visible light, such as an In--Sn--Zn--O-based metal
oxide, an In--Al--Zn--O-based metal oxide, an Sn--Ga--Zn--O-based
metal oxide, an Al--Ga--Zn--O-based metal oxide, an
Sn--Al--Zn--O-based metal oxide, an In--Zn--O-based metal oxide, an
Sn--Zn--O-based metal oxide, an Al--Zn--O-based metal oxide, an
In--Sn--O-based metal oxide, an In--O-based metal oxide, an
Sn--O-based metal oxide, or a Zn--O-based metal oxide, can be
employed. The thickness of the oxide conductive film is selected as
appropriate in the range of greater than or equal to 1 nm and less
than or equal to 300 nm. In the case of employing a sputtering
method, film deposition is performed with the use of a target
containing SiO.sub.2 at greater than or equal to 2 wt % and less
than or equal to 10 wt %, so that SiO.sub.x (x>0) which hinders
crystallization is contained in the light-transmitting conductive
film. Thus, the oxide semiconductor layer 213 can be prevented from
being crystallized in heat treatment for dehydration or
dehydrogenation performed later.
[0118] For example, in the case where an In--Ga--Zn--O-based film
is used for the oxide semiconductor layer, the oxide semiconductor
layer 213 serving as a channel formation region and the oxide
conductive layer 214a and the oxide conductive layer 214b can be
separately formed under different deposition conditions.
[0119] For example, in the case where the deposition is performed
by a sputtering method, the oxide conductive layer 214a and the
oxide conductive layer 214b which are formed using an oxide
semiconductor film formed in an argon gas each have n-type
conductivity and an activation energy (.DELTA.E) of greater than or
equal to 0.01 eV and less than or equal to 0.1 eV.
[0120] Note that in this embodiment, the oxide conductive layers
214a and 214b are In--Ga--Zn--O-based films and include at least
amorphous components. Moreover, the oxide conductive layers 214a
and 214b may include crystal grains (nanocrystals). The crystal
grains in the oxide conductive layers 214a and 214b have a diameter
of approximately 1 nm to 10 nm, typically approximately 2 nm to 4
nm.
[0121] The oxide conductive layers 214a and 214b are not
necessarily provided, but when the oxide conductive layers 214a and
214b are provided between the oxide semiconductor layer 213 serving
as a channel formation region and the conductive layers 215a to
215d serving as a source terminal and a drain terminal, good
electrical junctions can be obtained and the transistor 251 can
operate stably. Further, excellent mobility can be maintained even
when the drain voltage is high.
[0122] Alternatively, the transistor in FIGS. 8A and 8B may have a
structure in FIGS. 9A and 9B, in which a conductive layer 217 is
provided over the oxide semiconductor layer 213 with the oxide
insulating layer 207 therebetween (with the oxide insulating layer
207 and the nitride insulating layer therebetween in the case where
the nitride insulating layer is provided). FIGS. 9A and 9B
illustrate an example of the structure of the transistor of this
embodiment. FIG. 9A is a top diagram of the transistor and FIG. 9B
is a cross-sectional diagram taken along line Z1-Z2 in FIG. 9A. The
conductive layer 217 functions as a second gate terminal. A second
gate voltage is applied to the conductive layer 217, whereby the
threshold voltage of the transistor 251 can be controlled. In the
case where a planarization insulating layer is provided, the
conductive layer 217 can be provided over the planarization
insulating layer.
[0123] For example, when a voltage of the second gate terminal is
higher than that of the source terminal, the threshold voltage of
the transistor is shifted to a negative side; when a voltage is
lower than that of the source terminal, the threshold voltage of
the transistor is shifted to a positive side.
[0124] As illustrated in FIGS. 8A to 8C and FIGS. 9A and 9B as
examples, the transistors of this embodiment are transistors each
including an oxide semiconductor layer serving as a channel
formation region. Accordingly, the transistors have mobility higher
than that of the conventional transistor including amorphous
silicon in a channel formation region, and thus high-speed
operation is possible.
[0125] One embodiment in the case where a plurality of transistors
in FIGS. 8A and 8C is used will be described with reference to
FIGS. 10A and 10B. As an example, FIGS. 10A and 10B illustrate a
structure of the plurality of transistors which are applicable to
the shift register of one embodiment of the present invention. FIG.
10A is a top diagram of two transistors and FIG. 10B is a
cross-sectional diagram taken along line X1-X2 in FIG. 10A.
[0126] The transistor 251 and a transistor 252 are illustrated in
FIG. 10A. Note that as an example, a structure is illustrated in
which an oxide conductive layer is provided between an oxide
semiconductor layer and the conductive layer serving as a source
terminal or a drain terminal.
[0127] The transistor 251 is the transistor illustrated in FIGS. 8A
and 8C. Therefore, the above description is to be referred to.
[0128] The transistor 252 includes a conductive layer 211 over the
substrate 201; an insulating layer 202 over the conductive layer
211; an oxide semiconductor layer 213 over the insulating layer
202; oxide conductive layers 214a and 214b over the oxide
semiconductor layer 213; and conductive layers 215a and 215b.
[0129] In the transistor 252, the conductive layer 211 serves as a
gate terminal, the insulating layer 202 serves as a gate insulating
layer, the oxide conductive layers 214a and 214b the conductivity
of which is higher than that of the oxide semiconductor layer 213
each serve as a source region (also referred to as a low-resistance
source region) or a drain region (also referred to as a
low-resistance drain region), and the conductive layers 215a and
215b each serve as a source terminal or a drain terminal. The oxide
semiconductor layer 213 includes a channel formation region. Note
that the oxide semiconductor layer 213 is subjected to dehydration
or dehydrogenation treatment when the oxide semiconductor layer 213
is formed.
[0130] The transistors 251 and 252 in FIGS. 10A and 10B are
provided with an oxide insulating layer 207 to be in contact with
part of the oxide semiconductor layer 213 and part of the oxide
semiconductor layer 2132, respectively, in addition to performing
dehydration or dehydrogenation treatment on the oxide semiconductor
layers.
[0131] Further, the conductive layer 211 of the transistor 251 is
in contact with the conductive layer 215b in an opening portion
formed in the gate insulating layer 202. Accordingly, favorable
contact can be obtained, which leads to reduction in contact
resistance. Thus, the number of openings can be reduced, which
results in reducing the area occupied by openings. Therefore, a
logic circuit (e.g., an inverter) which has this structure with the
use of two transistors can be formed, for example.
[0132] As illustrated in FIGS. 10A and 10B, in the shift register
described in Embodiment 1, a conductive layer serving as a gate
terminal of a transistor may be electrically connected to a source
terminal or a drain terminal of another transistor in an opening
portion formed in an insulating layer serving as a gate insulating
layer.
[0133] Next, an example of a method for manufacturing the
transistor illustrated in FIGS. 8A and 8B is described with
reference to FIGS. 11A to 11D. FIGS. 11A to 11D are cross-sectional
diagrams illustrating an example of a method for manufacturing the
transistor illustrated in FIGS. 8A and 8B.
[0134] In the below description, the word "film" means something
formed on the entire surface of a substrate to be processed into a
desired shape in a subsequent photolithography step or the like,
and something before the processing. The word "layer" means
something obtained by processing and shaping a "film" into a
desired shape by a photolithography step or the like, or something
that is to be formed on the entire surface of a substrate.
[0135] First, the substrate 201 is prepared. A conductive film is
formed over the substrate 201, and then the conductive layer 211 is
formed by a first photolithography step (see FIG. 11A). Note that
the conductive layer 211 is preferably tapered. When the conductive
layer 211 is tapered, adhesion between the conductive layer 211 and
a film formed on the conductive layer 211 can be increased.
[0136] The substrate 201 needs to have an insulating surface and
heat resistance high enough to withstand at least heat treatment to
be performed later. As the substrate 201, a glass substrate or the
like can be used, for example.
[0137] As the glass substrate, in the case where the temperature of
the heat treatment to be performed later is high, the one whose
strain point is 730.degree. C. or higher is preferably used. As the
glass substrate, a glass material such as aluminosilicate glass,
aluminoborosilicate glass, or barium borosilicate glass is used. In
general, by containing a larger amount of barium oxide (BaO) than
boric acid (B.sub.2O.sub.3), more practical heat-resistant glass
can be obtained. Therefore, a glass substrate containing BaO and
B.sub.2O.sub.3 so that the amount of BaO is larger than that of
B.sub.2O.sub.3 is preferably used.
[0138] Note that a substrate formed of an insulator, such as a
ceramic substrate, a quartz substrate, or a sapphire substrate may
be used for the substrate 201, instead of the glass substrate.
Alternatively, a crystallized glass substrate or the like may be
used.
[0139] An insulating layer serving as a base layer may be provided
between the substrate 201 and the conductive layer 211. The base
layer has a function of preventing diffusion of an impurity element
from the substrate 201 and can be formed to have a single-layer
structure or a stacked structure of one or more of a silicon
nitride layer, a silicon oxide layer, a silicon nitride oxide
layer, and a silicon oxynitride layer.
[0140] As an example of a material of the conductive film for
forming the conductive layer 211, a metal material such as
molybdenum, titanium, chromium, tantalum, tungsten, aluminum,
copper, neodymium, or scandium or an alloy material containing any
of these materials as a main component can be used. The conductive
film for forming the conductive layer 211 can be formed with a
single film containing one or more of these materials or a stacked
film thereof.
[0141] As the conductive film for forming the conductive layer 211,
a stacked film of three layers in which a titanium layer, an
aluminum layer, and a titanium layer are stacked in this order, or
a stacked film of three layers in which a molybdenum layer, an
aluminum layer, and a molybdenum layer are stacked in this order is
preferably used. A single-layer film, a stacked film of two layers,
or a stacked film of four or more layers may also be used as the
conductive film. When a stacked conductive film of a titanium film,
an aluminum film, and a titanium film is used as the conductive
film, etching can be performed by a dry etching method with the use
of a chlorine gas.
[0142] Next, the insulating layer 202 is formed over the conductive
layer 211.
[0143] The insulating layer 202 can be formed to have a single
layer of a silicon oxide layer, a silicon nitride layer, a silicon
oxynitride layer, or a silicon nitride oxide layer or a stacked
layer thereof by a plasma CVD method, a sputtering method, or the
like. For example, a silicon oxynitride layer may be formed by a
plasma CVD method with the use of SiH.sub.4, oxygen, and nitrogen
as a deposition gas. The thickness of the insulating layer 202 is
greater than or equal to 100 nm and less than or equal to 500 nm;
in the case where the insulating layer 202 is formed using a
stacked layer, for example, a first insulating layer with a
thickness of greater than or equal to 50 nm and less than or equal
to 200 nm and a second insulating layer with a thickness of greater
than or equal to 5 nm and less than or equal to 300 nm are stacked.
When a silicon oxide film which is formed using a silicon target
doped with phosphorus or boron is used for the insulating layer
202, entry of impurities (such as moisture, hydrogen ions, and
OH.sup.-) can be suppressed.
[0144] In this embodiment, the insulating layer 202 is formed using
a silicon nitride film with a thickness of 200 nm by a plasma CVD
method as an example.
[0145] Next, an oxide semiconductor film is formed over the
insulating layer 202. The thickness of the oxide semiconductor film
is preferably greater than or equal to 2 nm and less than or equal
to 200 nm. For example, when the thickness of the oxide
semiconductor film is as small as 50 nm or less, the oxide
semiconductor film can be in an amorphous state even when heat
treatment for dehydration or dehydrogenation is performed after
formation of the oxide semiconductor film. By making the thickness
of the oxide semiconductor film small, crystallization of the oxide
semiconductor film can be suppressed when heat treatment is
performed after the formation of the oxide semiconductor film.
[0146] Note that before the oxide semiconductor film is formed by a
sputtering method, particles which are attached on a surface of the
insulating layer 202 may be removed by reverse sputtering in which
an argon gas is introduced and plasma is generated. The reverse
sputtering refers to a method in which, without application of a
voltage to a target side, an RF power source is used for
application of a voltage to a substrate side in an argon atmosphere
so that plasma is generated in the vicinity of the substrate to
modify a surface. Note that instead of an argon atmosphere, a
nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or
the like may be used.
[0147] As the oxide semiconductor film, any of the following can be
used: an In--Ga--Zn--O-based oxide semiconductor film, an
In--Sn--Zn--O-based oxide semiconductor film, an
In--Al--Zn--O-based oxide semiconductor film, an
Sn--Ga--Zn--O-based oxide semiconductor film, an
Al--Ga--Zn--O-based oxide semiconductor film, an
Sn--Al--Zn--O-based oxide semiconductor film, an In--Zn--O-based
oxide semiconductor film, an Sn--Zn--O-based oxide semiconductor
film, an Al--Zn--O-based oxide semiconductor film, an
In--Sn--O-based oxide semiconductor film, an In--O-based oxide
semiconductor film, an Sn--O-based oxide semiconductor film, and a
Zn--O-based oxide semiconductor film. In this embodiment, the oxide
semiconductor film is formed using an In--Ga--Zn--O-based metal
oxide semiconductor target by a sputtering method. Alternatively,
the oxide semiconductor film can be formed by a sputtering method
in a rare gas (typically, argon) atmosphere, an oxygen atmosphere,
or a mixed atmosphere of a rare gas (typically, argon) and oxygen.
In the case of employing a sputtering method, film deposition may
be performed using a target containing SiO.sub.2 at greater than or
equal to 2 wt % and less than or equal to 10 wt % and SiOx (x>0)
which hinders crystallization may be contained in the oxide
semiconductor film. Accordingly, crystallization of the oxide
semiconductor layer can be suppressed in heat treatment for
dehydration or dehydrogenation which is to be performed later.
[0148] Here, the oxide semiconductor film is formed using a metal
oxide semiconductor target for film deposition including In, Ga,
and Zn (at a composition ratio of
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 [mol], In:Ga:Zn=1:1:0.5
[at]) under the following condition: the distance between the
substrate and the target is 100 mm, the pressure is 0.6 Pa, the
direct-current (DC) power supply is 0.5 kW, and the atmosphere is
oxygen (the proportion of the oxygen flow is 100%). Note that a
pulse direct-current (DC) power supply is preferable because powder
substances (also referred to as a particle generated at the time of
film deposition can be reduced and the film thickness can be made
uniform. In this embodiment, as the oxide semiconductor film, an
In--Ga--Zn--O-based film is formed by a sputtering method with the
use of an In--Ga--Zn--O-based metal oxide semiconductor target for
film deposition.
[0149] As the composition ratio of the metal oxide semiconductor
target, In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:0.5 [mol],
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:0.25 [at],
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:2 [mol],
In.sub.2O.sub.3:Ga.sub.2O.sub.3:ZnO=1:1:1 [at], or the like can be
employed instead of the above-mentioned composition ratio.
[0150] Examples of a sputtering method include an RF sputtering
method in which a high-frequency power source is used as a
sputtering power source, a DC sputtering method, and a pulsed DC
sputtering method in which a bias is applied in a pulsed manner. An
RF sputtering method is mainly used in the case of forming an
insulating film, and a DC sputtering method is mainly used in the
case of forming a metal conductive film.
[0151] Further, there is also a multi-source sputtering apparatus
in which a plurality of targets of different materials can be set.
With the multi-source sputtering apparatus, films of different
materials can be formed to be stacked in the same chamber, or a
film of plural kinds of materials can be formed by electric
discharge at the same time in the same chamber.
[0152] Moreover, there are a sputtering apparatus provided with a
magnet system inside a chamber and used for a magnetron sputtering
method, and a sputtering apparatus used for an ECR sputtering
method in which plasma generated with the use of microwaves is used
without using glow discharge.
[0153] As a film deposition method employing a sputtering method,
there are a reactive sputtering method in which a target substance
and a sputtering gas component are chemically reacted with each
other during film formation to form a thin film of a compound
thereof, and a bias sputtering method in which voltage is also
applied to a substrate during film deposition.
[0154] As an evacuation means of the deposition chamber where
sputtering is performed, a cryopump is preferably used. When the
cryopump is used for evacuation, impurities such as moisture in the
deposition chamber can be removed.
[0155] Next, the oxide semiconductor film is processed into an
island shape by a second photolithography step to form the oxide
semiconductor layer 213 (see FIG. 11B). Note that after the second
photolithography step, the oxide semiconductor layer 213 may be
subjected to heat treatment (at higher than or equal to 400.degree.
C. and lower than 750.degree. C.) in an inert gas atmosphere (e.g.,
nitrogen, helium, neon, or argon) so that impurities such as
hydrogen and water contained in the layer are removed.
[0156] Next, the oxide semiconductor layer 213 is dehydrated or
dehydrogenated. First heat treatment for dehydration or
dehydrogenation is performed at a temperature higher than or equal
to 400.degree. C. and lower than 750.degree. C., preferably higher
than or equal to 425.degree. C. and lower than 750.degree. C. Note
that in the case of the temperature that is 425.degree. C. or
higher, the heat treatment time may be one hour or shorter, whereas
in the case of the temperature lower than 425.degree. C., the heat
treatment time is longer than one hour. In this embodiment, the
substrate is introduced into an electric furnace, which is one of
heat treatment apparatuses, and heat treatment is performed on the
oxide semiconductor layer 213 in a nitrogen atmosphere. Then, the
oxide semiconductor layer 213 is not exposed to air, which prevents
entry of water and hydrogen into the oxide semiconductor layer 213.
In this embodiment, slow cooling is performed from the heating
temperature at which the oxide semiconductor layer 213 is subjected
to dehydration or dehydrogenation to a temperature low enough to
prevent water from entering again, specifically to a temperature
that is lower than the heating temperature by 100.degree. C. or
more, in one furnace. The atmosphere is not limited to a nitrogen
atmosphere, and any atmosphere may be employed as long as it is an
inert gas atmosphere such as helium, neon, or argon.
[0157] Note that the heat treatment apparatus is not limited to an
electric furnace, and may have a device for heating an object by
heat conduction or heat radiation from a heating element such as a
resistance heating element. For example, a rapid thermal anneal
(RTA) apparatus such as a gas rapid thermal anneal (GRTA) apparatus
or a lamp rapid thermal anneal (LRTA) apparatus can be used. An
LRTA apparatus is an apparatus for heating an object to be
processed by radiation of light (an electromagnetic wave) emitted
from a lamp such as a halogen lamp, a metal halide lamp, a xenon
arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high
pressure mercury lamp. A GRTA apparatus is an apparatus for
performing heat treatment with a high-temperature gas. As the gas,
an inert gas which does not react with an object to be processed by
heat treatment is used. For example, nitrogen or a rare gas such as
argon is used.
[0158] When the oxide semiconductor layer 213 is subjected to heat
treatment at a temperature of higher than or equal to 400.degree.
C. and lower than 750.degree. C., the dehydration or
dehydrogenation of the oxide semiconductor layer can be achieved;
thus, water (H.sub.2O) can be prevented from being contained again
in the oxide semiconductor layer later.
[0159] In the first heat treatment, it is preferable that water,
hydrogen, and the like be not contained in nitrogen or a rare gas
such as helium, neon, or argon. Further, the purity of nitrogen or
a rare gas such as helium, neon, or argon which is introduced into
the heat treatment apparatus is preferably greater than or equal to
6N (99.9999%), more preferably greater than or equal to 7N
(99.99999%) (i.e., the impurity concentration is preferably less
than or equal to 1 ppm, more preferably less than or equal to 0.1
ppm).
[0160] The oxide semiconductor layer 213 includes microcrystalline
grains and an amorphous region, or includes only microcrystalline
grains, depending on conditions of the first heat treatment or a
material of the oxide semiconductor layer 213. For example, the
oxide semiconductor layer 213 may become a microcrystalline
semiconductor layer having a crystallinity of 90% or more, or 80%
or more. Further, depending on conditions of the first heat
treatment or a material of the oxide semiconductor layer 213, the
oxide semiconductor layer 213 may become an amorphous oxide
semiconductor layer containing no crystalline grains.
[0161] The oxide semiconductor layer 213 is changed to an
oxygen-deficient type after the first heat treatment so as to be
low-resistance. The oxide semiconductor film after the first heat
treatment has a higher carrier concentration than the oxide
semiconductor layer shortly after the formation and preferably has
a carrier concentration of 1.times.10.sup.18/cm.sup.3 or more.
[0162] Note that the conductive layer 211 becomes a
microcrystalline layer or a polycrystalline layer depending on the
conditions of the first heat treatment or the material of the
conductive layer 211 in some cases. For example, in the case of
using a film of an alloy of indium oxide and tin oxide as the
conductive layer 211, it is crystallized by the first heat
treatment at 450.degree. C. for one hour, whereas in the case of
using a film of an alloy of indium oxide and tin oxide containing
silicon oxide as the conductive layer 211, it is not
crystallized.
[0163] The first heat treatment can be performed on the oxide
semiconductor layer 213 which has not been processed into the
island-shaped oxide semiconductor layer. In that case, the
substrate is taken out of the heat treatment apparatus after the
first heat treatment, and then a photolithography step is
performed.
[0164] Next, a conductive film is formed over the insulating layer
202 and the oxide semiconductor layer 213.
[0165] For the conductive film, an element selected from titanium
(Ti), molybdenum (Mo), tungsten (W), aluminum (Al), chromium (Cr),
copper (Cu), and tantalum (Ta), an alloy containing any of these
elements as a component, a compound containing any of these
elements in combination, or the like is used. The conductive film
is not limited to a single layer containing the above element and
can be formed in a stacked layer of two or more layers. In this
embodiment, a three-layer conductive film in which a titanium film
(with a thickness of 100 nm), an aluminum film (with a thickness of
200 nm), and a titanium film (with a thickness of 100 nm) are
stacked is formed. Instead of a titanium film, a titanium nitride
film may be used.
[0166] In the case where heat treatment at 200.degree. C. to
600.degree. C. is performed later, it is preferable that the
conductive film have heat resistance high enough to withstand the
heat treatment. For example, it is preferable to use an aluminum
alloy to which an element for preventing hillocks is added or a
conductive film stacked with a heat-resistance conductive film. As
the formation method of the conductive film, a sputtering method, a
vacuum evaporation method (e.g., an electron beam evaporation
method), an arc discharge ion plating method, or a spray method is
used. Alternatively, the conductive film may be formed by
discharging a conductive nanopaste of silver, gold, copper, or the
like by a screen printing method, an ink-jet method, or the like
and baking the nanopaste.
[0167] Next, a third photolithography step is performed so that a
resist mask 233a and a resist mask 233b are formed. Then the
conductive film is selectively etched, so that the conductive layer
215a and the conductive layer 215b are formed (see FIG. 11C).
[0168] In the third photolithography step, only part of the
conductive film which is on the oxide semiconductor layer 213 is
selectively removed. For example, when an ammonia peroxide mixture
(hydrogen peroxide:ammonia:water=5:2:2 in a weight ratio) or the
like is used as an alkaline etchant in order to selectively remove
only part of the metal conductive film, which is on the
In--Ga--Zn--O-based oxide semiconductor layer, the metal conductive
film can be selectively removed, and the oxide semiconductor layer
formed of an oxide semiconductor can be left.
[0169] In the third photolithography step, an exposed region of the
oxide semiconductor layer 213 is etched in some cases depending on
an etching condition. In that case, the oxide semiconductor layer
in a region which is sandwiched between the conductive layer 215a
and the conductive layer 215b is thinner than the oxide
semiconductor layer in a region which overlaps with the conductive
layer 215a and the conductive layer 215b over the conductive layer
211.
[0170] Next, the oxide insulating layer 207 is formed over the
insulating layer 202 and the oxide semiconductor layer 213. At this
stage, part of the oxide semiconductor layer 213 is in contact with
the oxide insulating layer 207. Note that a region of the oxide
semiconductor layer, which overlaps with the conductive layer 211
with the insulating layer 202 therebetween, is a channel formation
region.
[0171] The oxide insulating layer 207 can be formed to have a
thickness of at least 1 nm by a method with which impurities such
as water and hydrogen are not mixed into the oxide insulating
layer, such as a sputtering method, as appropriate. In this
embodiment, a silicon oxide film is formed as the oxide insulating
layer by a sputtering method. The substrate temperature in
deposition may be higher than or equal to room temperature and
lower than or equal to 300.degree. C. The substrate temperature is
10 .degree. C. in this embodiment. The silicon oxide film can be
formed by a sputtering method in a rare gas (typically argon)
atmosphere, an oxygen atmosphere, or an atmosphere containing a
rare gas (typically argon) and oxygen. Moreover, a silicon oxide
target or a silicon target can be used as a target. For example,
with the use of a silicon target, a silicon oxide film can be
formed by a sputtering method in a mixed atmosphere containing
oxygen and a rare gas. The oxide insulating layer which is formed
to be in contact with the oxide semiconductor layer whose
resistance is reduced is formed using an inorganic insulating film
which does not contain impurities such as moisture, hydrogen ions,
and OH.sup.- and blocks entry of such impurities from the outside;
a silicon oxide film, a silicon nitride oxide film, an aluminum
oxide film, an aluminum oxynitride film, or the like is typically
used. Note that an oxide insulating layer formed by a sputtering
method is particularly dense, and even a single layer can be used
as a protective film for suppressing a phenomenon in which
impurities are diffused into a layer in contact therewith. Also, a
target doped with phosphorus (P) or boron (B) can be used so that
phosphorus (P) or boron (B) is added to the oxide insulating
layer.
[0172] In this embodiment, the film deposition is performed by a
pulsed DC sputtering method using a columnar polycrystalline,
boron-doped silicon target which has a purity of 6N (the
resistivity is 0.01 .OMEGA.cm), in which the distance between the
substrate and the target (T-S distance) is 89 mm, the pressure is
0.4 Pa, the direct-current (DC) power source is 6 kW, and the
atmosphere is oxygen (the proportion of the oxygen flow is 100%).
The film thickness is 300 nm.
[0173] The oxide insulating layer 207 is provided on the channel
formation region of the oxide semiconductor layer 213 and also
functions as a channel protective layer.
[0174] Next, second heat treatment (for example, at higher than or
equal to 200.degree. C. and lower than or equal to 400.degree. C.,
preferably, at higher than or equal to 250.degree. C. and lower
than or equal to 350.degree. C.) may be performed in an inert gas
atmosphere or a nitrogen atmosphere. For example, the second heat
treatment is performed in a nitrogen atmosphere at 250.degree. C.
for one hour. When the second heat treatment is performed, the
oxide semiconductor layer 213 is heated while part thereof is in
contact with the oxide insulating layer 207 and other parts thereof
are in contact with the conductive layer 215a and the conductive
layer 215b.
[0175] When the second heat treatment is performed while the oxide
semiconductor layer 213 whose resistance is reduced in the first
heat treatment is in contact with the oxide insulating layer 207,
the region in contact with the oxide insulating layer 207 becomes
in an oxygen-excess state. Accordingly, the region of the oxide
semiconductor layer 213, which is in contact with the oxide
insulating layer 207, increases resistance (becomes an i-type) in a
depth direction of the oxide semiconductor layer 213 (see FIG.
11D).
[0176] The timing of performing the second heat treatment is not
limited to the timing shortly after the third photolithography step
as long as it is after the third photolithography step.
[0177] Thus, the transistor illustrated in FIGS. 8A and 8B.
[0178] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 3
[0179] In this embodiment, another example of a transistor which is
applicable to the shift register described in Embodiment 1 and
different from the transistor described in Embodiment 2 will be
described.
[0180] An example of a structure of the transistor of this
embodiment will be described with reference to FIGS. 12A and 12B.
FIGS. 12A and 12B illustrate an example of a structure of the
transistor of this embodiment. FIG. 12A is a top diagram of the
transistor and FIG. 12B is a cross-sectional diagram taken along
line Z1-Z2 in FIG. 12A.
[0181] The transistor illustrated in FIGS. 12A and 12B includes the
conductive layer 211 over the substrate 201; the insulating layer
202 over the conductive layer 211; the conductive layers 215a and
215b over the insulating layer 202; the oxide semiconductor layer
213 over the insulating layer 202 and the conductive layers 215a
and 215b.
[0182] In the transistor, the conductive layer 211 serves as a gate
terminal, the insulating layer 202 serves as a gate insulating
layer, one of the conductive layers 215a and 215b serves as a
source terminal, and the other of the conductive layers 215a and
215b serves as a drain terminal. The oxide semiconductor layer 213
includes a channel formation region. Note that the oxide
semiconductor layer 213 is subjected to dehydration or
dehydrogenation treatment when the oxide semiconductor layer 213 is
formed.
[0183] Further, as for the transistor in FIGS. 12A and 12B, the
oxide semiconductor layer 213 is subjected to dehydration or
dehydrogenation treatment, and moreover, the oxide insulating layer
207 is formed to be in contact with part of the oxide semiconductor
layer 213. A transistor including the oxide semiconductor layer
213, which undergoes dehydration or dehydrogenation treatment and
then brought into contact with the oxide insulating layer 207
formed thereon, as a channel formation region has high reliability
because a threshold voltage (V-th) shift due to a long-term use or
high load hardly occurs.
[0184] Note that a nitride insulating layer may be provided over
the oxide insulating layer 207. It is preferable that the nitride
insulating layer be in contact with the insulating layer 202
provided below the oxide insulating layer 207 or an insulating
layer serving as a base, so that impurities such as moisture,
hydrogen ions, and OH.sup.- from the vicinity of side surfaces of
the substrate are prevented from entering. In particular, it is
effective to use a silicon nitride layer for the insulating layer
202 in contact with the oxide insulating layer 207 or the
insulating layer serving as a base. That is, when the silicon
nitride layers are provided so as to surround a lower surface, an
upper surface, and side surfaces of the oxide semiconductor layer
213, reliability of the transistor is improved.
[0185] Further, a planarization insulating layer may be provided
over the oxide insulating layer 207 (over the nitride insulating
layer in the case where the nitride insulating layer is
provided).
[0186] As in FIGS. 9A and 9B, the transistor 251 in FIGS. 12A and
12B may have a structure in which a conductive layer is provided in
a region which overlaps with the oxide semiconductor layer 213 over
the oxide insulating layer 207 (over the planarization insulating
layer in the case where the planarization insulating layer is
provided). The conductive layer serves as a second gate terminal.
Second gate voltage is applied to the conductive layer, whereby the
threshold voltage of the transistor can be controlled.
[0187] Note that the planarization insulating layer is not
necessarily provided. When the planarization insulating layer is
not provided, a conductive layer serving as the second gate
terminal can be formed over the oxide insulating layer 207 (over
the nitride insulating layer in the case where the nitride
insulating layer is formed).
[0188] For example, when the potential of the second gate terminal
is higher than the potential of the source terminal, the threshold
voltage of the transistor shifts in a negative direction. When the
potential of the second gate terminal is lower than the potential
of the source terminal, the threshold voltage of the transistor
shifts in a positive direction.
[0189] As illustrated in FIGS. 12A and 12B, the transistor of this
embodiment is a so-called bottom-contact transistor in which an
oxide semiconductor layer is provided over a conductive layer
serving as a source terminal or a drain terminal. Therefore,
high-speed operation can be performed since the transistor of this
embodiment has higher mobility than that of the conventional
transistor including amorphous silicon in a channel formation
region. Further, the bottom-contact transistor is applied, so that
an area where the oxide semiconductor layer is in contact with the
conductive layer serving as the source terminal or the drain
terminal can be increased, leading to prevention of peeling or the
like.
[0190] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 4
[0191] In this embodiment, another example of a transistor which is
applicable to the shift register described in Embodiment 1 and
different from the transistors described in Embodiments 2 and 3
will be described.
[0192] The example of the structure of the transistor of this
embodiment will be described with reference to FIGS. 13A and 13B.
FIGS. 13A and 13B illustrate an example of the structure of the
transistor of this embodiment. FIG. 13A is a top diagram and FIG.
13B is a cross-sectional diagram taken along line Z1-Z2 in FIG.
13A.
[0193] As in the transistors illustrated in FIGS. 8A to 8C, the
transistor illustrated in FIGS. 13A and 13B includes the conductive
layer 211 over the substrate 201, the insulating layer 202 over the
conductive layer 211, the oxide semiconductor layer 213 over the
insulating layer 202, the conductive layer 215a and the conductive
layer 215b over the oxide semiconductor layer 213.
[0194] In the transistor, the conductive layer 211 serves as a gate
terminal, the insulating layer 202 serves as a gate insulating
layer, one of the conductive layers 215a and 215b serves as a
source terminal, and the other of the conductive layers 215a and
215b serves as a drain terminal. The oxide semiconductor layer 213
includes a channel formation region. Note that the oxide
semiconductor layer 213 is subjected to dehydration or
dehydrogenation treatment when the oxide semiconductor layer 213 is
formed.
[0195] Further, as for the transistor illustrated in FIGS. 13A and
13B, the oxide semiconductor layer 213 is subjected to dehydration
or dehydrogenation treatment, and moreover, the oxide insulating
layer 207 is provided below the conductive layers 215a and 215b so
that the oxide insulating layer 207 is in contact with part of the
oxide semiconductor layer 213. The oxide insulating layer 207 in
FIGS. 13A and 13B has a function of a channel protective layer.
[0196] A nitride insulating layer may be provided over the oxide
insulating layer 207 and the conductive layers 215a and 215b. It is
preferable that the nitride insulating layer 207 be in contact with
the insulating layer 202 provided below the oxide insulating layer
207 or an insulating layer serving as a base, so that impurities
such as moisture, hydrogen ions, and OH.sup.- from the vicinity of
side surfaces of the substrate are prevented from entering. In
particular, it is effective to use a silicon nitride layer for the
insulating layer 202 in contact with the oxide insulating layer 207
or the insulating layer serving as a base. That is, when the
silicon nitride layers are provided so as to surround a lower
surface, an upper surface, and side surfaces of the oxide
semiconductor layer 213, reliability of the transistor is
improved.
[0197] Further, a planarization insulating layer may be provided
over the oxide insulating layer 207 and the conductive layers 215a
and 215b (over the nitride insulating layer in the case where the
nitride insulating layer is provided).
[0198] Further, a conductive layer may be provided over the oxide
insulating layer 207 (over the planarization insulating layer in
the case where the planarization insulating layer is provided) so
that the oxide insulating layer 207 is sandwiched between the
conductive layer and the oxide semiconductor layer 213. The
conductive layer serves as a second gate terminal. Second gate
voltage is applied to the conductive layer, whereby the threshold
voltage of the transistor 251 can be controlled.
[0199] Note that the planarization insulating layer is not
necessarily provided. When the planarization insulating layer is
not provided, a conductive layer can be formed over the oxide
insulating layer 207 (over the nitride insulating layer in the case
where the nitride insulating layer is formed).
[0200] For example, when the potential of the second gate terminal
is higher than the potential of the source terminal, the threshold
voltage of the transistor shifts in a negative direction. When the
potential of the second gate terminal is lower than the potential
of the source terminal, the threshold voltage of the transistor
shifts in a positive direction.
[0201] Further, as in the transistor in FIG. 8C, the transistor of
this embodiment may have a structure in which a pair of oxide
conductive layers serving as buffer layers is provided over parts
of the oxide semiconductor layer 213, and the conductive layers
215a and 215b which serve as a pair of electrodes are provided so
as to be in contact with the pair of the oxide conductive
layers.
[0202] As described above, the transistors of this embodiment are
so-called channel protective transistors each including an
insulating layer serving as a channel formation layer over part of
the oxide semiconductor layer. Accordingly, the transistors have
mobility higher than that of the conventional transistor including
amorphous silicon in a channel formation region, and thus
high-speed operation is possible.
[0203] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 5
[0204] In this embodiment, an example of a display device including
the shift register described in Embodiment 1 is described with
reference to FIGS. 14A to 14C.
[0205] A variety of display devices, such as a liquid crystal
display device or an electroluminescence (hereinafter also referred
to as EL) display device, is given as the display device including
the shift register described in Embodiment 1. A structure of a
display device in this embodiment is described with reference to
FIG. 14A. FIG. 14A is a block diagram illustrating a structure of
the display device in this embodiment.
[0206] The display device illustrated in FIG. 14A includes a pixel
portion 701, a scan line driver circuit 702, and a signal line
driver circuit 703.
[0207] The pixel portion 701 has a dot matrix structure in which a
plurality of pixels 704 is provided. Specifically, the plurality of
pixels 704 is arranged in the row and column directions. Each of
the pixels 704 is electrically connected to the scan line driver
circuit 702 through a scan line 705 and electrically connected to
the signal line driver circuit 703 through a signal line 706.
[0208] The scan line driver circuit 702 is a circuit for selecting
the pixel 704 to which a data signal is inputted, and outputs a
selection signal to the pixel 704 through the scan line 705. p The
signal line driver circuit 703 is a circuit for outputting data
written to the pixel 704 as a signal, and outputs pixel data as a
signal through the signal line 706 to the pixel 704 selected by the
scan line driver circuit 702.
[0209] The pixel 704 includes at least a display element and a
switching element. A liquid crystal element or a light-emitting
element such as an EL element can be applied to the display
element, for example. A transistor can be applied to the switching
element, for example.
[0210] Next, an example of structures of the scan line driver
circuit 702 and the signal line driver circuit 703 is described
with reference to FIGS. 14B and 14C. FIGS. 14B and 14C are block
diagrams illustrating a structure of the driver circuit. FIG. 14B
is a block diagram illustrating a structure of the scan line driver
circuit 702. FIG. 14C is a block diagram illustrating a structure
of the signal line driver circuit 703.
As illustrated in FIG. 14B, the scan line driver circuit 702
includes a shift register 900, a level shifter 901, and a buffer
902.
[0211] Signals such as a scan line driver circuit start pulse
signal (GSP) and a scan line driver circuit reference clock signal
(GCK) are inputted to the shift register 900, and selection signals
are sequentially output from sequential logic circuits. As
illustrated in Embodiment 1, the shift register 900 in this
embodiment includes a plurality of wirings for supplying scan line
driver circuit reference clock signals (GCK) in part of the
operation period.
[0212] As illustrated in FIG. 14C, the signal line driver circuit
703 includes a shift register 903, a first latch circuit 904, a
second latch circuit 905, a level shifter 906, and a buffer
907.
[0213] Signals such as a signal line driver circuit start pulse
signal (SSP) and a signal line driver circuit reference clock
signal (SCK) are inputted to the shift register 903, and selection
signals are sequentially output from sequential logic circuits. As
in Embodiment 1, the shift register 903 in this embodiment includes
a plurality of wirings by which signal line driver circuit
reference signals (SCK) are supplied in part of the operation
period.
[0214] Note that the shift register described in Embodiment 1 may
be employed for only one of the shift register 900 and the shift
register 903.
[0215] A data signal (DATA) is inputted to the first latch circuit
904. The first latch circuit 904 can be constituted using a logic
circuit.
[0216] The buffer 907 has a function of amplifying a signal and
includes an operational amplifier or the like. The buffer 907 can
be constituted using a logic circuit.
[0217] The second latch circuit 905 can hold a latch (LAT) signal
temporally and outputs the held latch signals all at once to the
pixel portion 701 in FIG. 14A. This is referred to as line
sequential driving. Therefore, in the case of using a pixel in
which not line sequential driving but dot sequential driving is
performed, the second latch circuit 905 is not necessary. The
second latch circuit 905 can be constituted using a logic
circuit.
[0218] Next, operation of the display device in this embodiment is
described.
[0219] First, the scan line 705 is selected by the scan line driver
circuit 702. A data signal is inputted from the signal line driver
circuit 703 to the pixel 704 electrically connected to the selected
scan line 705, via the signal line 706. Accordingly, data is
written to the pixel 704, and the pixel 704 enters into a display
state. The scan lines 705 are sequentially selected by the scan
line driver circuit 702; thus, data is written to all the pixels
704. The above is the operation of the display device in this
embodiment.
[0220] The circuits of the display device illustrated in FIGS. 14A
to 14C can be provided over one substrate. Further, the circuits of
the display device illustrated in FIGS. 14A to 14C can be
configured using transistors having the same conductivity type.
Providing the circuits over one substrate leads to the reduction in
the size of the display device, and using transistors having the
same conductivity type can simplify the process.
[0221] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 6
[0222] In this embodiment, as an example of the display device
described in Embodiment 5, a liquid crystal display device is
described with reference to FIGS. 15A to 15C.
[0223] FIG. 15A is a circuit diagram of a pixel included in the
liquid crystal display device in this embodiment. The pixel
illustrated in FIG. 15A includes a transistor 821, a liquid crystal
element 822, and a capacitor 823.
[0224] A gate terminal of the transistor 821 is electrically
connected to a scan line 804, and a first terminal of the
transistor 821 is electrically connected to a signal line 805. Note
that the transistor 821 serves as a selection transistor which
controls application of voltage to a liquid crystal element 822
included in the pixel.
[0225] One terminal of the liquid crystal element 822 is
electrically connected to a second terminal of the transistor 821
and the other terminal of the liquid crystal element 822 is
electrically connected to a wiring for supplying a common potential
(Vcom) (hereinafter also referred to as a common potential line).
The liquid crystal element 822 includes a first electrode which
serves as part or the whole of one terminal, a second electrode
which serves as part or the whole of the other terminal, and a
layer including liquid crystal molecules whose alignment is changed
by applying voltage between the first electrode and the second
electrode (such a layer is referred to as a liquid crystal
layer).
[0226] One terminal of the capacitor 823 is electrically connected
to the second terminal of the transistor 821 and the other terminal
of the capacitor 823 is electrically connected to the common
potential line. The capacitor 823 includes a first electrode which
serves as part or the whole of one terminal, a second electrode
which serves as part or the whole of the other terminal, and a
dielectric layer provided between the first electrode and the
second electrode. The capacitor 823 has a function of a storage
capacitor in a pixel. Note that although the capacitor 823 is not
necessarily provided, the provision of the capacitor 823 can reduce
adverse effects due to leakage current of the transistor 821.
[0227] As a method for driving liquid crystals of the liquid
crystal display device in this embodiment, a twisted nematic (TN)
mode, an in-plane-switching (IPS) mode, a fringe field switching
(FFS) mode, a multi-domain vertical alignment (MVA) mode, a
patterned vertical alignment (PVA) mode, an axially symmetric
aligned micro-cell (ASM) mode, an optically compensated
birefringence (OCB) mode, a ferroelectric liquid crystal (FLC)
mode, an antiferroelectric liquid crystal (AFLC) mode, and the like
are given.
[0228] Alternatively, blue-phase liquid crystal for which an
alignment film is not necessary may be used. The blue phase is a
kind of liquid crystal phase and appears just before phase
transition from a cholesteric phase to an isotropic phase when
temperature of cholesteric liquid crystal rises. Since the blue
phase appears only in a narrow temperature range, a liquid crystal
composition in which 5 wt. % or more of a chiral material is mixed
is used for the liquid crystal layer in order to improve the
temperature range. The liquid crystal composition which contains
blue-phase liquid crystal and the chiral material has the following
characteristics: the response speed is 10 .mu.s to 100 .mu.s, which
is short; alignment treatment is not necessary due to optical
isotropy; and viewing angle dependence is low.
[0229] When a signal is inputted to a pixel, first, a pixel to
which data is written is selected, and in the selected pixel, the
transistor 821 is turned on by a signal inputted from the scan line
804.
[0230] At this time, a data signal from the signal line 805 is
inputted to the pixel via the transistor 821, and the potential of
one terminal of the liquid crystal element 822 corresponds to the
potential of the data signal. Therefore, the alignment-state of the
liquid crystal element 822 is set depending on the voltage applied
between one terminal and the other terminal. After data writing,
the transistor 821 is turned off by a signal inputted from the scan
line 804, the alignment-state of the liquid crystal element 822 is
maintained during a display period, and the pixel enters into a
display state. The above operation is sequentially performed per
scan line 804, and in all the pixels included in the liquid crystal
display device.
[0231] In displaying moving images in a liquid crystal display
device, there is a problem in that an afterimage or motion blur
occurs because of slow response of liquid crystal molecules
themselves. In order to improve moving image characteristics of the
liquid crystal display device, there is a driving technique called
black insertion, in which the entire screen is displayed as black
every other frame.
[0232] Moreover, there is a driving technique called double-frame
rate driving, in which a vertical synchronizing frequency is 1.5
times or more, preferably 2 times or more as high as a normal
vertical synchronizing frequency.
[0233] Further, in order to improve the moving image
characteristics of the liquid crystal display device, there is a
driving technique in which a plurality of LED (light-emitting
diode) light sources, a plurality of EL light sources, or the like
are used as backlights to form an area light source, and the light
sources forming the area light source are independently lit
intermittently in one frame period. For the area light source, LEDs
of three kinds or more or an LED which emits white light may be
used. Since a plurality of LEDs can be independently controlled,
the timing when the LED emits light can be synchronized with the
timing when optical modulation of the liquid crystal layer is
changed. Part of the LEDs can be turned off in this driving
technique, so that power consumption can be reduced particularly in
the case of displaying an image in which a black display region
occupies a large area in one screen.
[0234] By combining these driving techniques, display
characteristics such as moving image characteristics of the liquid
crystal display device can be improved as compared to those of a
conventional liquid crystal display device.
[0235] Next, a structure of the display device in this embodiment,
which includes the above pixel, is described with reference to
FIGS. 15B and 15C. FIGS. 15B and 15C illustrate a structure of the
pixel in the display device in this embodiment. FIG. 15B is a top
diagram of the pixel, and FIG. 15C is a cross-sectional diagram
taken along line A1-A2 and line B1-B2 of FIG. 15B.
[0236] The liquid crystal display device illustrated in FIGS. 15B
and 15C includes, in the cross section A1-A2, a conductive layer
2001 provided over a substrate 2000; an insulating layer 2002
provided over the conductive layer 2001; an oxide semiconductor
layer 2003 provided over the insulating layer 2002; a pair of
conductive layers 2005a and 2005b provided over the oxide
semiconductor layer 2003; an oxide insulating layer 2007 provided
over the conductive layers 2005a and 2005b and the oxide
semiconductor layer 2003; and a transparent conductive layer 2020
which is in contact with the conductive layer 2005b through an
opening provided in the oxide insulating layer 2007.
[0237] Note that the conductive layer 2001 serves as a gate
terminal, the insulating layer 2002 serves as a gate insulating
layer, one of the conductive layers 2005a and 2005b serves as a
first terminal, and the other of the conductive layers 2005a and
2005b serves as a second terminal. Although the transistor
described in Embodiment 2 (see FIG. 8B) is employed here, the
transistor described in Embodiment 3 or 4 can be employed as the
transistor.
[0238] The liquid crystal display device illustrated in FIGS. 15B
and 15C includes, in the cross section B1-B2, a conductive layer
2008 provided over the substrate 2000; the insulating layer 2002
provided over the conductive layer 2008; the oxide insulating layer
2007 provided over the insulating layer 2002; and the transparent
conductive layer 2020 provided over the oxide insulating layer
2007.
[0239] Further, the liquid crystal display device in this
embodiment includes a conductive layer 2022 and a transparent
conductive layer 2029, a conductive layer 2023, a conductive layer
2024, and a transparent conductive layer 2028 which serve as an
electrode or a wiring for connecting flexible printed circuits
(FPC).
[0240] The transparent conductive layers 2020, 2029, and 2028 are
formed using indium oxide (In.sub.2O.sub.3), an alloy of indium
oxide and tin oxide (In.sub.2O.sub.3--SnO.sub.2, referred to as
ITO), or the like by a sputtering method, a vacuum evaporation
method, or the like. Such a material is etched with a hydrochloric
acid-based solution. Note that since etching of ITO particularly
tends to leave residue, an alloy of indium oxide and zinc oxide
(In.sub.2O.sub.3--ZnO) may be used in order to improve the etching
processability.
[0241] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 7
[0242] In this embodiment, as an example of the display device
described in Embodiment 5, a light-emitting display device
including a light-emitting element utilizing electroluminescence is
described with reference to FIGS. 16A and 16B and FIGS. 17A and
17B.
[0243] Light-emitting elements utilizing electroluminescence are
classified according to whether a light-emitting material is an
organic compound or an inorganic compound. In general, the former
is referred to as an organic EL element, and the latter as an
inorganic EL element.
[0244] In an organic EL element, by application of voltage to the
light-emitting element, electrons and holes are separately injected
from a pair of electrodes into a layer containing a light-emitting
organic compound, and there flows a current. Then, the carriers
(electrons and holes) are recombined, thereby emitting light. Based
on such a mechanism, such a light-emitting element is referred to
as a current-excitation light-emitting element.
[0245] Inorganic EL elements are classified, according to the
element structures, into a dispersion inorganic EL elements and
thin-film inorganic EL elements. A dispersion inorganic EL element
includes a light-emitting layer where particles of a light-emitting
material are dispersed in a binder, and its light emission
mechanism is donor-acceptor recombination light emission utilizing
a donor level and an acceptor level. A thin-film inorganic EL
element has a structure in which a light-emitting layer is
sandwiched between dielectric layers, which are further sandwiched
between electrodes, and its light emission mechanism is localized
light emission utilizing inner-shell electron transition of metal
ions. Note that, an organic EL element is described as a
light-emitting element here.
[0246] FIG. 16A is a circuit diagram illustrating a circuit
configuration of a pixel in the light-emitting display device in
this embodiment.
[0247] As illustrated in FIG. 16A, the pixel of the display device
in this embodiment includes a transistor 851, a capacitor 852
serving as a storage capacitor in the pixel, a transistor 853, and
a light-emitting element 854.
[0248] A gate terminal of the transistor 851 is electrically
connected to a scan line 855. A first terminal of the transistor
851 is electrically connected to a signal line 856.
[0249] One terminal of the capacitor 852 is electrically connected
to a second terminal of the transistor 851. The other terminal of
the capacitor 852 is electrically connected to the low power supply
potential line.
[0250] A gate terminal of the transistor 853 is electrically
connected to the second terminal of the transistor 851 and the one
terminal of the capacitor 852. A first terminal of the transistor
853 is electrically connected to the low power supply potential
line.
[0251] A first terminal of the light-emitting element 854 is
electrically connected to a second terminal of the transistor 853.
A second terminal of the light-emitting element 854 is electrically
connected to the high power supply potential line.
[0252] When a signal is inputted to a pixel, first, a pixel to
which data is written is selected. In the selected pixel, the
transistor 851 is turned on by a scan signal inputted from the scan
line 855, and a video signal (also referred to as a data signal)
which is a fixed voltage is inputted from the signal line 856 to
the gate terminal of the transistor 853.
[0253] The transistor 853 is turned on or off by a potential in
response to the data signal inputted to the gate terminal. At this
time, current flows depending on the voltage applied between one
terminal and the other terminal of the light-emitting element 854,
and the light-emitting element 854 emits light with luminance in
response to the amount of current flowing therethrough. Further,
the gate voltage of the transistor 853 is held for a certain period
of time by the capacitor 852; thus, the light-emitting element 854
maintains a light-emitting state for a certain period of time.
[0254] When the data signal inputted from the signal line 856 to
the pixel is digital, the light-emitting state of the pixel is
controlled by switching on and off of the transistor. Accordingly,
gradation can be expressed by an area ratio grayscale method or a
time ratio grayscale method. An area ratio grayscale method refers
to a driving method by which one pixel is divided into a plurality
of subpixels and each of the subpixels with the structure
illustrated in FIG. 16A is independently driven based on a data
signal so that gradation is expressed. Further, a time ratio
grayscale method refers to a driving method by which a period
during which a pixel is in a light-emitting state is controlled so
that gradation is expressed.
[0255] Since the response speed of light-emitting elements is
higher than that of liquid crystal elements or the like, the
light-emitting elements are suitable for a time ratio grayscale
method as compared to the liquid crystal elements. Specifically,
when display is performed by a time gray scale method, one frame
period is divided into a plurality of subframe periods. Then, in
accordance with video signals, the light-emitting state of the
light-emitting element is controlled in each subframe period.
By dividing one frame period into a plurality of subframe periods,
the total length of a period in which pixels actually emit light in
one frame period can be controlled with video signals, and
gradation can be expressed.
[0256] Next, a structure of a light-emitting element is described
with reference to FIGS. 16B to 16D. Here, a cross-sectional
structure of a pixel in the case where the transistor 853 is an
n-channel transistor is described as an example. Note that the
transistor 853 used in a light-emitting display device illustrated
in FIGS. 16B to 16D is a driving transistor.
[0257] In order to extract light emitted from the light-emitting
element 854, at least one of an anode and a cathode needs to be
transparent. A transistor and a light-emitting element are formed
over a substrate. There are light-emitting elements having a top
emission structure in which light is extracted through the surface
opposite to the substrate, having a bottom emission structure in
which light is extracted through the surface on the substrate side,
and having a dual emission structure in which light is extracted
through the surface on the substrate side and the surface opposite
to the substrate. The pixel structure of the present invention can
be applied to a light-emitting element having any of these emission
structures.
[0258] A light-emitting element having a top emission structure is
described with reference to FIG. 16B.
[0259] FIG. 16B is a cross-sectional diagram of a pixel in the case
where the transistor 853, which is a driving transistor, is an
n-channel transistor and light emitted from the light-emitting
element 854 passes through an anode 7005. In FIG. 16B, a cathode
7003 of the light-emitting element 854 and the transistor 853,
which is the driving transistor, are electrically connected to each
other, and a light-emitting layer 7004 and the anode 7005 are
sequentially stacked over the cathode 7003. As the cathode 7003,
any conductive film can be used as long as it has a low work
function and reflects light. For example, Ca, Al, CaF, MgAg, AlLi,
or the like is preferably used. The light-emitting layer 7004 may
be formed using a single layer or by stacking a plurality of
layers. When the light-emitting layer 7004 is formed using a
plurality of layers, the light-emitting layer 7004 is formed by
stacking an electron-injection layer, an electron-transport layer,
a light-emitting layer, a hole-transport layer, and a
hole-injection layer sequentially over the cathode 7003. Note that
it is not necessary to form all of these layers. The anode 7005 is
formed using a light-transmitting conductive material such as a
material of indium oxide containing tungsten oxide, indium zinc
oxide containing tungsten oxide, indium oxide containing titanium
oxide, indium tin oxide containing titanium oxide, indium tin oxide
(hereinafter also referred to as ITO), indium zinc oxide, or indium
tin oxide to which silicon oxide is added.
[0260] The light-emitting element 854 corresponds to a region where
the light-emitting layer 7004 is sandwiched between the cathode
7003 and the anode 7005. In the pixel illustrated in FIG. 16B,
light is emitted from the light-emitting element 854 to the anode
7005 side as shown by an arrow.
[0261] Next, a light-emitting element having a bottom emission
structure is described with reference to FIG. 16C. FIG. 16C is a
cross-sectional diagram of a pixel in the case where the transistor
853 is an n-channel transistor and light is emitted from the
light-emitting element 854 to a cathode 7013 side. In FIG. 16C, the
cathode 7013 of the light-emitting element 854 is formed over a
light-transmitting conductive layer 7017 which is electrically
connected to the transistor 853, and a light-emitting layer 7014
and an anode 7015 are sequentially stacked over the cathode 7013.
Note that when the anode 7015 has a light-transmitting property, a
light-blocking layer 7016 for reflecting or blocking light may be
formed so as to cover the anode 7015. As in the case of FIG. 16B, a
variety of materials can be used for the cathode 7013 as long as a
material is a conductive material having a low work function. Note
that the cathode 7013 has a thickness that can transmit light
(preferably has approximately 5 nm to 30 nm). For example, a
20-nm-thick aluminum layer can be used as the cathode 7013. The
light-emitting layer 7014 may be formed using a single layer or by
stacking a plurality of layers as in FIG. 16B. The anode 7015 is
not necessary to transmit light, but can be formed using a
light-transmitting conductive material as in FIG. 16B. The
light-blocking layer 7016 can be formed using, for example, metal
or the like which reflects light; however, one embodiment of the
present invention is not limited to metal. For example, a resin to
which a black pigment is added can be used.
[0262] The light-emitting element 854 corresponds to a region where
the light-emitting layer 7014 is sandwiched between the cathode
7013 and the anode 7015. In the pixel illustrated in FIG. 16C,
light is emitted from the light-emitting element 854 to the cathode
7013 side as shown by an arrow.
[0263] Next, a light-emitting element having a dual emission
structure is described with reference to FIG. 16D. In FIG. 16D, a
cathode 7023 of the light-emitting element 854 is formed over a
light-transmitting conductive layer 7027 which is electrically
connected to the transistor 853, and a light-emitting layer 7024
and an anode 7025 are sequentially stacked over the cathode 7023.
As in the case of FIG. 16B, a variety of materials can be used for
the cathode 7023 as long as a material is a conductive material
having a low work function. Note that the cathode 7023 has a
thickness that can transmit light. For example, a 20-nm-thick
aluminum layer can be used as the cathode 7023. The light-emitting
layer 7024 may be formed using a single layer or by stacking a
plurality of layers as in FIG. 16B. The anode 7025 can be formed
using a light-transmitting conductive material as in FIG. 16B.
[0264] The light-emitting element 854 corresponds to a region where
the cathode 7023, the light-emitting layer 7024, and the anode 7025
overlap with each other. In the pixel illustrated in FIG. 16D,
light is emitted from the light-emitting element 854 to both the
anode 7025 side and the cathode 7023 side as shown by arrows.
[0265] Note that, although the organic EL elements are described
here as the light-emitting elements, an inorganic EL element can
also be provided as a light-emitting element.
[0266] Note that in this embodiment, the example is described in
which a transistor (also referred to as a driving transistor) which
controls driving of a light-emitting element is electrically
connected to the light-emitting element; alternatively, a structure
may be employed in which a transistor for current control is
connected between the driving transistor and the light-emitting
element.
[0267] Next, the appearance and cross section of the light-emitting
display device (also referred to as a light-emitting panel) in this
embodiment are described with reference to FIGS. 17A and 17B. FIG.
17A is a top diagram of the light-emitting display device, in which
a transistor and a light-emitting element formed over a first
substrate are sealed between the first substrate and a second
substrate by a sealing material. FIG. 17B is a cross-sectional
diagram taken along line H-I of FIG. 17A.
[0268] A sealing material 4505 is provided so as to surround a
pixel portion 4502, a signal line driver circuits 4503a and 4503b,
and scan line driver circuits 4504a and 4504b which are provided
over a first substrate 4501. Moreover, a second substrate 4506 is
provided over the pixel portion 4502, the signal line driver
circuits 4503a and 4503b, and the scan line driver circuits 4504a
and 4504b. Accordingly, the pixel portion 4502, the signal line
driver circuits 4503a and 4503b, and the scan line driver circuits
4504a and 4504b are sealed, together with a filler 4507, by the
first substrate 4501, the sealing material 4505, and the second
substrate 4506. In such a manner, it is preferable to pack (seal)
the pixel portion 4502, the signal line driver circuits 4503a and
4503b, and the scan line driver circuits 4504a and 4504b with a
protective film (such as an attachment film or an ultraviolet
curable resin film) or a cover material with high air-tightness and
little degasification so that the pixel portion 4502, the signal
line driver circuits 4503a and 4503b, and the scan line driver
circuits 4504a and 4504b are not exposed to the air.
[0269] The pixel portion 4502, the signal line driver circuits
4503a and 4503b, and the scan line driver circuits 4504a and 4504b,
which are formed over the first substrate 4501, each include a
plurality of transistors. In FIG. 17B, a transistor 4510 included
in the pixel portion 4502 and transistors 4509 and 4555 included in
the signal line driver circuit 4503a are illustrated as an
example.
[0270] As the transistors 4509, 4510, and 4555, any of the highly
reliable transistors described in Embodiments 2 to 4 which include
an oxide semiconductor layer as a semiconductor layer, can be used.
In this embodiment, the transistors 4509, 4510, and 4555 are
n-channel transistors. An insulating layer 4542 is formed over the
transistors 4509, 4510, and 4555. An insulating layer 4544 is
formed over the insulating layer 4542. A conductive layer 4540 is
formed over the transistor 4509 with the insulating layers 4542 and
4544 provided therebetween. The conductive layer 4540 has a
function of a second gate terminal.
[0271] Note that in the pixel portion 4502, a planarization
insulating layer 4545 is provided over the insulating layer 4542,
and an insulating layer 4543 is provided over the planarization
insulating layer 4545.
[0272] Moreover, reference numeral 4511 denotes a light-emitting
element. A first electrode 4517 which is a pixel electrode included
in the light-emitting element 4511 is electrically connected to a
second terminal of the transistor 4510. Note that the
light-emitting element 4511 has a stacked structure of the first
electrode 4517, a light-emitting layer 4512, and a second electrode
4513; however, the structure of the light-emitting element is not
limited to that shown in this embodiment. The structure of the
light-emitting element 4511 can be changed as appropriate depending
on the direction in which light is extracted from the
light-emitting element 4511, or the like.
[0273] A bank 4520 is formed using an organic resin film, an
inorganic insulating film, or organic polysiloxane. In particular,
it is preferable that the bank 4520 be formed using a
photosensitive material to have an opening portion over the first
electrode 4517, and a sidewall of the opening portion be formed as
an inclined surface with a continuous curvature.
[0274] The light-emitting layer 4512 may be formed using a single
layer or a stack of a plurality of layers.
[0275] In order to prevent oxygen, hydrogen, moisture, carbon
dioxide, or the like from entering the light-emitting element 4511,
a protective layer may be formed over the second electrode 4513 and
the bank 4520. As the protective layer, a silicon nitride layer, a
silicon nitride oxide layer, a DLC (diamond like carbon) layer, or
the like can be formed.
[0276] Further, a variety of signals and voltages are supplied to
the signal line driver circuits 4503a and 4503b, the scan line
driver circuits 4504a and 4504b, or the pixel portion 4502 from
FPCs 4518a and 4518b.
[0277] In the light-emitting display device illustrated in FIGS.
17A and 17B, a connection terminal electrode 4515 is formed using
the same conductive film as the conductive film for forming the
first electrode 4517 included in the light-emitting element 4511. A
terminal electrode 4516 is formed using the same conductive film as
the conductive film for forming the source electrodes and the drain
electrodes of the transistors 4509, 4510, and 4555.
[0278] The connection terminal electrode 4515 is electrically
connected to a terminal of the FPC 4518a through an anisotropic
conductive layer 4519.
[0279] The substrate located in the direction in which light is
extracted from the light-emitting element 4511 needs to have a
light-transmitting property. In that case, a light-transmitting
material such as a glass, a plastic, a polyester film, or an
acrylic film is used as the substrate.
[0280] As the filler 4507, an ultraviolet curable resin or a
thermosetting resin can be used other than an inert gas such as
nitrogen or argon. For example, polyvinyl chloride (PVC), acrylic,
polyimide, an epoxy resin, a silicone resin, polyvinyl butyral
(PVB), or ethylene vinyl acetate (EVA) can be used. In this
embodiment, nitrogen is used for the filler 4507.
[0281] If necessary, an optical film such as a polarizing plate, a
circularly polarizing plate (including an elliptically polarizing
plate), a retardation plate (a quarter-wave plate or a half-wave
plate), or a color filter may be provided as appropriate on a
light-emitting surface of the light-emitting element. Further, a
polarizing plate or a circularly polarizing plate may be provided
with an anti-reflection film. Further, a polarizing plate or a
circularly polarizing plate may be provided with an anti-reflection
film.
[0282] As the signal line driver circuits 4503a and 4503b and the
scan line driver circuits 4504a and 4504b, a driver circuit formed
using a single crystal semiconductor layer or a polycrystalline
semiconductor layer may be mounted on a substrate separately
prepared. Alternatively, only the signal line driver circuits 4503a
and 4503b or part thereof, or the scan line driver circuits 4504a
and 4504b, or part thereof may be separately formed to be mounted.
This embodiment is not limited to the structure in FIGS. 17A and
17B.
[0283] Through the above steps, a light-emitting display device
(display panel) can be manufactured.
[0284] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 8
[0285] In this embodiment, as an example of the display device
described in Embodiment 5, electronic paper which can display
images without an external connection line such as an FPC is
described with reference to FIGS. 18A to 18C and FIG. 19.
[0286] Note that the electronic paper in this embodiment has a
period for holding an image (an image holding period) and a period
for rewriting an image (an image rewriting period). In the image
holding period, electronic power for holding the image is not
needed. Accordingly, the electronic paper is a display device with
less power consumption.
[0287] The electronic paper includes a display element; display can
be controlled by application of voltage to the display element and
can be maintained in a state where voltage is not applied to the
display element. Examples of the display element include an element
using electrophoresis (an electrophoretic element), a particle
rotation type element using a twisting ball, a particle transfer
type element using a charged toner or Electronic Liquid Powder
(registered trademark), a magnetophoretic element in which
gradation is expressed by magnetism, a liquid transfer type
element, a light-scattering element, and a phase-change element. In
this embodiment, electronic paper including an electrophoretic
element is described as an example of electronic paper.
[0288] An example of the electrophoretic elements is an element
having a microcapsule containing positively charged first
particles, negatively charged second particles which exhibit color
different from that of the first particles, and liquid serving as a
solvent. By application of voltage to the electrophoretic element,
the first particles or the second particles are collected on one
side of the microcapsule; thus, display can be performed. Note that
in a state where voltage is not applied to the electrophoretic
element, the first particles and the second particles are not
transferred. That is, the display of the electrophoretic element is
maintained. Alternatively, as the electrophoretic element, an
element having a microcapsule containing positively or negatively
charged particles and liquid which exhibits color different from
that of the particles and serves as a solvent can be used, for
example.
[0289] Note that for the positively or negatively charged particles
in the microcapsule, one or a composite material of a conductive
material, an insulating material, a semiconductor material, a
magnetic material, a liquid crystal material, a ferroelectric
material, an electroluminescent material, an electrochromic
material, or a magnetophoretic material may be used.
[0290] Next, an example of a structure of the electronic paper in
this embodiment is described with reference to FIGS. 18A to 18C.
Note that FIG. 18A is a circuit diagram of a pixel of the
electronic paper, FIG. 18B is a top diagram of the pixel, and FIG.
18C is a cross-sectional diagram taken along line A-B of FIG.
18B.
[0291] The pixel of the electronic paper in this embodiment
includes a transistor 601, a capacitor 602, and an electrophoretic
element 603 (see FIG. 18A). A gate terminal of the transistor 601
is electrically connected to a scan line 630. A first terminal of
the transistor 601 is electrically connected to a signal line 631.
One terminal of the capacitor 602 is electrically connected to a
second terminal of the transistor 601. The other terminal of the
capacitor 602 is electrically connected to a common potential line.
One terminal of the electrophoretic element 603 is electrically
connected to the second terminal of the transistor 601 and one
terminal of the capacitor 602. The other terminal of the
electrophoretic element 603 is electrically connected to the common
potential line. Note that in this embodiment, a ground potential,
0V, or the like is given as a common potential (V.sub.com).
[0292] The pixel includes a substrate 600, the transistor 601 and
the capacitor 602 which are provided over the substrate 600, the
electrophoretic element 603 provided over the transistor 601 and
the capacitor 602, and a substrate 604 provided over the
electrophoretic element 603 (see FIGS. 18B and 18C). Note that the
electrophoretic element 603 is not illustrated in FIG. 18B.
[0293] The transistor 601 includes a conductive layer 610 which is
electrically connected to the scan line 630, an insulating layer
611 over the conductive layer 610, a semiconductor layer 612 over
the insulating layer 611, and conductive layers 613 and 614 which
are electrically connected to the signal line 631 over the
semiconductor layer 612. Note that the conductive layer 610 serves
as a gate terminal, the insulating layer 611 serves as a gate
insulating layer, the conductive layer 613 serves as a first
terminal, and the conductive layer 614 serves as a second terminal.
Further, the conductive layer 610 and the conductive layer 613 can
be expressed as part of the scan line 630 and part of the signal
line 631, respectively.
[0294] The capacitor 602 includes the conductive layer 614, the
insulating layer 611, and a conductive layer 615 which is
electrically connected to a common potential line 632. Note that
the conductive layer 614 serves as one terminal, the insulating
layer 611 serves as a dielectric, and the conductive layer 615
serves as the other terminal. The conductive layer 615 can be
expressed as part of the common potential line 632.
[0295] The electrophoretic element 603 includes a pixel electrode
616 which is electrically connected to the conductive layer 614 in
an opening portion provided in an insulating layer 620, a counter
electrode 617 to which the same potential as that of the conductive
layer 615 is applied, and a layer 618 which includes charged
particles and is provided between the pixel electrode 616 and the
counter electrode 617. Note that the pixel electrode 616 serves as
one terminal and the counter electrode 617 serves as the other
terminal.
[0296] In the electronic paper in this embodiment, voltage applied
to the layer 618 including the charged particles is controlled;
thus, transfer of the charged particles which are dispersed in the
layer 618 can be controlled. The counter electrode 617 and the
substrate 604 of the electronic paper in this embodiment have a
light-transmitting property. That is, the display device in this
embodiment is a reflective display device in which the substrate
604 side corresponds to a display surface.
[0297] Materials which can be used for components of the electronic
paper in this embodiment are given below.
[0298] Examples of the substrate 600 include a semiconductor
substrate (e.g., a single crystal substrate or a silicon
substrate), an SOI substrate, a glass substrate, a quartz
substrate, a conductive substrate whose top surface is provided
with an insulating layer, flexible substrates such as a plastic
substrate, a bonding film, paper containing a fibrous material, and
a base film. As examples of the glass substrate, there are a barium
borosilicate glass substrate, an aluminoborosilicate glass
substrate, and a soda lime glass substrate. As examples of the
flexible substrate, there are flexible synthetic resins such as
plastics typified by polyethylene terephthalate (PET), polyethylene
naphthalate (PEN), polyether sulfone (PES), and acrylic.
[0299] As the conductive layers 610 and 615, the scan line 630, and
the common potential line 632, an element selected from aluminum
(Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten (W),
molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc),
an alloy containing any of these elements, or a nitride containing
any of these elements can be used. A stacked structure of these
materials can also be used.
[0300] As the insulating layer 611, an insulator such as silicon
oxide, silicon nitride, silicon oxynitride, silicon nitride oxide,
aluminum oxide, or tantalum oxide can be employed. A stacked
structure of these materials can also be used. Note that silicon
oxynitride refers to a substance which contains more oxygen than
nitrogen and contains oxygen, nitrogen, silicon, and hydrogen at
given concentrations ranging from 55 atomic % to 65 atomic %, 1
atomic % to 20 atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic
% to 10 atomic %, respectively, where the total percentage of atoms
is 100 atomic %. Further, silicon nitride oxide refers to a
substance which contains more nitrogen than oxygen and contains
oxygen, nitrogen, silicon, and hydrogen at given concentrations
ranging from 15 atomic % to 30 atomic %, 20 atomic % to 35 atomic
%, 25 atomic % to 35 atomic %, and 15 atomic % to 25 atomic %,
respectively, where the total percentage of atoms is 100 atomic
%.
[0301] The semiconductor layer 612 can be formed using any of the
following semiconductor material, for example: a material
containing an element belonging to Group 14 of the periodic table,
such as silicon (Si) or germanium (Ge), as its main component; a
compound such as silicon germanium (SiGe) or gallium arsenide
(GaAs); oxide such as zinc oxide (ZnO) or zinc oxide containing
indium (In) and gallium (Ga); or an organic compound exhibiting
semiconductor characteristics can be used. A stacked structure of
layers formed using these semiconductor materials can also be
used.
[0302] As the conductive layers 613 and 614, and the signal line
631, an element selected from aluminum (Al), copper (Cu), titanium
(Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr),
neodymium (Nd), and scandium (Sc), an alloy containing any of these
elements, or a nitride containing any of these elements can be
used. A stacked structure of these materials can also be used.
[0303] As the insulating layer 620, an insulator such as silicon
oxide, silicon oxynitride, silicon nitride, silicon nitride oxide,
aluminum oxide, or tantalum oxide can be used. Further, the
insulating layer 620 can also be formed using an organic material
such as polyimide, polyamide, polyvinylphenol, benzocyclobutene,
acrylic, or epoxy; a siloxane material such as a siloxane resin; an
oxazole resin; or the like. Note that a siloxane material
corresponds to a material having a Si--O--Si bond. Siloxane has a
skeleton structure with a bond of silicon (Si) and oxygen (O). As a
substituent, an organic group (e.g., an alkyl group or aromatic
hydrocarbon) or a fluoro group may be used. The organic group may
include a fluoro group.
[0304] As the pixel electrode 616, an element selected from
aluminum (Al), copper (Cu), titanium (Ti), tantalum (Ta), tungsten
(W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium
(Sc), an alloy containing any of these elements, or a nitride
containing any of these elements can be used. A stacked structure
of these materials can also be used. Further, a light-transmitting
conductive material such as indium oxide containing tungsten oxide,
indium zinc oxide containing tungsten oxide, indium oxide
containing titanium oxide, indium tin oxide containing titanium
oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to
which silicon oxide is added can be used.
[0305] As the charged particles contained in the layer 618
including charged particles, titanium oxide or the like can be used
as positively charged particles, and carbon black or the like can
be used as negatively charged particles. In addition, a single
material selected from a conductive material, an insulating
material, a semiconductor material, a magnetic material, a liquid
crystal material, a ferroelectric material, an electroluminescent
material, an electrochromic material, or a magnetophoretic
material, or a composite material formed using any of these
materials can also be used.
[0306] As the counter electrode 617, a light-transmitting
conductive material such as indium oxide containing tungsten oxide,
indium zinc oxide containing tungsten oxide, indium oxide
containing titanium oxide, indium tin oxide containing titanium
oxide, indium tin oxide, indium zinc oxide, or indium tin oxide to
which silicon oxide is added can be used.
[0307] As the substrate 604, a light-transmitting substrate can be
used. The light-transmitting substrate is typified by a glass
substrate formed using barium borosilicate glass,
aluminoborosilicate glass, soda-lime glass, or the like; or a
flexible substrate formed using polyethylene terephthalate (PET) or
the like.
[0308] The electronic paper in this embodiment can be used in
electronic appliances of various fields, which display information.
For example, the electronic paper can be applied to e-book readers
(electronic books), posters, advertisements on vehicles such as
trains, or displays on a variety of cards such as credit cards. An
example of such an electronic appliance is illustrated in FIG. 19.
FIG. 19 illustrates an example of an e-book reader.
[0309] As illustrated in FIG. 19, an e-book reader 2700 has two
housings 2701 and 2703. The housings 2701 and 2703 are united with
an axis portion 2711, and the e-book reader 2700 can be opened and
closed with the axis portion 2711 as an axis. With such a
structure, the e-book reader 2700 can be operated like a paper
book.
[0310] A display portion 2705 is incorporated into the housing
2701. A display portion 2707 is incorporated into the housing 2703.
The display portions 2705 and 2707 may display one image or
different images. When the display portions display different
images, text can be displayed on the right display portion (the
display portion 2705 in FIG. 19) and an image can be displayed on
the left display portion (the display portion 2707 in FIG. 19), for
example.
[0311] Further, FIG. 19 illustrates an example where the housing
2701 is provided with an operation portion and the like. For
example, the housing 2701 is provided with a power supply switch
2721, operation keys 2723, a speaker 2725, and the like. Pages can
be turned by the operation key 2723. Note that a keyboard, a
pointing device, or the like may be provided on the same side as
the display portion in the housing. Moreover, a terminal for
external connection (e.g., an earphone terminal, a USB terminal,
and a terminal capable of connecting a variety of cables such as an
AC adapter and a USB cable), a portion for inserting recording
media, or the like may be provided on a rear surface or a side
surface of the housing. Furthermore, the e-book reader 2700 may
functions as an electronic dictionary.
[0312] In addition, the e-book reader 2700 may wirelessly transmit
and receive information. The e-book reader 2700 can have a
structure where desired book data or the like is wirelessly
purchased and downloaded from an e-book server.
[0313] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
Embodiment 9
[0314] The display devices described in Embodiments 5 to 8 can be
applied to a variety of electronic appliances (including amusement
machines). Examples of electronic appliances are television devices
(also referred to as televisions or television receivers), monitors
for computers and the like, cameras such as digital cameras and
digital video cameras, digital photo frames, mobile phone devices
(also referred to as mobile phones or cellular phones), portable
game machines, portable information terminals, sound reproducing
devices, and large game machines such as pachinko machines.
[0315] FIG. 20A illustrates an example of a television device. In a
television device 9600, a display portion 9603 is incorporated into
a housing 9601. The display portion 9603 can display an image.
Further, the housing 9601 is supported by a stand 9605 here.
[0316] The television device 9600 can be operated with an operation
switch of the housing 9601 or a separate remote controller 9610.
Channels and volume can be controlled with an operation key 9609 of
the remote controller 9610 so that an image displayed on the
display portion 9603 can be controlled. Further, the remote
controller 9610 may be provided with a display portion 9607 for
displaying data output from the remote controller 9610.
[0317] Note that the television device 9600 is provided with a
receiver, a modem, and the like. With the receiver, general
television broadcast can be received. Further, when the television
device 9600 is connected to a communication network by wired or
wireless connection via the modem, one-way (from a transmitter to a
receiver) or two-way (between a transmitter and a receiver or
between receivers) data communication can be performed.
[0318] FIG. 20B illustrates an example of a digital photo frame.
For example, in a digital photo frame 9700, a display portion 9703
is incorporated into a housing 9701. The display portion 9703 can
display a variety of images. For example, the display portion 9703
can display data of an image taken with a digital camera or the
like and function as a normal photo frame.
[0319] Note that the digital photo frame 9700 is provided with an
operation portion, an external connection portion (e.g., a USB
terminal, or a terminal which can be connected to various cables
such as a USB cable), a recording medium insertion portion, and the
like. Although these components may be provided on the surface on
which the display portion is provided, it is preferable to provide
them on the side surface or the rear surface for the design of the
digital photo frame 9700. For example, a memory storing data of an
image taken with a digital camera is inserted in the recording
medium insertion portion of the digital photo frame, and the image
data can be transferred and then displayed on the display portion
9703.
[0320] Further, the digital photo frame 9700 may be configured to
transmit and receive data wirelessly. The structure may be employed
in which desired image data is transferred wirelessly to be
displayed.
[0321] FIG. 21A is a portable game machine and includes two
housings of a housing 9881 and a housing 9891, which are connected
with a joint portion 9893 so that the portable game machine can be
opened and folded. A display portion 9882 is incorporated into the
housing 9881, and a display portion 9883 is incorporated into the
housing 9891. Moreover, the portable game machine illustrated in
FIG. 21A is provided with a speaker 9884, a recording medium
insertion portion 9886, an LED lamp 9890, input means (operation
keys 9885, a connection terminal 9887, a sensor 9888 (having a
function of measuring force, displacement, position, speed,
acceleration, angular velocity, rotation number, distance, light,
liquid, magnetism, temperature, chemical substance, sound, time,
hardness, electric field, current, voltage, electric power, radial
ray, flow rate, humidity, gradient, vibration, odor, or infrared
ray), and a microphone 9889), and the like. It is needless to say
that the structure of the portable game machine is not limited to
that described above. The portable game machine may have a
structure in which additional accessory equipment is provided as
appropriate as long as at least a display device is provided. The
portable game machine in FIG. 21A has a function of reading a
program or data stored in a recording medium to display it on the
display portion, and a function of sharing information with another
portable game machine by wireless communication. Note that a
function of the portable game machine in FIG. 21A is not limited to
those described above, and the portable game machine can have a
variety of functions.
[0322] FIG. 21B illustrates an example of a slot machine, which is
a large amusement machine. In a slot machine 9900, a display
portion 9903 is incorporated into a housing 9901. Moreover, the
slot machine 9900 is provided with operation means such as a start
lever and a stop switch, a coin slot, a speaker, and the like.
Needless to say, the structure of the slot machine 9900 is not
limited to the above structure. The slot machine may have a
structure in which additional accessory equipment is provided as
appropriate as long as at least the display device described in any
of the above embodiments is provided.
[0323] FIG. 22A illustrates an example of a mobile phone. A mobile
phone 9000 is provided with a display portion 9002 incorporated
into a housing 9001, an operation button 9003, an external
connection port 9004, a speaker 9005, a microphone 9006, and the
like.
[0324] When the display portion 9002 of the mobile phone 9000
illustrated in FIG. 22A is touched with a finger or the like, data
can be inputted into the mobile phone 9000. Further, operation such
as making calls and texting can be performed by touching the
display portion 9002 with a finger or the like.
[0325] There are mainly three screen modes of the display portion
9002. The first mode is a display mode mainly for displaying an
image. The second mode is an input mode mainly for inputting data
such as text. The third mode is a display-and-input mode, which is
a combination of the two modes, that is, a combination of the
display mode and the input mode.
[0326] For example, in the case of making a call or texting, a text
input mode mainly for inputting text is selected for the display
portion 9002 so that characters displayed on a screen can be
inputted. In that case, it is preferable to display a keyboard or
number buttons on almost all area of the screen of the display
portion 9002.
[0327] When a detection device including a sensor for detecting
inclination, such as a gyroscope or an acceleration sensor, is
provided inside the mobile phone 9000, display on the screen of the
display portion 9002 can be automatically changed by determining
the orientation of the mobile phone 9000 (whether the mobile phone
9000 stands upright or is laid down on its side).
[0328] The screen modes are changed by touching the display portion
9002 or using the operation button 9003 of the housing 9001.
Alternatively, the screen modes may be changed depending on the
kind of the image displayed on the display portion 9002. For
example, when a signal of an image displayed on the display portion
9002 is data of moving images, the screen mode is changed to the
display mode. When the signal is text data, the screen mode is
changed to the input mode.
[0329] Further, in the input mode, when input by touching the
display portion 9002 is not performed for a certain period while a
signal detected by an optical sensor in the display portion 9002 is
detected, the screen mode may be controlled so as to be changed
from the input mode to the display mode.
[0330] The display portion 9002 can also function as an image
sensor. For example, an image of a palm print, a fingerprint, or
the like is taken when the display portion 9002 is touched with a
palm or a finger, whereby personal identification can be performed.
Further, when a backlight or a sensing light source which emits
near-infrared light is provided in the display portion, an image of
a finger vein, a palm vein, or the like can be taken.
[0331] FIG. 22B illustrates another example of a mobile phone. The
mobile phone in FIG. 22B includes a display device 9410 in a
housing 9411, which includes a display portion 9412 and operation
buttons 9413; and a communication device 9400 in a housing 9401,
which includes scan buttons 9402, an external input terminal 9403,
a microphone 9404, a speaker 9405, and a light-emitting portion
9406 that emits light when receiving a call. The display device
9410 having a display function can be detached from and attached to
the communication device 9400 having a telephone function in two
directions shown by arrows. Accordingly, short axes of the display
device 9410 and the communication device 9400 can be attached to
each other, or long axes of the display device 9410 and the
communication device 9400 can be attached to each other. Further,
when only a display function is necessary, the display device 9410
may be detached from the communication device 9400 so that the
semiconductor device 9410 can be used by itself. The communication
device 9400 and the display device 9410 can transmit and receive
images or input information to/from each other by wireless
communication or wired communication, and each of the communication
device 9400 and the display device 9410 has a rechargeable
battery.
[0332] Note that this embodiment or part of this embodiment can be
freely combined with the other embodiments or part of the other
embodiments.
This application is based on Japanese Patent Application serial no.
2009-234845 filed with Japan Patent Office on Oct. 9, 2009, the
entire contents of which are hereby incorporated by reference.
EXPLANATION OF REFERENCE
[0333] 101: transistor, 102: transistor, 103: transistor, 104:
transistor, 105: transistor, 106: transistor, 111: clock signal
selection transistor, transistor, 112: clock signal selection
transistor, transistor, 113: clock signal selection transistor,
114: clock signal selection transistor, 115: clock signal selection
transistor, 116: clock signal selection transistor, 121: low power
supply potential selection transistor, 122: low power supply
potential selection transistor, 123: low power supply potential
selection transistor, 124: low power supply potential selection
transistor, 125: low power supply potential selection transistor,
126: low power supply potential selection transistor, 131:
transistor, 132: transistor, 133: transistor, 134: transistor, 201:
substrate, 202: insulating layer, 207: oxide semiconductor layer,
211: conductive layer, 213: oxide semiconductor layer, 214a: oxide
conductive layer, 214b: oxide conductive layer, 215a: conductive
layer, 215b: conductive layer, 215c: conductive layer, 217:
conductive layer, 233a: resist mask, 233b: resist mask, 251:
transistor, 252: transistor, 600: substrate, 601: transistor, 602:
capacitor, 603: electrophoretic element, 604: substrate, 610:
conductive layer, 611: insulating layer, 612: semiconductor layer,
613: conductive layer, 614: conductive layer, 615: conductive
layer, 616: pixel electrode, 617: counter electrode, 618: layer
including charged particles, 620: insulating layer, 630: scan line,
631: signal line, 632: common potential line, 701: pixel portion,
702: scan line driver circuit, 703: signal line driver circuit,
704: pixel, 705: scan line, 706: signal line, 804: scan line, 805:
signal line, 821: transistor, 822: liquid crystal element, 823:
capacitor, 851: transistor, 852: capacitor, 853: transistor, 854:
light-emitting element, 855: scan line, 856: signal line, 900:
shift register, 901: level shifter, 902: buffer, 903: shift
register, 904: latch circuit, 905: latch circuit, 906: level
shifter, 907: buffer, 2000: substrate, 2001: conductive layer,
2002: insulating layer, 2003: oxide semiconductor layer, 2005a:
conductive layer, 2005b: conductive layer, 2007: oxide
semiconductor layer, 2008: conductive layer, 2020: transparent
conductive layer, 2022: conductive layer, 2023: conductive layer,
2024: conductive layer, 2028: transparent conductive layer, 2029:
transparent conductive layer, 2112: conductive layer, 2132: oxide
semiconductor layer, 2142a: oxide conductive layer, 2142b: oxide
conductive layer, 2700: e-book reader, 2701: housing, 2703:
housing, 2705: display portion, 2707: display portion, 2711: axis
portion, 2721: power supply switch, 2723: operation key, 2725:
speaker, 4501: substrate, 4502: pixel portion, 4503a: signal line
driver circuit, 4503b: signal line driver circuit, 4504a: scan line
driver circuit, 4504b: scan line driver circuit, 4505: sealing
material, 4506: substrate, 4507: filler, 4509: transistor, 4510:
transistor, 4511: light-emitting element, 4512: light-emitting
layer, 4513: electrode, 4515: connection terminal electrode, 4516:
terminal electrode, 4517: electrode, 4518a: FPC, 4518b: FPC, 4519:
anisotropic conductive layer, 4520: bank, 4540: conductive layer,
4542: insulating layer, 4543: insulating layer, 4544: insulating
layer, 4545: planarization insulating layer, 4555: transistor,
7003: cathode, 7004: light-emitting layer, 7005: anode, 7013:
cathode, 7014: light-emitting layer, 7015: anode, 7016:
light-blocking layer, 7017: conductive layer, 7023: cathode, 7024:
light-emitting layer, 7025: anode, 7027: conductive layer, 9000:
mobile phone device, 9001: housing, 9002: display portion, 9003:
operation button, 9004: external connection port, 9005: speaker,
9006: microphone, 9400: communication device, 9401: housing, 9402:
operation button, 9403: external input terminal, 9404: microphone,
9405: speaker, 9406: light-emitting portion, 9410: display device,
9411: housing, 9412: display portion, 9413: operation button, 9600:
television device, 9601: housing, 9603: display portion, 9605:
stand, 9607: display portion, 9609: operation key, 9610: remote
controller, 9700: digital photo frame, 9701: housing, 9703: display
portion, 9881: housing, 9882: display portion, 9883: display
portion, 9884: speaker, 9885: operation key, 9886: recording medium
insertion portion, 9887: connection terminal, 9888: sensor, 9889:
microphone, 9890: LED lamp, 9891: housing, 9893: joint portion,
9900: slot machine, 9901: housing, 9903: display portion
* * * * *