U.S. patent application number 09/736139 was filed with the patent office on 2001-05-03 for active matrix display device having multiple gate electrode portions.
This patent application is currently assigned to Semiconductor Energy Laboratory Co., Ltd. Invention is credited to Hayakawa, Masahiko, Tsukamoto, Yosuke.
Application Number | 20010000627 09/736139 |
Document ID | / |
Family ID | 18183767 |
Filed Date | 2001-05-03 |
United States Patent
Application |
20010000627 |
Kind Code |
A1 |
Hayakawa, Masahiko ; et
al. |
May 3, 2001 |
Active matrix display device having multiple gate electrode
portions
Abstract
In a thin-film transistor of multi-gate structure, the width of
a channel forming region 108 closest to a drain region 102 is made
the narrowest. This prevents a transistor structure closest to the
drain region from first deteriorating. Further, the channel length
at the vicinity of a center of an active layer is intentionally
widened, so that the amount of current flowing through the vicinity
of the center of the active layer is decreased and the
deteriorating phenomenon due to heat accumulation is prevented.
Therefore, a semiconductor device with a high reliability is
realized.
Inventors: |
Hayakawa, Masahiko;
(Kanagawa, JP) ; Tsukamoto, Yosuke; (Kanagawa,
JP) |
Correspondence
Address: |
SCOTT C. HARRIS
Fish & Richardson P.C.
4350 La Jolla Village Drive, Suite 500
San Diego
CA
92122
US
|
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd
|
Family ID: |
18183767 |
Appl. No.: |
09/736139 |
Filed: |
December 13, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09736139 |
Dec 13, 2000 |
|
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|
08970542 |
Nov 14, 1997 |
|
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|
6184559 |
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Current U.S.
Class: |
257/347 ; 257/59;
257/E27.111; 257/E29.137; 438/149 |
Current CPC
Class: |
H01L 29/78696 20130101;
H01L 27/12 20130101; G02F 1/1368 20130101; H01L 29/42384
20130101 |
Class at
Publication: |
257/347 ; 257/59;
438/149 |
International
Class: |
H01L 021/00; H01L
031/0392; H01L 029/04; H01L 027/12; H01L 031/036; H01L 027/01; H01L
031/0376; H01L 031/20; H01L 021/84 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 1996 |
JP |
8-326069 |
Claims
1. An EL display device comprising at least a thin film transistor
including: a semiconductor island including: a source region, a
drain region, and at least a first channel forming region, a second
channel forming region, and a third channel forming region, each
being formed between the source region and the drain region; a gate
insulating film; and a gate electrode, adjacent to the
semiconductor island and having the gate insulating film between
said semiconductor island and said gate electrode, wherein the gate
electrode is divided into at least a first gate electrode portion,
a second gate electrode portion and a third gate electrode portion,
wherein the first gate electrode portion is formed over the first
channel forming region and the closest to the drain region, the
first channel forming region being the closest to the drain region,
wherein the first gate electrode has the narrowest width so that
the first channel forming region has the shortest channel length,
wherein the third gate electrode portion is formed over the third
channel forming region and the closest to the source region thereby
the third channel forming region being defined the closest to the
source region, wherein the third gate electrode portion has the
widest width so that the third channel forming region has the
longest channel length, wherein a current flows from the source
region to the drain region through each of the first, second and
third channel forming regions in a channel length direction.
2. A device according to claim 1, wherein the semiconductor island
comprises crystalline silicon.
3. A device according to claim 1, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
4. A device according to claim 1, wherein the EL display device is
used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
5. An EL display device comprising at least a thin film transistor
including: a semiconductor island with a serpentine shape, said
semiconductor island including: a source region, a drain region,
and at least a first channel forming region, a second channel
forming region and a third channel forming region, each being
formed between the source region and the drain region; a gate
insulating film; and a gate electrode adjacent to the serpentine
shaped semiconductor island the gate insulating film between said
semiconductor island and said gate electrode, wherein the gate
electrode has at least a first gate electrode portion, a second
gate electrode portion and a third gate electrode portion, wherein
the first gate electrode portion is formed over the first channel
forming region and the closest to the drain region thereby the
first channel forming region being defined the closest to the drain
region, wherein the first gate electrode has the narrowest width so
that the first channel forming region has the shortest channel
length, wherein the third gate electrode portion is formed over the
third channel forming region and the closest to the source region
thereby the third channel forming region being defined the closest
to the source region, wherein the third gate electrode portion has
the widest width so that the third channel forming region has the
longest channel length, wherein a current flows from the source
region to the drain region through each of the first, second and
third channel forming regions in a channel length direction.
6. A device according to claim 5, wherein the semiconductor island
comprises crystalline silicon.
7. A device according to claim 5, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
8. A device according to claim 5, wherein the EL display device is
used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
9. An EL display device comprising: a driving circuit portion; and
a pixel matrix circuit portion, said pixel matrix circuit portion
including at least a pixel thin film transistor, said pixel thin
film transistor comprising: a semiconductor island including: a
source region, a drain region, and at least a first channel forming
region, a second channel forming region and a third channel forming
region, each being formed between the source region and the drain
region; a gate insulating film; and a gate electrode adjacent to
the semiconductor island having the gate insulating film
therebetween, wherein the gate electrode has a symmetrical
structure and is divided into at least a first gate electrode
portion, a second gate electrode portion and a third gate electrode
portion, wherein the first gate electrode portion is formed over
the first channel forming region and the closest to one of the
source and drain regions, wherein the third gate electrode portion
is formed over the third channel forming region and the closest to
the other of the source and drain regions, wherein the second gate
electrode portion is formed over the second channel forming region
and between the first and third gate electrode portion thereby the
second channel forming region being defined between the first and
third channel forming regions, wherein the second gate electrode
portion has the widest width so that the second channel forming
region has the longest channel length, wherein each of the first
and third gate electrodes has a same width which is narrower than a
width of the second gate electrode portion so that each of the
first and third channel forming regions has a same length which is
shorter than a width of the second channel forming region, wherein
a current flows from the source region to the drain region through
each of the first, second and third channel forming regions in a
channel length direction, wherein the source region and the drain
region are interchanged with each other every time of a charge and
a discharge.
10. A device according to claim 9, wherein the semiconductor island
comprises crystalline silicon.
11. A device according to claim 9, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
12. A device according to claim 9, wherein the EL display device is
used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
13. An EL display device comprising at least a thin film transistor
including: a semiconductor island including: a source region, a
drain region, and a channel forming region being formed between the
source and drain regions; a gate insulating film; and a gate
electrode adjacent to the semiconductor island having the gate
insulating film therebetween, wherein the gate electrode has two
end portions and a center portion formed between the two end
portions, each of the end and center portions being located in a
channel width direction, wherein the center portion of the gate
electrode has the widest width so that a center portion of the
channel forming region has the longest channel length, wherein a
current flows from the source region to the drain region through
the channel forming region in a channel length direction.
14. A device according to claim 13, wherein the semiconductor
island comprises crystalline silicon.
15. A device according to claim 13, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
16. A device according to claim 13, wherein the EL display device
is used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
17. An EL display device comprising at least a thin film transistor
including: a semiconductor island including: a source region, a
drain region, and a channel forming region being formed between the
source region and the drain region; a gate insulating film; and a
gate electrode adjacent to the semiconductor island having the gate
insulating film therebetween, wherein the gate electrode has two
end portions and a middle portion formed between the two end
portions, each of the end and middle portions being located in a
channel width direction, wherein the channel forming region has a
plurality of slits therein so that a plurality of active regions
are defined and electrically connected in parallel to each other,
wherein the middle portion of the gate electrode has the widest
width so that a middle portion of the channel forming region has
the longest channel length, wherein a current flows from the source
region to the drain region through the channel forming region in a
channel length direction.
18. A device according to claim 17, wherein the semiconductor
island comprises crystalline silicon.
19. A device according to claim 17, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
20. A device according to claim 17, wherein EL display device is
used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
21. An EL display device comprising at least a thin film transistor
including: a semiconductor island including: a source region; a
drain region; at least a first channel forming region, a second
channel forming region and a third channel forming region, each
being formed between the source and drain regions; a gate
insulating film; a gate electrode adjacent to the semiconductor
island having the gate insulating film therebetween; wherein the
gate electrode is divided into at least a first gate electrode
portion, a second gate electrode portion and a third gate electrode
portion; wherein the first gate electrode portion is formed over
the first channel forming region and the closest to the drain
region thereby the first channel forming region being defined the
closest to the drain region; wherein the first gate electrode
portion has the narrowest width so that the first channel forming
region has the shortest channel length; wherein the third gate
electrode portion is formed over the third channel forming region
and the closest to the source region thereby the third channel
forming region being defined the closest to the source region;
wherein the third gate electrode portion has the widest width so
that the third channel forming region has the longest channel
length; wherein each of the first, second and third gate electrode
portions has two end portions and a middle portion formed between
the two end portions, each of the end and middle portions being
located in a channel width direction; wherein the middle portion of
each of the first, second and third gate electrodes has the widest
width so that a middle portion of each of the first, second and
third channel forming regions has the longest channel length; and
wherein a current flows from the source region to the drain region
through each of the first, second, and third channel forming
regions in a channel length direction.
22. A device according to claim 21, wherein the semiconductor
island comprises crystalline silicon.
23. A device according to claim 21, wherein the gate electrode
overlaps with the semiconductor island having the gate insulating
film therebetween.
24. A device according to claim 21, wherein the EL display device
is used in one selected from the group consisting of a TV camera, a
head mount display, a car navigation system, a projector, a video
camera, a personal computer.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
2. The present invention relates to a semiconductor device using a
thin-film semiconductor. More particularly, the present invention
relates to a structure of a gate electrode in an insulated gate
type transistor.
3. 2. Description of the Related Art
4. As a semiconductor device using a thin-film semiconductor,
attention is attached to a thin-film transistor (TFT). Especially,
in recent years, a TFT capable of performing high speed operation
by using a crystalline silicon film (for example, polysilicon
film), has been put into a practical use.
5. Although a thin-film transistor using a crystalline silicon film
as an active layer has a high mobility (field effect mobility), it
has such a defect that an off-state current (current flowing when
the TFT is in an off-state) is large. Further, the thin-film
transistor has a problem that when the mobility becomes high, a
withstand voltage becomes low so that deterioration becomes
noticeable.
6. As means for solving such problems, there is known a technique
disclosed in Japanese Examined Patent Publication No. Hei 5-44195.
According to this technique, a thin-film transistor is made
equivalently to have such a structure (also called as a multi-gate
structure) that a plurality of thin-film transistors are connected
in series to each other, so that an applied voltage is distributed
to each of the plurality of thin-film transistors.
7. FIG. 4 is a structural view showing an active layer and a gate
electrode of a thin-film transistor manufactured by using the
technique disclosed in the above publication. In FIG. 4, reference
numeral 401 denotes a source region, and 402 denotes a drain
region. Gate electrodes 403 to 406 are formed above the active
layer through a gate insulating film (not shown). At this time, the
gate electrodes 403 to 406 are formed integrally so that they are
connected electrically.
8. Channel forming regions 407 to 410 are formed just under the
gate electrodes 403 to 406 into shapes corresponding to those of
the gate electrodes 403 to 406. It is characterized in that the
structure is substantially composed of a plurality of thin-film
transistors commonly connected in series.
9. However, according to experiments carried out by the present
inventors by using the TFTs having the structure as shown in FIG.
4, it has been found that the thin-film transistor closest to the
drain region 402 deteriorates most intensely. Also, it has been
found that when a high voltage is applied between the source and
drain, breakdown or deterioration proceeds sequentially from a
transistor at the side close to the drain region.
10. According to another experiment, it has been found that in a
TFT constituted by an active layer with a wide channel width, the
vicinity of a center of an active layer (vicinity of the center in
the channel width direction) deteriorates most intensely.
SUMMARY OF THE INVENTION
11. An object of the present invention is to prevent breakdown or
deterioration of a semiconductor device equivalently having such a
structure that a plurality of semiconductor devices are connected
in series to each other, by relieving the concentration of electric
field on one of the plurality of semiconductor devices close to a
drain side.
12. Another object of the present invention is to prevent
deterioration from occurring at a center portion of an active
layer, by suppressing an electric current flowing through the
vicinity of the center of the active layer.
13. According to a structure of a first invention, a semiconductor
device is comprised of: an active layer including source and drain
regions and channel forming regions; a gate insulating film; and a
gate electrode overlapping with the active layer through the gate
insulating film, and is characterized in that the gate electrode
has a structure which can be regarded substantially as a plurality
of gate electrodes integrally formed, and that among said plurality
of gate electrodes, the one closest to the drain region has the
narrowest width.
14. In the above structure, the fact that the width of the gate
electrode closest to the drain region is the narrowest implies the
fact that the width of the channel forming region (in other words,
channel length) formed just under the gate electrode is the
narrowest.
15. According to another structure of the first invention, a
semiconductor device is comprised of: an active layer including
source and drain regions and channel forming regions; a gate
insulating film; and a gate electrode overlapping with the active
layer through the gate insulating film, and is characterized in
that the gate electrode has a structure which can be regarded
substantially as a plurality of gate electrodes integrally formed,
and that the widths of the plurality of gate electrodes
sequentially become narrower as it comes to close to the drain
region.
16. Also in this case, the above feature implies that the widths of
the channel forming regions sequentially become narrower as it
comes close to the drain region.
17. These structures are intended to decrease the resistance
component of the channel forming region by narrowing the width of
the gate electrode close to the drain region, that is, the width of
the channel forming region, so that a voltage applied to the
channel forming region is lowered.
18. According to a structure of a second invention, a semiconductor
device is comprised of: an active layer including source and drain
regions and channel forming regions; a gate insulating film; and a
gate electrode overlapping with said active layer through said gate
insulating film, and is characterized in that a width of the gate
electrode is varied in a channel width direction of the active
layer.
19. According to another structure of the second invention, a
semiconductor device is comprised of: an active layer including
source and drain regions and channel forming regions; a gate
insulating film; and a gate electrode overlapping with said active
layer through said gate insulating film, and is characterized in
that a width of the gate electrode becomes wider as it comes close
to a center portion of the active layer from an end of the active
layer in a channel width direction.
20. The above two structures are intended to suppress the amount of
flowing current by widening the width of the gate electrode at the
vicinity of the center of the active layer so that the channel
forming region is widened and the resistance component is increased
at the vicinity of the center of the active layer.
21. As described above, the gist of the present invention is to
intentionally change the width of the channel forming region in the
active layer, so that the resistance component of the channel
forming region is set to have a desired characteristic. That is,
the present invention is a technique to distribute a voltage
applied to the channel forming region and to control the amount of
current flowing through a specified portion of the channel forming
region.
BRIEF DESCRIPTION OF THE DRAWINGS
22. FIG. 1 is a view for explaining the structure of an active
layer and a gate electrode of Embodiment 1;
23. FIG. 2 is a view for explaining the structure of an active
layer and a gate electrode of Embodiment 2;
24. FIG. 3 is a view for explaining the structure of an active
layer and a gate electrode of Embodiment 3;
25. FIG. 4 is a view for explaining the structure of an active
layer and a gate electrode of the prior art;
26. FIG. 5 is a view for explaining the structure of an active
layer and a gate electrode of Embodiment 4;
27. FIG. 6 is a view for explaining the structure of an active
layer and a gate electrode of Embodiment 5; and
28. FIGS. 7A to 7F are views for explaining examples of electronic
instruments of Embodiment 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
29. A first invention is a technique to prevent an electric field
from concentrating onto a channel forming region close to a drain
region in a thin-film transistor having a multi-gate structure. In
order to realize the technique, the structure as shown in FIG. 1 is
adopted.
30. In an active layer, channel forming regions 106 to 108 are
formed into the shapes corresponding to that of gate electrodes.
Since gate widths become narrow in the sequence of the gate
electrodes 103, 104 and 105, channel lengths become shorter in the
sequence of the channel forming regions 106, 107 and 108 (the
channel forming region 108 is the shortest).
31. With this structure, in accordance with Ohm's law, a voltage
applied to the channel forming region 108 becomes the lowest, so
that an electric field formed concentrically in an end portion at a
drain side of the channel forming region 108 becomes low. As a
result, it is possible to relieve the phenomenon that an electric
field is more concentrated onto a portion as it comes close to a
drain region.
32. A second invention is a technique to prevent deterioration or
breakdown from proceeding first in the vicinity of a center of an
active layer. In order to realize the technique, the structure as
shown in FIG. 5 is adopted.
33. In FIG. 5, a gate electrode 503 is patterned into such a shape
that the width of the gate electrode in the vicinity of the center
of the active layer becomes the widest. As a result, a channel
length of a channel forming region 504 becomes the widest in the
vicinity of the center of the active layer.
34. With the above structure, the amount of current flowing through
the vicinity of the active layer can be suppressed so that the
amount of generated heat can be decreased. Accordingly, it is
possible to prevent the deterioration phenomenon which appears to
be caused by heat accumulation.
Embodiment 1
35. In this embodiment, a thin-film transistor will be exemplified
as a semiconductor device, and the structures of an active layer
and a gate electrode of a thin-film transistor using the first
invention will be described. In this embodiment, although a gate
electrode has a triple-gate type multi-gate electrode structure in
which a gate electrode is divided into three at a region where the
gate electrode overlaps with an active layer, the gate electrode is
not limited to this structure.
36. In FIG. 1, reference numeral 101 denotes a source region, and
102 denotes a drain region, which are formed by adding impurity
elements (phosphorus, boron, etc.) imparting one conductivity.
Also, reference numeral 103 denotes a gate electrode with a width
"a", 104 denotes a gate electrode with a width "b", and 105 denotes
a gate electrode with a width "c". As shown in FIG. 1, the gate
electrodes 103 to 105 are electrically connected to each other.
37. Further, regions 106 to 108 are channel forming regions formed
correspondingly to the gate electrodes 103 to 105, and are
substantially intrinsic regions (undoped region) where impurity
elements are not intentionally added.
38. The structure of the first invention is featured that the
widths of the gate electrodes (widths of the channel forming
regions) become narrower as it comes close to the drain region 102.
In FIG. 1, the widths of the gate electrodes become narrower in the
sequence of "a", "b" and "c".
39. Incidentally, the scope of the first invention is not limited
to the shape of the active layer and the gate electrodes shown in
FIG. 1, but a user may arbitrarily determine the shape. Also, it is
necessary for a user to experimentally obtain specific values of
the widths of the channel forming regions and the like.
40. Further, in this embodiment, the description is made of the
structure in which the widths of the gate electrodes sequentially
become narrower as it comes close to the drain region. However, the
same effect as in the first invention can be obtained even if only
the gate electrode closest to the drain region is made thinner than
all of the other gate electrodes and all of the other gate
electrodes are made to have the same width.
41. Now, the description is made of the process until the present
inventors reached the first invention. Since a channel forming
region is substantially intrinsic, it behaves as a region having
high resistance. Thus, even if a thin-film transistor is in an
on-state, it is conceivable that as a channel length becomes
longer, its resistance component becomes higher. That is, in the
structure shown in FIG. 1, it is conceivable that the resistance of
the channel forming region 108 is the lowest.
42. Then, if the amount of current flowing between a source and a
drain is constant, according to Ohm's law, the higher the
resistance of a region is, the larger a voltage applied to the
region is. That is, a voltage applied to the channel forming region
108 becomes the lowest.
43. Further, it is conceivable that a voltage applied to both ends
of a channel forming region is concentrically applied to the end
portion (channel/drain connection portion) close to the drain side
of the channel forming region so that a high electric field is
formed. Thus, it can be said that as the voltage applied to the
channel forming region becomes lower, the electric field
concentrated on the end portion at the drain side becomes
lower.
44. When the above consideration is summarized, in the structure
shown in FIG. 1, it is understood that the electric fields formed
in the end portions at the drain side become lower in the sequence
of the channel forming regions 106, 107 and 108.
45. Conventionally, a higher electric field is apt to be formed at
the channel/drain connection portion closer to the drain region, so
that deterioration or breakdown tends to occur. However, by
effecting the first invention, the electric field applied to the
channel/drain connection portion can be made low as it comes close
to the drain region, so that the deterioration can be relieved.
Embodiment 2
46. In this embodiment, an example in which the shape of an active
layer is different from that of the first embodiment, will be
described with reference to FIG. 2. Portions in FIG. 2
corresponding to those in FIG. 1 will be designated by the same
reference numerals.
47. In the structure shown in FIG. 2, a first difference from FIG.
1 is that the active layer has a zigzag or serpentine shape. Such a
shape is effective in lowering an area occupied by the active
layer. A second difference from FIG. 1 is the shape of a gate
electrode.
48. By adjusting the design pattern of the gate electrode, a
channel forming region with a desired width can be formed. In this
embodiment, in order to form a channel forming region 106 with a
channel length "a", a gate electrode portion indicated by reference
numeral 201 is formed. Also, in order to form a channel forming
region 107 with a channel length "b" and a channel forming region
108 with a channel length "c", gate electrode portions 202 and 203
are formed, respectively.
49. Of course, the shape of the active layer and the shape of the
gate electrode to which the first invention can be applied, are not
limited to those shown in this embodiment. It is needless to say
that a user may adequately determine the shape according to the
necessity.
50. By employing the gate electrode described above, it is possible
to form the active layer in which the widths of the channel forming
regions become narrower as it comes close to the drain region 102
(a>b>c in the drawing).
Embodiment 3
51. The structure shown in the first or second embodiment is
effective when the positions of a source region and a drain region
are fixed. For example, the source and drain regions are fixed when
a driving circuit of an active matrix type electro-optical device
is formed.
52. However, since pixel TFTs arranged in a pixel matrix circuit of
the active matrix type electro-optical device, repeat charge and
discharge of an electric charge, the source region and the drain
region are counterchanged with each other every time the charge and
discharge are made. In this case, the first invention can not be
effected by the structures described in the first and second
embodiments.
53. Therefore, in the case described above, as shown in FIG. 3, it
is necessary to form a structure in which gate electrodes 303 and
305 at the sides close to a source region (or drain region) 301 and
a drain region (or source region) 302, respectively, are made
narrower than a gate electrode 304.
54. In this embodiment, when a channel length of a channel forming
region 307 is "b", channel lengths of channel forming regions 306
and 307 are made a channel length "a" which is shorter than the
channel length "b". If the gate electrodes are made to have a
symmetrical structure between a source side and a drain side, it is
desirable for keeping the symmetry of a TFT operation.
Embodiment 4
55. In this embodiment, a structure of an active layer and a gate
electrode of a thin-film transistor using a second invention will
be described with reference to FIG. 5.
56. In FIG. 5, reference numeral 501 denotes a source region, 502
denotes a drain region, and 503 denotes a gate electrode. The gate
electrode 503 has a structure in which a width of electrode is
locally widened. As a result, the width of a channel forming region
504 which is formed in correspondence with the shape of the gate
electrode 503 becomes wider as it comes close to the center of an
active layer from end portions of the active layer in a channel
width direction (direction shown by arrows in the drawing).
57. Here, the description is made of the process until the present
inventors found the second invention. The present inventors
considered the phenomenon that deterioration started from the
vicinity of a center of an active layer in a thin-film transistor
using the active layer having a wide channel width, and assumed
that the phenomenon was greatly affected by heat accumulation which
was caused by difficulty of heat radiation from the vicinity of the
center of the active layer.
58. For that reason, it is necessary to decrease the amount of
current flowing through the vicinity of the center of the active
layer to suppress heat generation. Therefore, the inventors
considered it to be important that the channel length at the
vicinity of the center of the active layer should be elongated to
form a region having a large resistance component to suppress the
amount of current flowing therethrough.
59. This embodiment shows a technique invented based on the above
findings of the present inventors, and shows an example in which
the shape of the gate electrode 503 above the active layer is
locally changed (widened), so that a large current is prevented
from flowing through the vicinity of the center of the active
layer.
60. Incidentally, as described above, the gist of the second
invention is that the channel length close to the center of the
active layer is elongated so as to suppress the heat generation due
to a large electric current. Therefore, if the gist is kept, the
structure and shape of the gate electrode may be arbitrarily
designed according to the necessity of a user.
Embodiment 5
61. In this embodiment, an example in which the second invention
shown in the fourth embodiment is combined with a heat dissipation
effect due to the shape of an active layer, will be described with
reference to FIG. 6.
62. The feature of an active layer shown in FIG. 6 is that slits
are locally provided. That is, parts of the active layer are
hollowed out, so that three active layers with narrow channel
widths are substantially electrically connected in parallel to each
other. The number of the slits may be appropriately changed.
63. In FIG. 6, reference numeral 601 denotes a source region, 602
denotes a drain region, 603 denotes a gate electrode, and 604 to
606 denote channel forming regions formed just under the gate
electrode 603. The channel forming regions 604 and 606 have channel
lengths of the same width, and the channel forming region 605 has a
channel length longer than other regions.
64. The feature of this embodiment is that since the slits are
provided in the active layer, generated heat can be easily
dissipated. Thus, the amount of flowing current can be decreased by
the second invention so that the generation of intense heat is
suppressed, and heat dissipation can be further effectively carried
out by providing the slits.
Embodiment 6
65. By combining the first invention described in the first to
third embodiments with the second invention described in the fourth
and fifth embodiments, a thin-film transistor of multi-gate
structure having higher reliability can be manufactured.
66. That is, deterioration of a thin-film transistor close to a
drain region can be prevented by the first invention, and
deterioration from the vicinity of a center of an active layer due
to heat generation can be prevented by the second invention.
67. This embodiment is an especially useful technique for a
thin-film transistor and the like for a driving circuit which
handles a large current and brings high speed operation.
Embodiment 7
68. A thin-film transistor described in the first to sixth
embodiments can constitute an active matrix type electro-optical
device (liquid crystal display device, EL display device, EC
display device, and the like). For example, in a liquid crystal
display device in which a pixel matrix circuit and a driving
circuit are integrally formed on the same substrate, the first
invention is effective for the pixel matrix circuit to which a high
voltage is applied, and the second invention is effective for the
driving circuit which handles a large current.
69. U.S. Pat. No. 5,569,936, the disclosure of which is herein
incorporated by reference, discloses an active matrix type liquid
crystal display device, which the thin film transistor formed
through the first to sixth embodiments of the present invention can
be applied to.
70. A thin-film transistor using the present invention can be
applied to electronic instruments and the like in which the above
electro-optical device is used as a display medium. The electronic
instruments will be described below with reference to drawings.
71. As semiconductor devices using the present invention, there are
enumerated a TV camera, a head mount display, a car navigation
system, a projector, a video camera, a personal computer, and the
like. Brief description thereof will be presented with reference to
FIGS. 7A to 7F.
72. FIG. 7A shows a mobile computer which is constituted by a main
body 2001, a camera portion 2002, an image receiving portion 2003,
an operation switch 2004, and a display device 2005. The present
invention can be applied to the display device 2005 and an
integrated circuit incorporated into the device.
73. FIG. 7B shows a head mount display which is constituted by a
main body 2101, a display device 2102, and a band portion 2103. The
display device 2102 is formed of two comparatively compact ones.
The present invention can be applied to the display device 2102 and
an integrated circuit incorporated into the device.
74. FIG. 7C shows a car navigation unit which is constituted by a
main body 2201, a display device 2202, an operation switch 2203,
and an antenna 2204. The present invention can be applied to the
display device 2202 and an integrated circuit inside the
device.
75. FIG. 7D shows a potable telephone which is constituted by a
main body 2301, an audio output portion 2302, an audio input
portion 2303, a display device 2304, an operation switch 2305, and
an antenna 2306. The present invention can be applied to the
display device 2304 and an integrated circuit inside the
device.
76. FIG. 7E shows a video camera which is constituted by a main
body 2401, a display device 2402, an audio input portion 2403, an
operation switch 2404, a battery 2405, and an image receiving
portion 2406. The present invention can be applied to the display
device 2402 and an integrated circuit inside the device.
77. FIG. 7F shows a front projector which is constituted by a main
body 2501, a light source 2502, a reflection type display device
2503, an optical system 2504, and a screen 2505. Since the screen
2505 is a large picture screen used for presentation, high
resolution is required for the display device 2503. The present
invention can be applied to the reflection type display device 2503
and an integrated circuit inside the device.
78. It should be noted that the term "semiconductor device" used in
the present specification implies "a driving device using a
semiconductor". The above electro-optical devices and electronic
instruments are also included in the category of the semiconductor
device.
79. As described above, by effecting the present invention, the
reliability of various semiconductor devices can be improved.
Accordingly, the present invention is a useful technique in
technology or industry.
80. If the present invention is effected, it is possible to relieve
the phenomenon in which an electric field is locally concentrated
in a thin-film transistor formed of a multi-gate structure. That
is, it becomes possible to prevent deterioration that tends to
occur at a high rate as it comes close to a drain region.
81. Further, if the amount of current flowing through the vicinity
of a center of an active layer is suppressed, it becomes possible
to decrease the breakdown or deterioration due to heat.
82. As described above, when the present invention is used, it is
possible to prevent the breakdown or deterioration of a
semiconductor device (semiconductor element) typified by a
thin-film transistor, and to constitute a semiconductor device
having high reliability by using such a semiconductor element.
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