Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
1999-09-17
2002-12-31
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S059000, C257S642000, C349S044000, C349S043000
Reexamination Certificate
active
06501097
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the configuration of a thin-film transistor used in an electro-optical device such as a liquid crystal display device.
In an electro-optical device that is represented by an. active matrix type liquid crystal display device, there is known a configuration in which thin-film transistors (TFTs) are used as drive elements and switching elements. Thin-film transistors are configured using a semiconductor (usually, silicon semiconductor) thin film that is formed on a glass substrate by vapor-phase deposition. Thin-film transistors are also used for an image sensor and other devices.
FIGS.
4
(A) and
4
(B) are a sectional view and a top view, respectively, of a one-pixel portion of an active matrix circuit using conventional thin-film transistors. FIG.
4
(A) is taken along line A—A′ in FIG.
4
(B). A thin-film transistor whose cross-section is shown in FIG.
4
(A) is composed of a glass substrate
401
, an amorphous silicon or crystalline silicon semiconductor active layer formed on the glass substrate
401
and having a source region
402
, a channel forming region
403
and a drain region
404
, a gate insulating film
405
made of silicon dioxide or silicon nitride, an interlayer film
407
made of silicon dioxide, a drain contact portion
412
, a source contact portion
411
, a drain electrode
410
, and a transparent conductive film (ITO film or the like)
408
that is connected to the drain electrode
410
and constitutes a pixel electrode. (The source and drain are reversed for a certain operation of the TFT.)
The source region
402
of the thin-film transistor shown in FIG.
4
(A) is connected to a source line
409
via the source contact portion
411
. A gate electrode
406
is connected to a gate line
413
. While usually the source line
409
and the gate line
413
are perpendicular to each other, this is not always the case. In general, the gate electrode
406
and the gate line
413
are made of a metal such as aluminum or a semiconductor such as polysilicon added with phosphorus.
FIGS.
4
(A) and
4
(B) show a single pixel. Actually, an active matrix circuit is formed by disposing at least one pixel as shown in FIGS.
4
(A) and
4
(B) at each intersection of the source lines and the gate lines. A liquid crystal panel is configured by sealing a liquid crystal material between an active matrix substrate on which the active matrix circuit is formed and an opposed substrate. There are following types of liquid crystal display devices that use the liquid crystal panel having the above configuration.
(1) Liquid crystal display is realized by applying light (back light) to the liquid crystal panel.
(2) A video image is produced by applying high-intensity light to the liquid crystal display panel and projecting, onto a screen, light that is transmitted from the liquid crystal panel. (Liquid crystal projector)
(3) A reflecting plate is disposed on the back side of the liquid crystal display panel, and display is effected by causing external light to be reflected by the reflecting plate.
In particular, where light is irradiated from the glass substrate side in cases (1) and (2) above, it is necessary to shield the active layer, particularly the channel forming region, from the illumination light. This originates from the fact that the active layer (semiconductor layer denoted by
402
-
404
in FIGS.
4
(A) and
4
(B)) is made of amorphous silicon or crystalline silicon such as polysilicon. In general, the resistance of a silicon semiconductor varies when it receives light. In particular., when an amorphous silicon or crystalline silicon film is used, which includes dangling bonds, its electrical characteristics are greatly varied by illumination with high-intensity light. Further, the resistance of an intrinsic semiconductor (used for the channel forming region) is varied more greatly by illumination with light than an N-type or P-type semiconductor (used for the source and drain regions). Therefore, it is absolutely necessary to prevent the channel forming region from being illuminated.
Where light
414
is incident from above the gate electrode
406
in the thin-film transistor (top gate TFT) having the structure in which the gate electrode
406
is located over the semiconductor active layer (see FIG.
4
(A)), it seems that no light may enters the channel forming region
403
due to the gate electrode
406
serving as a mask.
Actually, however, part of the incident light goes around the gate electrode
406
, to enter he channel forming region
403
. As a result, the conductivity of the channel forming region
403
is varied by the illumination and its characteristics are also varied.
That is, it is impossible for oily the gate electrode
406
to completely prevent light from entering the channel forming region
403
. This problem is remarkable when the channel forming region and the gate electrodes are formed in a self-aligned manner. To solve this problem, a light shielding layer or film is effectively used, but this increases the number of manufacturing steps.
SUMMARY OF THE INVENTION
An object of the present invention is to realize, without increasing the number of manufacturing steps, a structure of a thin-film transistor which is basically of the type shown in FIGS.
4
(A) and
4
(B) and in which light is not incident on nor enters the active layer, particularly the channel forming region. In particular, the invention is effective when applied to an active matrix circuit having thin-film transistors whose sources and drains are formed by a self-align process that produces a very small overlap of the gate electrode and the source and drain regions.
The invention is characterized in that in an active matrix circuit, a source line or an electrode or wiring line (they cannot be clearly discriminated from each other) extending from the source line is so formed as to cover a channel forming region, to use the source line or an electrode or wiring line as a light shielding layer for the channel forming region. Since the channel forming region is included in a portion where a gate electrode and a semiconductor active layer overlap with each other, forming a source line or an electrode or wiring line extending from the source line to cover such a portion is approximately equivalent to the above.
In the above structure, examples of a substrate having an insulative surface are a glass substrate, a plastic substrate, a metal or semiconductor substrate on whose surface an insulating film is formed.
FIG.
2
(D) shows an example of the structure in which the source line or the electrode or wiring line extending from the source line of the thin-film transistor is so formed as to cover at least the channel forming region. In FIG.
2
(D), a source line electrode/wiring line
112
that is connected to a source region
104
of a thin-film transistor via a contact portion
108
covers a channel forming region
105
. That is, in FIG.
2
(D), both of the electrode/wiring line
112
and the gate electrode
107
serve as light shielding films for the underlying channel forming region
105
.
It may be possible to realize a similar structure by using an electrode/wiring line for connecting the pixel electrode
103
and the drain region
106
. However, in such a case, a parasitic capacitance between the pixel electrode
103
and the gate electrode
107
becomes large and a potential variation of the gate line
107
affects the pixel electrode
107
, causing serious problems in the operation of the active matrix. (For example, refer to H. Ono Kikuo et al: “Flat. Panel Display '91,” page 109.)
On the other hand, in the invention, a capacitive coupling between the source line and the gate line due to a parasitic capacitance adversely affects the operation speed of the thin-film transistor, which does not influence the pixel potential. Therefore, no problem occurs in terms of the image display. Further, in the invention, a parasitic capacitance that occurs due to than source line laid over the gate electrode does not cause any
Robinson Eric J.
Robinson Intellectual Property Law Office PC
Semiconductor Energy Laboratory Co,. Ltd.
Tran Minh Loan
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