Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2000-06-29
2002-09-24
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C313S310000
Reexamination Certificate
active
06456014
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-186548, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a field emission device comprising means for limiting the current at the time of occurrence of a short circuit between the emitter and the gate.
Recently, the field emission device of the field emission type that is as small as the semiconductor device has been developed by use of the developed Si-semiconductor micromachining technique, and the application of the field emission device to a flat panel display and the like has been promoted. The report of C. A. Spindt et al. (Journal of Applied Physics, vol. 47, 5249, 1976) is known as a typical example of it.
As for the field emission device described in this report, as shown in
FIGS. 1A
to
1
D, an SiO
2
layer
2
is formed as an insulation layer on an Si-monocrystalline substrate
1
by thermal oxidation. After an Mo layer
3
that is to be a gate electrode is formed by vacuum evaporation, a hole
4
is formed by etching (FIG.
1
A). Then, Al is deposited by vacuum evaporation in the oblique direction while rotating the Si-monocrystalline substrate
1
, and an Al layer
5
is thereby formed (FIG.
1
B).
Next, Mo that is to be an emitter is deposited by vacuum evaporation on the Si-monocrystalline substrate
1
in the vertical direction, and Mo is stacked in a conical shape inside the hole
4
by taking advantage of the fact that the hole
4
becomes closed as a Mo layer
6
is stacked (FIG.
1
C). Finally, a conical emitter
7
is formed by removing the Al layer
5
and the Mo layer
6
(FIG.
1
D).
A resultant structure is held face to face with an anode (not shown) in a vacuum vessel so as to be the field emission device.
This field emission device applies the positive voltage to the gate to generate a large electric field at the tip of the emitter, and the electrons extracted from the interior of the emitter into the vacuum is collected by the anode (not shown). Actually, the field emission device has an array structure comprising many emitters to obtain a large current. However, as a large number of emitters are connected in parallel, there is a serious problem that the overall array cannot be operated if one emitter causes a short circuit with the gate. Thus the redundancy in a short circuit between the emitter and the gate needs to be improved.
To solve this problem, the device having the structure proposed in Extended Abstracts (The 54th Autumn Meeting, 1993): The Japan Society of Applied Physics 27P-Y-9. The structure of the device is shown in
FIGS. 2A
to
2
C. In the device shown in
FIG. 2A
, a lower part of an emitter
10
is formed by a high resistance layer
11
. In
FIG. 2B
, a gate potential is supplied to the high resistance gate
15
. In either case, when a short circuit occurs between the gate and the emitter, most of the voltage is applied to the high resistance portion and the overall device is prevented from becoming inoperable to maintain the redundancy.
In the device of
FIG. 2C
, the array is divided into blocks
17
and the gate potential is supplied to the gate of each block through a fuse
18
. When a short circuit occurs, the fuse is melted to electrically separate the blocks and the redundancy is thereby maintained.
A device having the structure as described in the Technical Digest of IVMC, 91, p. 200, 1991 is also proposed. The structure is illustrated in FIG.
3
.
After an n-type source region
21
and an n-type drain region
22
are formed on a p-type Si-substrate
20
, a thermally oxidized SiO
2
layer
23
is formed. Next, an emitter is formed in the above-explained manner in a drain region of a MOSFET produced by forming a source electrode
24
and a gate electrode
25
. In this device a current flowing in the emitter can be controlled by the MOSFET inserted at the emitter side. For this reason, a current flowing at the occurrence of a short circuit between the emitter and the gate is limited by the FET and the redundancy can be therefore maintained.
However, there is a serious problem in the above-described conventional method as explained below.
First, according to the method of limiting a current at the occurrence of a short circuit by a large resistance, such a resistance is needed as to be able to suppress the current in a short circuit. As a result, the operating speed of the device becomes remarkably lower. Further, when the resistor is inserted at the emitter side, there is another problem that the resistor causes the loss to be increased because of a large emitter current when the device is normally operated.
There is no problem about the resistance in the case of the fuse. However, it is difficult to produce a fuse blown at a low voltage with a small current, integrally with the substrate. For example, to prevent the heat from being diffused from the fuse, the fuse needs to be positioned apart from the substrate.
In addition, there is another problem that the blocks are separated at a low speed as the opening of the fuse requires a long time.
As for the structure of inserting the MOSFET at the emitter side, when the device is normally operated, the loss caused by the FET becomes large because of a large emitter current though the loss is not so serious as that in the case of the resistor. Further, as the ability to treat a large emitter current is required of the FET, a large area is needed for the FET and thereby the packing density of the device cannot easily be increased.
The structure of inserting the MOSFET at the emitter side has an advantage of simple production as the emitter can be formed on the drain region of the MOSFET as described above. When this structure is applied to a display device the display requires a comparatively small current and, therefore, the problem of the loss caused by the FET is not so serious. Furthermore, there is an advantage that the brightness does not vary as it is determined in accordance with the characteristics of the FET.
On the other hand, in a case where the field emission device is applied to the power switching device, the loss in the MOSFET is a serious problem when the MOSFET is inserted on the emitter side. For this reason, means for maintaining the redundancy relating to a short circuit between the emitter and the gate is required in the power switching device.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a field emission device which is capable of certainly separating a short-circuited portion at a high speed without causing the lowering of the operating speed of the field emission device or the increase in the power loss, and which is thereby suitable for a power switching device.
To achieve the object, a field emission device according to a first aspect of the present invention comprises:
a casing whose interior is kept to be a vacuum;
an anode plate provided inside the casing;
an emitter plate for emitting electrons held to face the anode plate in the casing, the emitter plate being composed of a conductive plate having a plurality of field emission portions facing the anode plate, the electrons being emitted to the anode plate so that a current flows between the anode plate and the emitter plate;
a gate plate for controlling the current between the anode plate and the emitter plate, the gate plate having a plurality of openings corresponding to the plurality of field emission portions of the emitter plate, the emitter plate and the gate plate being held to be insulated from one another;
a gate voltage supply terminal for supplying a gate voltage to the gate plate; and
a gate current limiting element inserted between the gate plate and the gate voltage supply terminal.
A field emission device according to a second aspect of the invention comprises:
a casing whose interior is kept to be a vacuum;
an anode plate provided inside the casing;
an emitter plate for emitting electrons hel
Ono Tomio
Sakai Tadashi
Zhang Li
A Minh D
Kabushiki Kaisha Toshiba
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