Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Having graded composition
Reexamination Certificate
1999-11-29
2002-02-26
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Having graded composition
C257S101000, C257S103000, C257S185000, C313S309000, C313S351000
Reexamination Certificate
active
06350999
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electron-emitting device comprising a surface layer composed of a material having a negative or substantially negative electron affinity.
A conventional electron-emitting device has a structure in accordance with a hot cathode system (an electron gun system) in which hot electrons are emitted from a solid into a vacuum by heating, to a high temperature, a cathode composed of a refractory metal material such as tungsten (W) which is disposed in spaced opposing relation to an anode. As a recent electron beam source substituting for an electron gun, attention has been focused on an extremely small electron-emitting device of cold cathode type. As typical electron-emitting devices of this type, there have been reported a device of field emission type and a device of pn junction type having a coating of a low-work-function material on the surface thereof.
The electron-emitting device of field emission type comprises a conic emitter portion formed of a refractory metal material such as silicon (Si) or molybdenum (Mo) and a withdrawn electrode disposed in spaced relation to the emitter portion. A voltage is applied to the withdrawn electrode to produce a high electric field (>1×10
9
V/m), which causes the emitter portion to emit electrons. The device of field emission type is advantageous in that it can be scaled down by using a micro-fabrication technique.
On the other hand, the electron-emitting device of pn junction type has a coating layer of a low-work-function material, such as cesium (Cs), on the surface of a p-type semiconductor layer, thereby producing a state with a negative electron affinity. The device is structured such that electrons are emitted into a vacuum from the p-type semiconductor layer with the application of a bias voltage to the device.
The characteristics equally required of the extremely small electron-emitting devices of cold cathode type are: easy emission of electrons with a relatively small operational voltage (e.g., an emitter material has a low electron affinity); easy control of the emitted electron beam; the chemically stable surface of the emitter portion owing to a stable emission property; and excellent wear resistance and excellent heat resistance.
However, the conventional electron-emitting devices mentioned above have the following disadvantages.
The device of field emission type has the disadvantage of a relatively high operational voltage because a high electric field is used to cause electron emission. The device also has the disadvantage of poor controllability of the amount of emitted current because the amount of emitted current is highly dependent on the configuration of the emitter portion and on the surface state and because it is difficult to form a large number of conic emitters with excellent uniformity.
The electron-emitting device of pn junction type using a semiconductor material is disadvantageous in terms of device stability and lifespan because the coating layer made of a low-work-function material, which is indispensable to the surface of the p-type semiconductor layer for achieving easy emission of electrons, is poor in stability.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a novel electron-emitting device of previously unknown structure, which is operable with a low voltage and excellent in emission property, by using a materiel having a negative or substantially negative electron affinity close to zero and by providing means for efficiently supplying electrons to the surface layer serving as an electron-emitting portion.
A first electron-emitting device of the present invention comprises: a semiconductor layer functioning as an electron supplying layer; a surface layer disposed in spaced relation to the semiconductor layer, the surface layer being composed of a material having a negative electron affinity or a positive electron affinity close to zero; a graded composition layer interposed between the semiconductor layer and the surface layer, the graded composition layer having a varying composition such that the electron affinity decreases in a direction from the semiconductor layer toward the surface layer; and a surface electrode disposed on the surface layer, the surface electrode applying a voltage such that electrons are supplied from the semiconductor layer to an outermost surface of the surface layer.
In the arrangement, the graded composition layer as the region in which the electrons move has the electron affinity decreasing with approach toward the surface layer. Accordingly, the electrons are easily emitted from the surface layer having a negative electron affinity into a vacuum by applying a voltage between the semiconductor layer functioning as the electron supplying layer and the surface electrode and thereby supplying the electrons to the surface layer through the graded composition layer. Since a voltage of about 10 V is sufficient as the maximum voltage to be applied, though it varies depending on the device structure, there can be implemented an electron-emitting device operable with a low voltage and capable of emitting the electrons with high efficiency.
In the first electron-emitting device, the electrons can be supplied efficiently from the electron supplying layer to the surface layer by using a non-doped semiconductor material for the graded composition layer.
The first electron-emitting device preferably uses the structure in which at least a part of the graded composition layer has a band gap enlarged nearly continuously from the semiconductor layer to the surface layer. In the structure, the electrons are allowed to move smoothly in the graded composition layer.
In the first electron-emitting device, the surface layer preferably contains aluminum nitride (AlN). The arrangement allows the formation of the device capable of emitting electrons with high efficiency by using the characteristic of negative electron affinity of the AlN surface.
Preferably, the first electron-emitting device further comprises a coating layer disposed on the surface layer, the coating layer being composed of a material different from the material composing the surface layer. The coating layer can control electron emission and protect the surface layer.
In the first electron-emitting device, the coating layer preferably contains a material having a negative electron affinity or a positive electron affinity close to zero. In the arrangement, the property of efficient electron emission can be maintained, while electron emission can be controlled and the surface layer can be protected.
In the first electron-emitting device, the coating layer preferably contains aluminum nitride (AlN).
In the first electron-emitting device, a region including the graded composition layer and the surface layer is preferably composed of Al
x
Ga
1−x
N (0≦x≦1) in which the proportion of Al increases with approach toward the outermost surface. The arrangement allows easy formation of the high-quality graded composition layer and surface layer.
In the first electron-emitting device, the surface electrode is preferably in Schottky contact with the surface layer. In the arrangement, the electrons can be supplied with excellent controllability from the electron supplying layer.
In the first electron-emitting device, the surface electrode preferably has first and second regions, an energy barrier between the surface layer and the second region being larger than an energy barrier between the surface layer and the first region. In the arrangement, the current of electrons can converge on the first region of the surface electrode and the density of the emitted current can be increased.
In the first electron-emitting device, the surface electrode preferably has a pattern for controlling the distribution of emitted electrons. As a result, a current of emitted electrons having a desired distribution can be generated with controllability.
The first electron-emitting device further comprises: an insulator layer overlying a part of
Deguchi Masahiro
Kitabatake Makoto
Setsune Kentaro
Tohda Takao
Uenoyama Takeshi
Loke Steven
Matsushita Electric - Industrial Co., Ltd.
Nadav Ori
Nixon & Peabody LLP
Robinson Eric J.
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