Electric lamp and discharge devices – Cathode ray tube – Image pickup tube
Reexamination Certificate
1998-11-24
2001-03-06
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Image pickup tube
C313S373000, C313S34600R, C257S010000, C257S011000, C257S077000
Reexamination Certificate
active
06198210
ABSTRACT:
The invention relates to a semiconductor device for generating electrons comprising a semiconductor body of a semiconductor material having at least one structure for emitting electrons, which structure is adjacent to a main surface of the semiconductor body and in which structure electrons can be generated by applying suitable electric voltages, which electrons emanate from the semiconductor body at the location of an emitting surface region.
The invention also relates to an electron tube provided with such a semiconductor device.
The electron tube can be used as a display tube or a camera tube, but it may also be constructed so as to be suitable for electrolithographic applications or electron microscopy.
A semiconductor device of the type mentioned hereinabove is shown in U.S. Pat. No. 4,303,930 (PHN 9532). In the semiconductor device, which is a so-called “cold cathode”, a p-n junction is operated in the reverse direction in such a manner that avalanche multiplication of chare carriers takes place. As a result of this, electrons can receive sufficient energy to exceed the work function. The emanation of the electrons is further stimulated by the presence of accelerating electrodes or gate electrodes and by providing the semiconductor surface, at the location of the emitting surface region, with a work function-reducing material, such as cesium.
Particularly the use of cesium as the work function-reducing material often leads to problems. This can be attributed to the fact that, for example, cesium is sensitive to the presence (in the operating environment) of oxidizing gases (such as water vapor, oxygen, CO
2
). In addition, as cesium has a high vapor pressure, it evaporates easily, which may be a drawback in applications where (semiconductor) substrates or preparations are situated in the vicinity of the cathode, as is the case in electron lithography or electron microscopy. In addition, ESD (Electron Stimulated Desorption) occurs; the electrons emitted by the cathode induce desorption of the cesium, in particular from slightly oxidied surfaces. A slight degree of oxidation occurs, for example, during spot-knocking of the electron tube.
One of the objects of the invention is to overcome one or more of the above-mentioned problems. To achieve this, a semiconductor device in accordance with the invention is characterized in that the structure for emitting electrons is covered with at least one layer of a further semiconductor material having a larger bandgap than the first semiconductor material.
The invention is based on the insight that notwithstanding the fact that the larger bandgap of the further semiconductor material constitutes an additional barrier to electrons, which are generated in the cold cathode, these electrons still reach, depending on the electric voltage applied between the further semiconductor material and the structure for emitting electrons, the surface of the layer of the further semiconductor material. Subsequently, the electrons are emitted from the further semiconductor material into the vacuum.
The invention further provides a number of measures for reducing the above-mentioned barrier. For example, a preferred embodiment of a semiconductor device in accordance with the invention is characterized in that the further semiconductor material is doped with dopants causing n-type conduction. As a result of this, said barrier is reduced so that a lower electric voltage between the further semiconductor material and the structure suffices to enable electrons to emanate. The reduction of the barrier is preferably such that an electric voltage is not necessary. The further semiconductor material preferably has a negative electron affinity (NEA). This is a condition in which the energy level of the vacuums at the surface is below the energy level of the minimum of the conduction band of the relevant semiconductor material. A similar situation is achieved by coating semiconductor material which does not intrinsically exhibit NEA properties with a layer of a work function-reducing material, such as cesium. Even if said coating with a layer of a work function-reducing material does not lead to NEA properties, the advantage that the above-mentioned ESD effect is precluded is nevertheless achieved (the layer of a further semiconductor material now serves, as it were, as a bonding layer for the work function-reducing material).
In another embodiment, the electric voltage is not applied between the further semiconductor material and the structure for emitting electrons, but between (an) electrode(s) provided near the main surface of the semiconductor body. The so-called Schottky effect also causes a reduction of the barrier. The electrode is situated, for example, on the surface of the semiconductor body (gate electrode). In another example, the electrode is a grid in the electron tube. A combination is possible too.
The electron-mission efficiency of the cold cathode thus formed is further increased by covering the further semiconductor material with a layer of a work function-reducing material, such as cesium. The above-mentioned ESD effect no longer occurs because the further semiconductor material is practically inert.
Suitable materials for the further semiconductor material have a bandgap of the order of 2 to 6.5 eV. The materials are preferably selected from the group formed by silicon carbide (BSiC, 4HSiC and various other poly-types), aluminium nitride (for example hexagonal AlN), cubic boron nitride (cBN), gallium-arsenic nitride (Al
x
Ga
y
N) and carbon-based materials ((semiconducting) diamond, diamond-like carbon material, monocrystalline and polycrystafline diamond, amorphous carbon).
To avoid bonding problems as well as mechanical stresses, if necessary, an additional layer of a material whose lattice constant lies between that of the semiconductor material and that of the further semiconductor material is situated between the semiconductor body and the further layer of semiconductor material.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 4616248 (1986-10-01), Khan et al.
patent: 5880481 (1999-03-01), Kroon et al.
Hijzen Erwin A.
Kroon Ron
Van Zutphen Tom
Haynes Mack
Kraus Robert J.
Patel Nimeshkumar D.
U.S. Philips Corporation
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