Semiconductor device manufacturing: process – Making conductivity modulation device
Reexamination Certificate
1999-11-09
2001-04-10
Turner, Archene (Department: 1775)
Semiconductor device manufacturing: process
Making conductivity modulation device
C427S077000
Reexamination Certificate
active
06214651
ABSTRACT:
BACKGROUND
Field of Invention
The present invention is related to cold cathode technology, and in particular a new use for Nitrogen-doped Chemical-Vapor-Deposited Diamond as a means of enhancing the performance of previously disclosed Vacuum Diode Heat Pumps and Vacuum Diode Thermionic Converters.
Background—Electron Devices
All vacuum electron devices require a physical source of electrons in the form of a cathode. Traditionally, cathodes for vacuum tubes and cathode ray tubes use thermionic emission to produce the electrons. This requires raising cathode materials to very high temperatures either by direct conduction of current or through the use of auxiliary heaters. The process is inefficient, requiring relatively high currents and dissipating much energy as heat to the surrounding area.
Recently, there has been substantial investigation of replacements for the heated thermionic cathodes. Specifically, “cold cathode” devices have attracted much attention. These cathodes may be very efficient because they eliminate the need to heat the cathode material. There are three types of cold cathode known to the art. In the field emission type of cold cathode device, electrons are emitted from the tip of an emitter cone. In the tunnel type of cold cathode device, electrons pass through a thin insulating film by the tunneling effect. In the avalanche type of cold cathode device, the electrons emitted are a fraction of a current that flows through a reverse biased p-n junction of a diode oriented such that the junction is parallel to the surface of the emitter.
While these cold cathode structures can be made in almost any size and may have many applications as single units, their best performance and major application is expected to come from extreme miniaturization, in dense structures.
Cold cathode structures are useful electron sources for applications such as flat panel displays, vacuum microelectronic devices, amplifiers, and electron microscopes. Additional electrodes may be, and commonly are, used to collect and/or control the electron current. This technology is presently undergoing extensive development, with many articles being published and numerous patents being issued. Work in the art has been focused on the development of better emissive structures and materials, the use of such devices in electronic applications, and enhanced methods of fabricating such devices as well as fabricating integrated devices.
Background—Thermionic Emissions
All material may be characterized by a “work function.” The work function is the quantity of energy required to move a single electron from the conduction band of a neutral sample of the material to free vacuum. Generally the work function is measured in electron volts. This work function may be considered a potential barrier to the escape of electrons from the material. A similar measure used to describe insulating materials is called “Electron Affinity,” so called because the conduction band of insulators is not occupied, and thus needs to be populated before a work function can be measured.
Electrons within materials may only occupy restricted energy bands, such as the low energy ‘valence’ band and the higher energy ‘conduction’ band in an insulator. In metals, the valence band is partially occupied, and thus forms the conduction band. In insulators, the valence band is fully occupied, and thus cannot conduct, and the next higher band forms the conduction band, but has no electrons in it and again cannot conduct. In semiconductors, the energy difference between the valence band and the conduction band is small enough that electrons may be ‘promoted’ to the conduction band, allowing some conduction.
Electrons are emitted from the highest occupied band, the conduction band. It is well known that dopant materials may be used to introduce electrons into the conduction band (N type doping) or to remove electrons from the valence band (P type doping). Higher temperatures will increase the number of electrons promoted to higher energy levels.
Where a material possesses a negative electron affinity, such that the bottom of the electronic conduction band lies above the ‘vacuum band’—the energy of a free electron in a vacuum—electrons can escape spontaneously from the material if they are promoted to the conduction band. These low or negative work function materials thus have the potential to act as cold cathodes.
The conduction band electrons of a conductor exhibits a distribution in kinetic energy, much as the individual molecules of a gas move at widely varying speeds. Some fraction of the electrons present in the conduction band of the conductor will be moving at such a speed and in such a direction that they may overcome the potential barrier of the work function, and escape the conductor. Positing a lone conductor in space, the escaping electrons will cause a negative charge to be built up in the region surrounding the conductor, while the conductor acquires a positive charge.
With negative electron affinity materials, all electrons in the conduction band are capable of escaping to vacuum. However, in this case, the electrons in the band which donates electrons to the conduction band will demonstrate a distribution of kinetic energies, and only a fraction will be able to overcome the potential barrier to the conduction band.
When additional circuit elements are added and an external electric field is applied, a current can be caused to flow; electrons escape from the cathode, are carried by the electric field to the anode, and are then carried back to the cathode via a conductor. If the source of electric potential is part of the return circuit, then the device is a standard vacuum diode. If the load is additionally part of the return circuit, then it is a vacuum thermionic converter, using the heat applied to the cathode in order to produce an electric current flow. This device is well known in the art.
Background—Thin Film Diamond
The desirability of materials with negative electron affinity has already been discussed. One such material is diamond. The conduction band for diamond is of high energy, depending upon impurities and crystal orientation, above vacuum energy, enabling the spontaneous emission of electrons. Methods for depositing a diamond film by high current density DC glow discharge are known in the art. These methods are capable of both forming a uniform positive column between a deposition cathode and a substrate, and keeping the positive column stable for a long time, thereby synthesizing a thick, high quality, large-area diamond film.
The technology of carbonaceous films is also disclosed in a patent titled “Fabrication of Amorphous Diamond Films,” by Steven Falabella, patent issue date: Dec. 12, 1995 U.S. Pat. No. 5,474,816. This patent discloses a method for coating a substrate with an amorphous diamond film by cooling a substrate; biasing the substrate: and condensing carbon ions thereon.
The object of this method is to reduce the intrinsic stress of amorphous diamond, in order to make it possible to provide a more durable coating in order to enhance the lifetime objects coated with the amorphous diamond. This method is disclosed as being important as a solid lubricant, in order to prevent or delay the failure of mechanical system.
The technology of low work-function cathodes is further disclosed in a patent titled “Electron Device Employing a Low/Negative Electron Affinity Electron Source,” by Xiaodong T. Zhu, et al., patent issue date Feb. 1, 1994, U.S. Pat. No. 5,283,501. They disclose the use of an electron source formed of a layer of single crystal diamond material in having a low or negative work-function cathode.
Background—Nitrogen-Doping of Thin Film Diamond Cathode Surfaces
It has been shown that diamond is a suitable material for the construction of surfaces which allow electrons to escape spontaneously from the surface of a cathode. However, in pure diamond the conduction band is empty and the material is an insulator. In order to cause electron emission, very high potential differences must be applied to r
Borealis Technical Limited
Turner Archene
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