Field emission device and field emission display employing the same

A field emission device (6), in accordance with a preferred embodiment, includes a cathode electrode (61), a gate electrode (64), a separator (62), and a number of emissive units (63) composed of an emissive material. The separator includes an insulating portion (621) and a number of conductive portions (622). The insulating portion of the separator is configured between the cathode electrode and the gate electrode for insulating the cathode electrode from the gate electrode. The emissive units are configured on the separator at positions proximate to two sides of the gate electrode. The emissive units are in connection with the cathode electrode via the conductive portions respectively. The emissive units are distributed on the separator adjacent to two sides of the gate electrode, which promotes an ability of emitting electrons from the emissive material and the emitted electrons to be guided by the gate electrode toward a smaller spot they bombard.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a field emission device for emitting electrons from an emissive material and, more particularly, to a field emission device having an improved electron emission performance, which can be used for high-resolution field emission display.

2. Discussion of the Related Art

Field emission displays (FEDs) are new, rapidly developing flat panel display technologies. Compared to conventional technologies, e.g., cathode-ray tube (CRT) and liquid crystal display (LCD) technologies, FEDs are superior in having a wider viewing angle, low energy consumption, a smaller size, and a higher quality display. In particular, carbon nanotube-based FEDs (CNTFEDs) have attracted much attention in recent years.

Carbon nanotube-based FEDs employ carbon nanotubes (CNTs) as electron emitters. Carbon nanotubes are very small tube-shaped structures essentially composed of a graphite material. Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes can have an extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (potentially greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). Thus, carbon nanotubes can transmit an extremely high electrical current and have a very low turn-on electric field (approximately 2 volts/micron) for emitting electrons. In summary, carbon nanotubes are one of the most favorable candidates for electrons emitters in electron emission devices and can play an important role in field emission display applications.

Generally, FEDs can be roughly classified into diode type structures and triode type structures. Diode type structures have only two electrodes, a cathode electrode and an anode electrode. Diode type structures can be used in characters display, but are unsatisfactory for applications requiring high-resolution displays, such as picture and graph display, because of their relatively non-uniform electron emissions and difficulty in controlling their electron emission. Triode type structures were developed from diode type structures by adding a gate electrode for controlling electron emission. Triode type structures can emit electrons at relatively lower voltages.

FIG. 1is a schematic view illustrating a conventional triode type field emission device4, which includes a cathode electrode40, an anode electrode45spaced from the cathode electrode40and a gate electrode43disposed between the cathode and the anode electrodes40,45. A barrier44is disposed between the cathode electrode40and the anode electrode45thereby separating the two electrodes40,45. Generally, an insulating layer42is deposited on the cathode electrode40for supporting the gate electrode43, i.e., the gate electrode43is formed on a top surface of the insulating layer42. The insulating layer42defines a cylindrical hole (not labeled) therein for exposing the cathode electrode40. An emissive material41, such as carbon nanotube, is disposed in the cylindrical hole on the exposed cathode electrode40. Furthermore, a phosphor material46is formed on a surface of the anode electrode45facing to the cathode electrode40. In the illustrated structure, the phosphor material46represents a picture element for displaying. A picture element means a minimum unit of an image displayed by the FED (i.e., a pixel). In a typical color FED, the color picture is obtained by a display system using three optical primary colors, i.e., R (red), G (green), and B (blue).

In use, different voltages are applied to the cathode electrode40, the anode electrode45and the gate electrode43. Electrons410are emitted from the emissive material41, and then travel through the cylindrical hole, finally reach to the anode electrode45and the phosphor material46. Therefore, the phosphor material46is activated and a visible light is produced.

The above field emission device, however, has a low resolution. Because electrons extracted from the emissive material41are diverged away from a central axis of the phosphor material46when they travel to the anode electrode45, thus, a spot that electrons bombard on the phosphor material46is enlarged. In addition, some of the diverged electrons are diverged at a large angle and bombard on a neighboring picture element (not shown), therefore an error display is occurred. Furthermore, a high voltage for extracting electrons from the emissive material is needed because of a large distance between the emissive material and the gate electrode.

Therefore, what is needed is a field emission device having a high resolution, lower voltage for emitting electrons, and a high emission efficiency.

SUMMARY

Accordingly, a field emission device, in accordance with a preferred embodiment, includes a cathode electrode, a gate electrode, a separator, and a number of emissive units composed of an emissive material. The separator includes an insulating portion and a number of conductive portions. The insulating portion of the separator is configured between the cathode electrode and the gate electrode for insulating the cathode electrode from the gate electrode. The emissive units are configured on the separator at positions proximate to two sides of the gate electrode. The emissive units are in connection with the cathode electrode via the conductive portions respectively. That the emissive units are distributed on the separator adjacent to two sides of the gate electrode promotes the ability of emitting electrons from the emissive material and the emitted electrons to be guided by the gate electrode toward a smaller spot they bombard.

The exemplifications set out herein illustrate at least one preferred embodiment of the present field emission device, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe preferred embodiments of the present field emission device, in detail.

Referring toFIGS. 2 and 3, an exemplarily field emission device6in accordance with a first preferred embodiment is shown. The field emission device6includes a bottom substrate60, a number of cathode electrodes61disposed on the bottom substrate60, a separator62disposed on the cathode electrodes61, a number of gate electrodes64(only one is shown inFIG. 2for illustration) disposed on the separator62, and a number of emissive units63distributed on the separator62. The emissive units63are respectively distributed proximate two sides of a gate electrode64associated therewith.

Generally, the bottom substrate60includes a sheet of insulative plate composed of an insulation material, such as glass, silicon, ceramic, etc. The cathode electrodes61are disposed parallel to each other along a first direction on the bottom substrate60, and can be made of a conductive material, such as indium-tin-oxide (ITO) and metallic material. Each of the cathode electrodes61can be made into elongated stripe-shaped thin film or layer and is spaced from each other. The separator62is configured on the cathode electrode61for holding the gate electrodes64and the emissive units63. The separator62is composed of an insulation portion621and a number of conductive portions622distributed in the insulation portion621. Each of the conductive portions622is respectively located at a position corresponding to an emissive unit63and is configured for electrically connecting the respective emissive unit63to a corresponding cathode electrode61. The insulation portion621, i.e., the rest part of the separator62other than the conductive portions622, is disposed between the cathode electrodes61and the gate electrodes64, thus the former is insulated from the latter. Further, referring toFIG. 3, each of the emissive units63has an inner periphery638with a first diameter and an outer periphery639with a second diameter, and each of the conductive portions622has a third diameter larger than the first diameter but smaller than the second diameter. Each of the conductive portions622is positioned under and within an area defined by the outer periphery639of one corresponding emissive unit63. In the present embodiment, the conductive portions622can be made, for example, by following method: manufacturing an insulative prototype separator, etching a number of through holes in the prototype separator at predetermined positions; filling a conductive material, such as copper, silver and other metals having a good conductivity, into the through holes, thus a separator having a number of conductive portions embedded therein is obtained.

The gate electrodes64are disposed parallel to each other and are placed on the separator62along a second direction perpendicular to the first direction, thus the gate electrodes64are perpendicular to the cathode electrodes61. The gate electrodes64can be made of a conductive material, preferably a metal having good conductivity Each of the gate electrodes64can be made into longitudinal strip-shaped thin film or layer and is spaced from each other. In the present embodiment, each of the gate electrodes64defines a top surface641, a bottom surface (not labeled) opposite to the top surface641, and two lateral surfaces640between the top surface641and the bottom surface.

The emissive units63are made of an electron emissive material, such as carbon nanotubes, carbon fibers and sharp-tipped elements comprised of at least one of graphite carbon, diamond carbon, silicon, and an emissive conductive metal. Each of the emissive units63can be structured into a desired form, such as a rectangular shape, as shown inFIG. 2. In the present embodiment, each of the emissive units63defines a top surface631, a bottom surface opposite to the top surface631, and a number of lateral surfaces630between the top surface631and the bottom surface. Advantageously, each of the emissive unites63is arranged adjacent the gate electrode64, such that at least one of the lateral surfaces630of the emissive unit63is proximate and facing to one of the lateral surface640of the gate electrode64. As such, a distance between the lateral surface640of the gate electrode64and the proximate lateral surface630of the emissive unit63can be minimized without short-circuiting therebetween. Preferably, such distance can be, for example, about several microns or less. Therefore, a minimum electric field between the gate electrode and emissive units required for extracting electrons from the emissive units can be lowered, i.e., a threshold voltage applied for the gate electrode can be lowered.

Advantageously, the emissive units63associated with a corresponding gate electrode64are regularly arranged in two columns aligned the second direction. Each emissive unit63has at least a portion of the lateral surface630directly facing the proximate lateral surface640of the corresponding gate electrode64, i.e., at least a portion of a projection of the lateral surface630can be projected onto the proximate lateral surface640of the corresponding gate electrode64. In the present embodiment, the entire lateral surface630of the emissive unit63is directly facing the proximate lateral surface640of the gate electrode64. The top surface631and the bottom surface of each emissive unit63are substantially coplanar with the top surface641and the bottom surface of the gate electrodes64, respectively.

Referring toFIG. 4, a field emission display device7employing the above field emission device6, according to another embodiment, is shown. In addition to the field emission device6, the field emission display device7further includes a top plate78opposite to the bottom substrate60, an anode electrode77formed on a surface of the top plate78, a phosphor layer76composed of a number of picture elements761formed on the anode electrode77, and a number of spacers75configured for separating the top plate78from the bottom substrate60. Generally, the anode electrode may be made of an ITO conductive thin film. Each of the picture elements761of the phosphor layer76corresponds to a gate electrode64and two emissive units63proximate the gate electrode64. Preferably, the gate electrode64is directly facing a central area of the picture element761of the phosphor layer76. As such, the two emissive units63associated with the picture element761are configured for facing two side areas of the picture element761and offsetting from the central area of the picture element761.

In operation, electrons632can be extracted from the emissive units63by a strong electric field generated by the corresponding gate electrode64and focused on the central area of the picture element761or a vicinity thereof. Thus, a size of spot that electrons bombarded on the picture element is lowered and a resolution of displaying is improved. Specifically, electrons632emitted from the emissive unit63located at a left side of the gate electrode64are attracted towards the central area of the picture element761or a right side thereof during their travel to the anode electrode77. Similarly, electrons632emitted from the emissive unit63located at a right side of the gate electrode64are attracted towards the central area of the picture element761or a left side thereof during their travel to the anode electrode77.

Referring toFIG. 5, a field emission display device8employing the field emission device, according to a third embodiment is shown. For purpose of simplifying description, only one pixel structure of the display device is illustrated. The pixel structure of the display device is composed of three primary color areas for emitting three primary colors, i.e., red (R), green (G) and blue (B). Each of the primary color areas corresponds to a gate electrode64′ and two emissive units63′ proximate two sides of the gate electrode64′. Preferably, the gate electrode64′ is directly facing a central area of a primary color area. As such, the two emissive units63′ associated with the primary color area are configured for facing two sides of the central area of the primary color area. Therefore, electron emission for bombarding each of the primary color area can be precisely controlled, and a higher resolution displaying is realized.