Patent Publication Number: US-7714493-B2

Title: Field emission device and field emission display employing the same

Description:
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. 1  is a schematic view illustrating a conventional triode type field emission device  4 , which includes a cathode electrode  40 , an anode electrode  45  spaced from the cathode electrode  40  and a gate electrode  43  disposed between the cathode and the anode electrodes  40 ,  45 . A barrier  44  is disposed between the cathode electrode  40  and the anode electrode  45  thereby separating the two electrodes  40 ,  45 . Generally, an insulating layer  42  is deposited on the cathode electrode  40  for supporting the gate electrode  43 , i.e., the gate electrode  43  is formed on a top surface of the insulating layer  42 . The insulating layer  42  defines a cylindrical hole (not labeled) therein for exposing the cathode electrode  40 . An emissive material  41 , such as carbon nanotube, is disposed in the cylindrical hole on the exposed cathode electrode  40 . Furthermore, a phosphor material  46  is formed on a surface of the anode electrode  45  facing to the cathode electrode  40 . In the illustrated structure, the phosphor material  46  represents 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 electrode  40 , the anode electrode  45  and the gate electrode  43 . Electrons  410  are emitted from the emissive material  41 , and then travel through the cylindrical hole, finally reach to the anode electrode  45  and the phosphor material  46 . Therefore, the phosphor material  46  is activated and a visible light is produced. 
   The above field emission device, however, has a low resolution. Because electrons extracted from the emissive material  41  are diverged away from a central axis of the phosphor material  46  when they travel to the anode electrode  45 , thus, a spot that electrons bombard on the phosphor material  46  is 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. 
   Other objects, advantages and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the present field emission device can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present device. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a schematic, cross-sectional view of a conventional field emission device; 
       FIG. 2  is a schematic, isometric view of a field emission device, according to a first preferred embodiment; 
       FIG. 3  is an partial cross-sectional view along line III-III of  FIG. 2 ; 
       FIG. 4  is a schematic, cross-sectional view of a field emission display, according to a second embodiment; and 
       FIG. 5  is a schematic, cross-sectional view of a field emission display, according to a third embodiment. 
   

   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 to  FIGS. 2 and 3 , an exemplarily field emission device  6  in accordance with a first preferred embodiment is shown. The field emission device  6  includes a bottom substrate  60 , a number of cathode electrodes  61  disposed on the bottom substrate  60 , a separator  62  disposed on the cathode electrodes  61 , a number of gate electrodes  64  (only one is shown in  FIG. 2  for illustration) disposed on the separator  62 , and a number of emissive units  63  distributed on the separator  62 . The emissive units  63  are respectively distributed proximate two sides of a gate electrode  64  associated therewith. 
   Generally, the bottom substrate  60  includes a sheet of insulative plate composed of an insulation material, such as glass, silicon, ceramic, etc. The cathode electrodes  61  are disposed parallel to each other along a first direction on the bottom substrate  60 , and can be made of a conductive material, such as indium-tin-oxide (ITO) and metallic material. Each of the cathode electrodes  61  can be made into elongated stripe-shaped thin film or layer and is spaced from each other. The separator  62  is configured on the cathode electrode  61  for holding the gate electrodes  64  and the emissive units  63 . The separator  62  is composed of an insulation portion  621  and a number of conductive portions  622  distributed in the insulation portion  621 . Each of the conductive portions  622  is respectively located at a position corresponding to an emissive unit  63  and is configured for electrically connecting the respective emissive unit  63  to a corresponding cathode electrode  61 . The insulation portion  621 , i.e., the rest part of the separator  62  other than the conductive portions  622 , is disposed between the cathode electrodes  61  and the gate electrodes  64 , thus the former is insulated from the latter. Further, referring to  FIG. 3 , each of the emissive units  63  has an inner periphery  638  with a first diameter and an outer periphery  639  with a second diameter, and each of the conductive portions  622  has a third diameter larger than the first diameter but smaller than the second diameter. Each of the conductive portions  622  is positioned under and within an area defined by the outer periphery  639  of one corresponding emissive unit  63 . In the present embodiment, the conductive portions  622  can 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 electrodes  64  are disposed parallel to each other and are placed on the separator  62  along a second direction perpendicular to the first direction, thus the gate electrodes  64  are perpendicular to the cathode electrodes  61 . The gate electrodes  64  can be made of a conductive material, preferably a metal having good conductivity Each of the gate electrodes  64  can be made into longitudinal strip-shaped thin film or layer and is spaced from each other. In the present embodiment, each of the gate electrodes  64  defines a top surface  641 , a bottom surface (not labeled) opposite to the top surface  641 , and two lateral surfaces  640  between the top surface  641  and the bottom surface. 
   The emissive units  63  are 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 units  63  can be structured into a desired form, such as a rectangular shape, as shown in  FIG. 2 . In the present embodiment, each of the emissive units  63  defines a top surface  631 , a bottom surface opposite to the top surface  631 , and a number of lateral surfaces  630  between the top surface  631  and the bottom surface. Advantageously, each of the emissive unites  63  is arranged adjacent the gate electrode  64 , such that at least one of the lateral surfaces  630  of the emissive unit  63  is proximate and facing to one of the lateral surface  640  of the gate electrode  64 . As such, a distance between the lateral surface  640  of the gate electrode  64  and the proximate lateral surface  630  of the emissive unit  63  can 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 units  63  associated with a corresponding gate electrode  64  are regularly arranged in two columns aligned the second direction. Each emissive unit  63  has at least a portion of the lateral surface  630  directly facing the proximate lateral surface  640  of the corresponding gate electrode  64 , i.e., at least a portion of a projection of the lateral surface  630  can be projected onto the proximate lateral surface  640  of the corresponding gate electrode  64 . In the present embodiment, the entire lateral surface  630  of the emissive unit  63  is directly facing the proximate lateral surface  640  of the gate electrode  64 . The top surface  631  and the bottom surface of each emissive unit  63  are substantially coplanar with the top surface  641  and the bottom surface of the gate electrodes  64 , respectively. 
   Referring to  FIG. 4 , a field emission display device  7  employing the above field emission device  6 , according to another embodiment, is shown. In addition to the field emission device  6 , the field emission display device  7  further includes a top plate  78  opposite to the bottom substrate  60 , an anode electrode  77  formed on a surface of the top plate  78 , a phosphor layer  76  composed of a number of picture elements  761  formed on the anode electrode  77 , and a number of spacers  75  configured for separating the top plate  78  from the bottom substrate  60 . Generally, the anode electrode may be made of an ITO conductive thin film. Each of the picture elements  761  of the phosphor layer  76  corresponds to a gate electrode  64  and two emissive units  63  proximate the gate electrode  64 . Preferably, the gate electrode  64  is directly facing a central area of the picture element  761  of the phosphor layer  76 . As such, the two emissive units  63  associated with the picture element  761  are configured for facing two side areas of the picture element  761  and offsetting from the central area of the picture element  761 . 
   In operation, electrons  632  can be extracted from the emissive units  63  by a strong electric field generated by the corresponding gate electrode  64  and focused on the central area of the picture element  761  or 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, electrons  632  emitted from the emissive unit  63  located at a left side of the gate electrode  64  are attracted towards the central area of the picture element  761  or a right side thereof during their travel to the anode electrode  77 . Similarly, electrons  632  emitted from the emissive unit  63  located at a right side of the gate electrode  64  are attracted towards the central area of the picture element  761  or a left side thereof during their travel to the anode electrode  77 . 
   Referring to  FIG. 5 , a field emission display device  8  employing 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 electrode  64 ′ and two emissive units  63 ′ proximate two sides of the gate electrode  64 ′. Preferably, the gate electrode  64 ′ is directly facing a central area of a primary color area. As such, the two emissive units  63 ′ 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. 
   It is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.