Patent Publication Number: US-5157309-A

Title: Cold-cathode field emission device employing a current source means

Description:
TECHNICAL FIELD 
     This invention relates generally to cold-cathode field emission devices and more specifically to methods and devices used to control electron emission from cold-cathode field emission devices. 
     BACKGROUND OF THE INVENTION 
     Cold-cathode field emission devices (FEDs) are known in the art. FEDs can be constructed by a variety of processes, virtually all of which yield structures that emit electrons from an emitter electrode. 
     A common problem with FEDs is that emitter electron emission is not accurately controllable, due at least in part to FED fabrication inconsistencies. Electronic devices that are comprised of arrays of large numbers of FEDs can yield a minority of heavily conducting field emission devices and a majority of non-conducting field emission devices. As such, various methods have been employed as attempts to realize FEDs with accurately controlled electron emission. 
     Known methods of controlling FED emission require that a controlling voltage be employed to modulate or limit the electron emission. Since FED emission characteristics are related to process variables, it is not practical to establish a voltage/emission relationship which will be applicable for successive FED fabrications or to individual FEDs within a group from a single fabrication. 
     Accordingly, there exists a need for accurately controlling electron emission from FEDs. 
     SUMMARY OF INVENTION 
     The need for controlling electron emission from FEDs is substantially met by employing a current source, coupled to the emitter electrode of an FED to control emitter electron emission. In one embodiment, the open circuit voltage of the current source is selected to induce emitter electron emission regardless of the gate voltage. In the preferred embodiment, the open circuit voltage of the current source is chosen to be insufficient to induce appreciable electon emission from the emitter electrode in the absence of an appropriate extraction potential on the gate. An appropriate extraction potential on the gate would be determined by the open circuit voltage of the emitter current source so as to produce a sufficient potential difference between the gate and the emitter to establish the electric field necessary to effect emitter electron emission. 
     In alternate embodiments of the invention that would include multiple FEDs forming an array of FEDs, such as a two-dimensional array of FEDs, a current source might be coupled to either the emitter of each device, or to the emitters of a group FEDs. Further, a plurality of current sources may be selectively independently coupled to individual emitters or groups of emitters in an array of FEDs. In such arrangements, the current sources can control electron emission from the FEDs. 
     (For the purposes of this disclosure, a current source can be considered to include any determinate source of electrons. Some exemplary current sources are briefly described herein.) 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 comprises a schematic diagram of an FED with an emitter current source and gate voltage source. 
     FIG. 2 comprises a top view of an array of clustered FEDs. Each FED cluster has four individual FEDs. 
     FIGS. 3 and 4 are schematic depictions of current sources. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 an FED circuit (100) for controlling FED electron emission is depicted that includes an FED having an emitter electrode (102), a gate electrode (103) and an anode (104). The emitter electrode (102) is coupled to a current source (101) that controls electron emission from the emitter electrode (102). Depending upon the open circuit voltage of the current source (101) an appropriate extraction potential (105) may be applied to the gate electrode to induce electron emission. (As stated above, the electrons supplied by the current source will be emitted from the emitter when the gate emitter potential is sufficient to induce emitter electon emission.) 
     In the embodiment shown in FIG. 1 an anode (104) collects at least some of the electrons emitted from the emitter (102). Other FED circuits might not utilize electron-collecting anodes. 
     FIG. 2 depicts a top view of an array (200) of FEDs (203), each FED being similar to the FED shown in FIG. 1. The plurality of FEDs (203) shown in FIG. 2 are symmetrically arranged along columns (C 1  -C 4 ) and rows (R A  -R B ) with respect to each other. The emitter electrodes (102) of FEDs along a column (C 1  for example) are operably coupled to a corresponding column (C 1 ) while the gate electrodes (103) of the FEDs along a row (R A  for example) are operably connected to a corresponding row (R A ). (In the embodiment shown in FIG. 2, at each cross-over of a column and row, four FEDs are shown. Alternate embodiments would include a single FED at each cross over as well as any number of FEDs at each cross over.) 
     Rotation of the structure shown in FIG. 2 by 90 degrees, alters the designation of rows and columns wherein references to columns and rows are interchanged. 
     The columns of interconnected emitter electrodes (102) of the FEDs (203) are formed during fabrication of the FEDs (203) by selectively connecting the emitter electrodes (102) of the corresponding FEDs (203) to column conductor stripes (201). The column conductor stripes (201) may be formed by any of the commonly known methodologies such as, for example: evaporation, sputtering, ion implantation, or diffusion doping, or any other appropriate technique. Rows of interconnected FEDs (203) are formed by selectively connecting the gate electrodes (103) of the corresponding FEDs (203) to row conductor stripes (202). The row conductor stripes (202) may be formed using any of the appropriate techniques as previously described for column conductor stripes (201). 
     The electronic device (200), depicted in FIG. 2, forms a matrix of FEDs addressed by row conductor stripes (202) and column conductor stripes (201), both of which may be selectively and independently energized to induce electron emission from one or more selected FEDs (203). Although the device shown in FIG. 2 depicts a plurality of FEDs (203) that can be selectively energized by any combination of a row conductor stripe (202) and column conductor stripe (201), alternative embodiments could provide for independently selecting a single FED (203) in an array of FEDs (203). 
     Electron emission in the FEDs shown in FIG. 2 is effected by coupling each column conductor stripe (201) to a current source (204). (Each column conductor stripe is connected to the emitter electrodes of its associated FEDs (203).) 
     The current source (204) provides a source of electrons that can be emitted by the emitter electrodes (102) of the FEDs (203), if an appropriate extraction potential is applied to at least one of the row conductor stripes (202). In the absence of an appropriate extraction potential (105) on any row conductor stripe (202), the output voltage of the current source (204) will increase, eventually reaching a pre-determined limit value. This open circuit voltage of the current source (204) should not be large enough to induce electron emission from the emitter (102) without the applied extraction potential (105). When an extraction potential is applied to at least one row conductor stripe (202), the output voltage of the current source (204) will assume a level necessary to induce electron emission, at the emitter electrodes of the FEDs (203), corresponding to the current level delivered by the current source (204). 
     Alternative embodiments might provide for electron emission to be induced independent of gate extraction potential; wherein the voltage level of the current source is not restricted to the pre-determined level as described above. Such alternative embodiments may provide that the gate electrode be operated at zero volts, or at a negative potential (less than zero), in which instance the operating voltage of the current source will be shifted correspondingly more negative so as to develop the prescribed gate to emitter potential differential required to establish the electric field necessary to effect electron emission. 
     As depicted in FIG. 2, each column conductor stripe (201) of a plurality of column conductor stripes (201) is connected to a single current source (204). Individual FEDs or, as depicted in FIG. 2 a plurality of FEDs (203) comprising a group of FEDs (203) or corresponding to a row conductor stripe (202) and a column conductor stripe (201) may be selected to emit an electron current prescribed by a current source (204). A plurality of columnarly independent FEDs (203) or groups of FEDs (203) can be simultaneously selected to emit an electron current prescribed by a plurality of current sources (204a-204d) that are each coupled to one of the plurality of columns by applying an appropriate extraction potential to a selected row conductor stripe (202a-202d). In this manner, a selected row of FEDs will emit an electron current with the emission level of each FED or group of FEDs (203) being modulated by the current source (204) connected to the column conductor stripe (201) associated with the FEDs (203) of the selected row and columns. 
     (Although a single current source is depicted as being coupled to each of the column conductor stripes, alternated embodiments might include multiple current sources coupled to a single column conductor stripe.) 
     Multi-row addressing of FEDs may be implemented by sequentially applying a single voltage source to each of the plurality of row conductor stripes or by selectively energizing each of a plurality of voltage sources coupled to each of the plurality fo row conductor stripes. If, while sequentially addressing each of the plurality of rows, the electron current to each of the plurality of columns is modulated, the resulting electron emission will be suitable for energizing an anode configured as a luminescent viewing screen. The resultant device is a cathodoluminescent display. 
     FIGS. 3 and 4 schematically depict possible embodiments of current sources that might be appropriate for implementing the current sources used in FIGS. 1 and 2. The current sources depicted are merely examples of some commonly known in the art and should not be considered as inclusive. Reference symbols in FIGS. 3, and 4 show current direction, rather than electron flow. 
     Referring to FIG. 3 a first embodiment of a current source (300) is shown that is comprised of a reference transistor (302), an output transistor (301), and a reference resistive circuit element (303), all of which are interconnected to provide a prescribed output transistor (301) collector current, I E . The magnitude of the open circuit output voltage is established by the power supply for the current source (300). 
     FIG. 4 depicts a current source (400) comprised of an operational amplifier (401), an output transistor (402), and a resistive circuit element (403), all of which are inter-coupled to provide a prescribed output transistor (402) drain current, 1 E .