Modulated intensity FED display

Abstract: An image display employing an array of field emission devices (860) each of which, whether individually or in concert with some others of the plurality, provides electron current to an anode (806) having a cathodoluminescent layer (870) and wherein the electron emission is substantially controlled by constant current sources (810, 811, 812, 813) associated therewith and wherein the luminous intensity of selected regions (850, 851, 852, 853, 854) of the cathodoluminescent layer (870) is determined by the duration of the ON state of each controlled current source (810, 811, 812, 813).

Description: Background of the Invention This invention relates, generally, to field emission devices and, more particularly, to field emission devices employed as image display devices, as disclosed in WO-A-9 205 571. Field emission devices (FEDs) are known and commonly include electron emitters which emit electrons into a free-space or vacuum region by means of an induced electric field near the surface of the electron emitter. The electric field in many instances is realized by providing an extraction electrode or gate electrode in close proximity to the electron emitter and applying a suitable potential therebetween. Emitted electrons are commonly, although not necessarily, collected by a distally disposed anode; however, in many instances, field emission devices are identified as electron emitters with only an associated extraction electrode. In the instances when field emission devices are employed as electron sources for display devices, it is desirable to effect a means to control electron emission to realize a preferred display image. For example, in order to provide an image on a viewing screen, electrons are emitted from some of a plurality of individually addressable field emission devices or some of an array of individually addressable field emission devices. Further, it may also be desirable, for some applications, to exercise a level of control over the brightness of each picture element or pixel of a plurality of picture elements or pixels which includes the display. It is known that by providing a select voltage between the extraction electrode of the field emission device and the electron emitter that the electron emission from the electron emitter will be prescribed in accordance with the electric field induced at an emitting surface of the electron emitter in accordance with the Fowler - Nordheim relation which may be generally expressed as: J = AE 2 /Øexp[BØ 3/2 /E] In the above relationship, it is seen that current density (J) from the electron emitter is a strong function of an induced electric field (E), which is directly related to an applied extraction voltage. However, a number of other factors determine a magnitude of the induced electric field. A first factor is proximity of the extraction electrode to the electron emitter. The closer the extraction electrode, for a given applied extraction voltage, the greater the magnitude of the induced electric field. A second factor inversely relates the magnitude of the induced electric field to the radius of curvature of the electron emitting structure or electron emitter. Electron emitters formed as sharp tips, edges, or cones provide for high electric field enhancement near the emitting tip which includes a region of geometric discontinuity having a very small radius of curvature. A third factor, as can be seen in the Fowler - Nordheim relation given above, the emitted current is a strong function of the material surface work function (Ø). Since each of these factors provide some variation to each field emission device of any array of field emission devices, it is not practical to effect emission control by adjusting the extraction voltage or gate voltage between the gate electrode and the electron emitter. That is to say the inventor has observed, the electron emission from the electron emitters of any two field emission devices in an array of field emission devices are dissimilar due to fabrication variables and complex methods used in the prior art to compensate for these and other variations, thus making the complex methods undesirable. An alternative technique employed to effect electron emission control from field emission devices is to provide a controllable determined current source to the electron emitters of each field emission device of the array of field emission devices. By determining the available current to each field emission device, it is not necessary to be concerned with fabrication variations because the voltage between the extraction electrode and the electron emitter will assume any required value (within the limits established by attendant voltage sources) to deliver the determined current. Controllable determined current source methods pose shortcomings to desired performance in some applications. Each field emission device (FED) electron emitter has associated therewith a capacitance that must be charged each time the corresponding FED is required to emit electrons. In some applications, the controlled current sources are required to provide dissimilar currents to each electron emitter of a plurality of FEDs in an array of FEDs in order to effect a gray scale capability for an image display. FEDs corresponding to pixel locations where the image display luminous intensity is desirably low will have imposed a requirement for a low electron emission and, therefore, a low determined current from the controlled determined current source associated therewith. The time required to charge the capacitance associated with the electron emitter of any FED is partially a function of a maximum available current into the capacitance. As such, controlled determined current sources provide an inadequacy in that determined current levels corresponding to desirable low FED emission levels are not able to charge the associated capacitance. Further, in applications employing controllable determinate current sources, wherein the gray scale is effected by distinctly dissimilar current levels, the associated FED emitter capacitance must charge to a different level for each controlled determined current level. This is readily apparent when considering that the emission current density is a function of the voltage between the gate electrode and the electron emitter and that in order to provide a prescribed or determined current the voltage will assume a corresponding value. That is, a high current, corresponding to a high luminous level, will dictate a higher voltage than will a low current, corresponding to a low luminous level. This variation in the current available for emission and coincidentally charging of the associated capacitance provides for dissimilar electron emission characteristics at each electron emitter of the array of FEDs. In many applications, variations are intolerable and limit utility of this method of operation for image displays. Accordingly, there exists a need for a method and a field emission device control circuitry which overcomes at least some of these shortcomings. Summary of the Invention This need and others are substantially met through provision of a field emission device including an electron emitter, for emitting electrons; an extraction electrode proximally disposed with respect to the electron emitter, and an anode for collecting at least some of the emitting electrons, distally disposed with respect to the electron emitter; each electron emitter having a capacitance associated therewith, the device comprising: a controlled current source operably coupled between the electron emitter and a reference potential, the controlled current source providing a pulse of constant current to the electron emitter for enhancing response time for emission of electrons from the electron emitter in response to current controlling signals applied to the external current controlling terminal; and; a controlling input line, for providing current controlling signals to the controlled current source, the controlling input line being operably coupled to the controlled current source for receiving signals determinative of the duration of the pulse current; voltages operably coupled between the extraction electrode and the reference potential, between the anode and the reference potential, and between the controlling input line and the reference potential so as to place the field emission device in an ON state. Brief Description of the Drawings FIG. 1 is a schematic representation of a field emission device with operably coupled externally provided voltage sources; FIG. 2 is a schematic representation of a field emission device with operably coupled voltage sources and constant current source; FIG. 3 is a partial schematic representation of a plurality of field emission devices; FIG. 4 is a schematic representation of a field emission device electron emitter circuit; FIGS. 5-7 are graphical representations of voltage vs. time relationships for field emission device electron emitter circuit operation; FIGS. 8-10 are graphical representations of current vs. time relationships for field emission device electron emitter circuit operation in accordance with the present invention; FIG. 11 is a schematic representation of a field emission device with operably coupled voltage sources and controlled constant current source in accordance with the present invention; and FIG. 12 is a schematic representation of a controlled constant current source field emission device image display in accordance with the present invention. Detailed Description of the Drawings FIG. 1 is a schematic representation of a field emission device 100, represented by dashed lined box, including an electron emitter 101 for emitting electrons, an extraction electrode or gate electrode 102, and an anode 103 for collecting at least some of any emitted electrons. In physical embodiments of field emission devices, extraction electrode 102 is proximally disposed with respect to electron emitter 101 and substantially symmetrically peripherally about a radius with respect to electron emitter 101. Anode 103 is distally disposed with respect to electron emitter 101 in which instance the schematic representation of FIG. 1 is further descriptive as a cross-sectional depiction. Also depicted is a first externally provided voltage source 104 that is operably coupled between gate electrode 102 and electron emitter 101, and a second externally provided voltage source 105 that is operably coupled between anode 103 and electron emitter 101. For purposes of the present illustration and as practicable, operable connection of first and second externally provided voltage sources 104 and 105 depicted as being made to electron emitter 101 may in fact be referred to as being operably coupled to a reference potential, such as a ground reference, in which instance electron emitter 101 would also be operably coupled to the reference potential. Operation of a field emission device such as that schematically depicted with reference to FIG. 1 is effected by providing a suitable voltage at extraction electrode 102 such as that which is provided by the operably coupled first externally provided voltage source 104. For example, voltages between extraction electrode 102 and the reference potential range from 5.0 volts to 200.0 volts. By virtue of voltage source 104 applied to extraction electrode 102, an electric field is induced at a surface of electron emitter 101 which gives rise to electron emission from electron emitter 101. When a suitable voltage is applied to anode 103 such as that which is applied by the operably coupled second voltage source 105 at least some emitted electrons are collected at anode 103. For example, voltages between anode 103 and the reference potential range from 5.0 volts to 20,000.0 volts. Emitted electron current is capable of being varied by modulating the magnitude of the voltage applied to extraction electrode 102. However, variations in physical realizations preclude a possibility that this method of modulating electron emission is adequately reproducible from one field emission device to another. FIG. 2 is a schematic representation of a field emission device 200, represented by dashed line box, including an electron emitter 201, for emitting electrons, an extraction electrode or gate electrode 202, and an anode 203, for collecting at least some of any emitted electrons. Also depicted is a first externally provided voltage source 204, operably coupled between gate electrode 102 and a reference potential 240, a second externally provided voltage source 205, operably coupled between anode 203 and reference potential 240, and a constant or determined current source 206 operably coupled between electron emitter 201 and reference potential 240. Operation of field emission device 200 is effected by providing a suitable extraction voltage to extraction electrode 202 as described previously with reference to FIG. 1 and by providing a constant current of electrons to electron emitter 201 by means of constant current source 206 operably coupled thereto. At least some emitted electrons are collected at anode 203 as described previously with reference to FIG. 1. For field emission device 200, it is observed that by providing constant current source 206 fabrication dependent emission characteristics will not affect electron emission within limits of constant current source 206 to compensate for the fabrication variations. Constant current source 206, typically, is a combination of electronic elements that work in combination to provide a predetermined current to a load. In addition, constant current sources typically function by providing a determined constant current to a connected load without restriction as to the load characteristics. Constant current source 206 effects this particular circuit operation by providing a constant current source output voltage, which is operably connected to the electron emitter 201, thus allowing a range of voltages to be provided to the load, e.g., electron emitter 201, with the desired constant current. With respect to field emission device 200 to which constant current source 206 is operably connected, the voltage applied to electron emitter 201 via constant current source 206 assumes a value which in conjunction with extraction electrode 202 voltage provides a desired electron emission in accordance with the Fowler - Nordheim relation described earlier. FIG. 3 is a partial schematic representation of a plurality of field emission devices 350, 351, and 352 each having associated therewith an electron emitter 301, 302, and 303 and a constant current source of a plurality of constant current sources 306, 307, and 308. An anode 313 for collecting at least some of any emitted electrons from any of electron emitters 301, 302, and 303 is depicted. Each constant current source 306, 307, and 308 may be selectively employed to provide the associated (operably connected) electron emitter or electron emitters 301, 302, and 303 a prescribed constant current which is capable of being dissimilar from any constant current which may be selectively provided by any other of the plurality of constant current sources 306, 307, and 308 to any other of electron emitters 301, 302, and 303 of the plurality of field emission devices 350, 351, and 352. FIG. 4 is a schematic representation of a part of a field emission device electron emitter circuit 400 wherein a circuit distributed impedance is depicted as a plurality of discrete shunt capacitances 402 and series resistances 401 operably connected to a gate-emitter capacitance 403. It will readily be observed that application of a constant current source to an embodiment of circuit 400 at a location referenced as a terminal IN causes capacitances 402 and 403 to charge in accordance with the current provided. In the instance of a large current (on the order of microAmperes) injected at terminal IN, capacitances 402 and 403 charge during a period of time and the corresponding voltage appearing in circuit 400 increases quickly and continues to increase until a level is reached whereat the electron emission from the attendant electron emitter (represented as gate-emitter capacitance 403) is substantially equal to the injected constant current. In the instance of a small current (on the order of nanoAmperes) injected at terminal IN, capacitances 402 and 403 charge during a longer period of time and the corresponding voltage appearing in circuit 400 increases slowly and continues to increase until a level is reached whereat the electron emission from the attendant electron emitter (represented as gate-emitter capacitance 403) is substantially equal to the injected constant current. FIGS. 5-7 are graphical representations of charging effects on the voltage which are developed in electron emitter circuit 400 as described previously with reference to FIG. 4. Referring now to the graphical representation of FIG. 5, it is observed that for a high injected current, electron emitter circuit 400 is charged rapidly to a desired voltage level 501, thus providing a prescribed constant current to the attendant electron emitter. For a moderate injected current, such as that which would be provided for less electron emission from a field emission device, electron emitter circuit 400 charges over a longer time interval, to charge to a desired voltage level 502 as depicted in FIG. 6, than that which is depicted in FIG. 5. For a low injected current, such as that which would be provided for low electron emission from a field emission device, the electron emitter circuit 400 charges very slowly as depicted in FIG. 7 and possibly will not reach a desired voltage level 503 during a prescribed period of time in order to provide that the prescribed constant current be emitted as electron current at the electron emitter of a field emission device. FIGS. 8-10 are graphical representations of preferred electron emission characteristics for field emission device electron emitters in accordance with the present invention. It is prescribed that in each instance a constant current source associated with an electron emitter of a plurality of field emission devices provides a substantially similar current as all other constant current sources associated with other electron emitters of the plurality of field emission devices. Further, it will be observed that FIGS. 8-10 depict that an aggregate of emitted currents is substantially a function of a duration of electron emission only. For example, as depicted in FIG. 8, an electron emission current is provided at a magnitude, (referenced as Imax) for a period of time starting at To and ending at Toff. In FIG. 9 an electron emission current of substantially similar magnitude (referenced as Imax) is provided for a shorter period of time starting at To and ending at Toff. In FIG. 10 an electron emission current of substantially similar magnitude (referenced as Imax) as shown in both FIGS. 8 and 9 is provided for a still shorter period of time starting at To and ending at Toff. It is evident that for the three distinct instances shown in FIGS. 8-10 a total electron charge is a function of the time duration during which the current, Imax, is sustained. FIG. 11 is a schematic representation of a field emission device 700, represented by dashed line box, including an electron emitter 701, an extraction electrode or a gate electrode 702, and an anode 703. A first externally provided voltage source 704 is depicted operably coupled between extraction electrode 702 and a reference potential 740 and a second externally provided voltage source 705 is depicted operably coupled between anode 703 and reference potential 740 as described previously with reference to FIG. 2. A determined controlled current source 710 is operably coupled between electron emitter 701 of field emission device 700 and reference potential 740. Typically, a current source is a network of electronic elements which provide a predetermined current to a load as described previously. Further, controlled current source 710 is a current source that is adjustable to a predetermined value of current. A controlling input line 711 is provided to controlled determined current source 710 to couple externally provided current duration information to controlled determined current source 710. Voltage versus time plot 751 depicts that current duration information is coupled to controlling input line 711 as a voltage of prescribed time duration. Alternatively, current versus time plot 752 depicts duration information is provided to controlled constant current source 710 by coupling onto controlling input line 711 a current of prescribed time duration. Coupling the time duration information onto controlling input line 711 will effectively place controlled constant current source 710 in an ON mode to deliver a prescribed or determined constant current to an electron emitter circuit of field emission device 700 for the time duration of the time duration information which, in concert with application of an enabling signal, such as a voltage, to the field emission device extraction electrode, effectively places field emission device 700 in the ON mode. Referring now to FIG. 12, there is depicted a field emission device image display in accordance with the present invention. An array or a plurality of field emission devices, represented within a dashed line box labeled 860, each of which is provided for selectively energizing a portion of an anode 806 is shown. Proximally disposed extraction electrodes of each field emission device in the plurality of field emission devices 860 are interconnected in a manner which forms rows 804 and 805 of extraction electrodes of interconnected field emission devices 860. Electron emitters 807 of the plurality of field emission devices 860 are selectively interconnected in a manner which forms columns 808 that correspond to emitters 807 of interconnected field emission devices 860. A controlled current source of a plurality of controlled current sources 810, 811, 812, and 813, is operably connected between each respective one column of the plurality of columns 808, and a reference potential. Each of the plurality of rows 804 and 805 of extraction electrodes is operably coupled to an output of a plurality of outputs 816 of a switch 802 which is provided to selectively enable a row of the plurality of rows 804 and 805 of extraction electrodes by operably coupling to a selected row an enabling signal means 803 operably coupled between switch 802 input 830 and the reference potential. Each of the controlled current sources of the plurality of controlled current sources 810, 811, 812 and 813 has operably connected thereto a controlling input line of a plurality of controlling input lines 840, 841, 842, and 843 whereon controlling signals are placed to selectively place the controlled current source attached thereto in an ON mode as described previously with reference to FIG. 11. The duration of the ON mode of a controlled current source is determined by the duration of the operably coupled controlling signal. Electron emission takes place from field emission devices of the plurality of field emission devices 860 corresponding to the selected row of the plurality of rows 804 and 805 or extraction electrodes. Each field emission device within the selected row of array 860 emits a substantially identical electron current as that of each other field emission device of the selected row and as determined by each of the controlled current sources. Effecting operation of the image display device in this manner eliminates performance variations which occur due to fabrication and materials inconsistencies. Emitted electrons are preferentially collected at distally disposed anode 806 which, for the image display now under consideration, includes at least a layer of cathodoluminescent material 870 disposed on a substantially transparent viewing screen 880. An externally provided voltage source 820 is operably connected between anode 806 and the reference potential to place an attractive voltage at anode 806 to facilitate collection of electrons. Anode 806 includes a plurality of regions 850, 851, 852, 853, and 854. Regions 850, 851, 852, and 853 are associated with the field emission devices that are identified as operably interconnected via the interconnected extraction electrodes that comprise row or extraction electrodes 804, which is depicted as selected by the switching means 802 and operably coupled to the enabling signal means 803. Each of the field emission devices of selected row of extraction electrodes 804 emits a substantially similar electron current as determined by each attendant controlled current source for a duration determined by the duration of the controlling signal input onto each respective controlling input line. For example, the field emission device associated with selected row of extraction electrodes 805 and controlled current source 810 emits electrons, corresponding to a preferred electron current determined by controlled current source 810, for a duration during which controlled current source 810 is in the ON mode as determined by the controlling signal coupled onto controlling input line 840. Emitted electrons are collected at anode 806 at region 850 that excites cathodoluminescent material 870 to a desired luminous intensity as depicted. The field emission device associated with row of extraction electrodes 805 and controlled current source 811 will also emit electrons, corresponding to the preferred electron current determined by controlled current source 811, for a duration during which controlled current source 811 is in the ON mode as determined by the controlling signal coupled onto controlling input line 841. Field emission devices associated with row 805 and respective controlled current sources 812 and 813 will similarly emit electrons corresponding to the preferred electron current and for a duration in accordance with the duration prescribed by the controlling signal applied to each controlling input line 842 and 843. It should be observed that the luminous intensity of a region of the plurality of regions 850, 851, 852, and 853 of anode 806 is directly related to the duration of controlled excitation by emitted electrons since each of the controlled current sources 810, 811, 812, and 813 provides substantially similar electron current to the associated field emission device to which it is operably connected. Further, region 850 provides greater luminous intensity than does region 851 and less luminous intensity than region 852 which is correlated to the duration of the controlling signal at each of the controlling input lines associated therewith. A controlling signal applied to controlling input line 840 which is of a longer duration than the controlling signal applied to controlling input line 841 and of shorter duration than the controlling signal applied to the controlling input line 842 produces a greater luminous intensity in region 850 then in region 851, and less luminous intensity in region 853 then in region 851. Although FIG. 12 depicts that at each intersection of row 804, 805 and columns 808 there is a single field emission device which will energize a corresponding region at anode 806 it should be understood that each anode picture element or pixel may be energized by a plurality of field emission devices in which instance the plurality of field emission devices is represented by the singular schematic depiction at each said intersection.

Claims: A field emission device (700) including an electron emitter (701), for emitting electrons; an extraction electrode (702) proximally disposed with respect to the electron emitter (701), and an anode (703) for collecting at least some of the emitting electrons, distally disposed with respect to the electron emitter (701); each electron emitter having a capacitance associated therewith, the device comprising: a controlled current source (710) operably coupled between the electron emitter (701) and a reference potential (740), the controlled current source providing a pulse of constant current to the electron emitter (701) for enhancing response time for emission of electrons from the electron emitter (701) in response to current controlling signals applied to the external current controlling terminal; and; a controlling input line (711), for providing current controlling signals to the controlled current source (710), the controlling input line (711) being operably coupled to the controlled current source (710) for receiving signals determinative of the duration of the pulse current; voltages operably coupled between the extraction electrode (702) and the reference potential (740), between the anode (703) and the reference potential (740), and between the controlling input line (711) and the reference potential (740) so as to place the field emission device (700) in an ON state. A field emission device (700) as claimed in claim 1 characterized by the ON state of the field emission device (700) is determined by a duration of the voltage operably coupled between the controlling input line (711) and the reference potential (740). A field emission device (700) as claimed in claim 1 further characterized by voltages operably coupled between the extraction electrode (702) and the reference potential (740) and between the anode (703) and the reference potential (740), and current operably coupled between the controlling input line (711) and the reference potential (740) so as to place the field emission device (700) in an ON state. A field emission device (700) as claimed in claim 3 characterized by the ON state of the field emission device (700) is determined by a duration of the current operably coupled between the controlling input line (711) and the reference potential (740). A field emission device (700) as claimed in claim 1 characterized by the anode (703), includes a substantially transparent viewing screen (880) having at least a cathodoluminescent layer (870) disposed thereon for collecting at least some of any emitted electrons and is distally disposed with respect to the electron emitter (701). A field emission device (700) as claimed in claim 5 further characterized by voltages operably coupled between the extraction electrode (702) and the reference potential (740), between the anode (703) and the reference potential (740), and between the controlling input line (711) and the reference potential (740) so as to place the field emission device (700) in an ON state. A field emission device (700) as claimed in claim 6 wherein the ON state of the field emission device (700) is determined by a duration of the voltage operably coupled between the controlling input line (711) and the reference potential (740). A field emission device (700) as claimed in claim 5 further comprised of voltages operably coupled between the extraction electrode (702) and the reference potential (740) and between the anode (703) and the reference potential (740), and a current operably coupled between the controlling input line (711) and the reference potential (740) so as to place the field emission device (700) in an ON state. A field emission device (700) as claimed in claim 8 characterized by the ON state of the field emission device (700) is determined by a duration of the current operably coupled between the controlling input line (711) and the reference potential (740).

Inventors: KANE ROBERT C, SMITH ROBERT T, KANE, ROBERT C., SMITH, ROBERT T.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1993
Main classification: H01J1
IPC codes: G09G3, H01J31, H01J1