Electron-emitting device and method of manufacturing the same

Abstract: An electron-emitting device comprises an electroconductive film including an electron-emitting region (2) disposed between a pair of electrodes (4,5) arranged on a substrate (1). The electron-emitting region (2) is formed close to a step portion formed by one of the electrodes (4,5) and the substrate (1).

Description: This invention relates to an electron-emitting device, particularly one of surface conduction type, and also to an electron source and an image forming apparatus comprising such electron-emitting devices. Examples of surface conduction electron-emitting device include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965). A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO 2 thin film for a device of this type, the use of Au thin film is proposed in [G. Dittmer: "Thin Solid Films", 9, 317 (1972)] whereas the use of In 2 O 3 /SnO 2 and that of carbon thin film are disclosed respectively in [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983)]. Fig. 43 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In Fig. 43, reference numeral 1 denotes a substrate. Reference numeral 3 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 2 when it is subjected to an electrically energizing process referred to as "energization forming" as will be described hereinafter. In Fig. 43, a pair of device electrodes are separated by a length L of 0.5 to 1[mm] and a width W' is 0.1[mm]. Conventionally, an electron emitting region 2 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 3 of the device to an electrically energizing process, which is referred to as energization forming. In the energization forming process, a DC voltage or a slowly rising voltage that rises typically at, for instance, a very slow rate of 1V/min. is applied to given opposite ends of the electroconductive thin film 3 to locally destroy, deform or structurally modify the film and produce an electron-emitting region 2 which is electrically highly resistive. Thus, the electron-emitting region 2 is part of the electroconductive thin film 3 that typically contains fissures therein so that electrons may be emitted from the fissures and their neighboring areas. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 2 whenever an appropriate voltage is applied to the electroconductive thin film 3 to make an electric current flow through the device. In an image display apparatus realized by arranging a large number of surface conduction electron-emitting devices of the above described type on a substrate and an anode electrode disposed above the substrate, a voltage is applied to the device electrodes of selected electron-emitting devices to cause their electron-emitting regions to emit electrons, while another voltage is applied to the anode electrode of the apparatus to attract electron beams emitted from the electron-emitting regions of the selected surface conduction electron-emitting devices. Under this condition, electrons emitted from the electron-emitting region of a surface conduction electron-emitting device form an electron beam, which move from the low potential side to the high potential side of the device electrode and, at the same time, toward the anode along a parabolic trajectory that is gradually spread before they finally get to the anode electrode. The trajectory of the electron beam is defined as a function of the potential difference of the voltages applied to the device electrodes of each device, the voltage applied to the anode electrode and the distance between the anode electrode and the electron-emitting devices. The image display apparatus is further provided with fluorescent members arranged on the anode electrode as so many pixels that emit light as emitted electrons collide with them. With this arrangement, the electron beam is required to have a profile that corresponds to the size of the pixel, or the target of the electron beam, but this requirement is not necessarily met in conventional image display apparatuses particularly in the case of high definition television sets comprising a large number of fine pixels. If such is the case, the electron beam can eventually hit adjacent pixels to produce unwanted colors on the screen to consequently degrade the quality of the display image. In addition, if the image display apparatus is very flat and has a large display screen that is tens of several inches wide as in the case of a so-called wall televisions set, it may be accompanied by another problem as described below. The surface conduction electron-emitting devices of such an image display apparatus is typically prepared by way of a patterning process using an aligner comprising a deep UV type light source, if the device electrodes of each surface conduction electron-emitting device is separated from other by less than 2 to 3µm, or a regular UV type light source, if the device electrodes are separated by more than 3µm, from the viewpoint of the performance of the aligner and the manufacturing yield. However, any known aligners have a relatively small exposure area that is several inches wide at most if they are of the deep UV type and are intrinsically not suited for a large exposure area because they are of the direct contact exposure type. The exposure area of aligners of the regular UV type does not generously exceed ten inches in the dimension and therefore they are by no means good for the manufacture of large screen apparatuses. In view of the above identified problem of aligners, the distance separating the device electrodes of each surface conduction electron-emitting device is preferably greater than 3µm and more preferably greater than tens of several µm in an electron source comprising a large number of such surface conduction electron-emitting devices or an image forming apparatus using such an electron source. On the other hand, as a result of the above described energization forming process, the produced electron-emitting region of the surface conduction electron-emitting device can become swerved particularly when the device electrodes are separated by a large distance to reduce the convergence of the electron beam emitted from there. Then, the energization forming process in the manufacture of surface conduction electron-emitting devices may lose accuracy in terms of the location and the profile of the electron-emitting region to produce devices that operate poorly. Thus, in an electron source comprising a large number of surface conduction electron-emitting devices having a large distance separating the device electrodes and an image forming apparatus using such an electron source, the electron-emitting devices do not operate uniformly for electron emission to consequently give rise to an uneven distribution of brightness nor the electron beams they emit converge in a desired way. The image displaying performance of such an apparatus is inevitably poor as it can provide only blurred images. Additionally, in the energization forming process for producing an electron-emitting region in the surface conduction electron-emitting device, each device consumes power normally between tens of several mW to several hundred mW, requiring a huge quantity of power for an electron source comprising a large number of surface conduction electron-emitting devices or an image forming apparatus using such an electron source. Then, in the energization forming process, there occurs a significant drop in the voltage applied to each device to additionally damage the uniformity in the performance of the produced devices. In certain cases, the substrate can be cracked during the energization forming process as a result of such lack of uniformity. An embodiment of the present invention aims to provide an electron-emitting device that emits electrons at a sufficiently high efficiency and produces a finely defined electron beam and an image forming apparatus comprising such electron-emitting devices and hence capable of producing highly defined, clear and bright images with high quality. An embodiment of the present invention aims to provide an image forming apparatus having a large display screen that can produce highly defined, clear and bright images even if the device electrodes of each electron-emitting device comprised therein is separated from each other by more than 3pm and preferably more than tens of several µm. An embodiment of the present invention aims to provide a method of manufacturing an image forming apparatus that can produce finely defined, clear and bright images by using an electron source that comprises a large number of surface conduction electron-emitting devices that are free from the above identified problems. In short, the present invention is intended to provide a novel surface conduction electron-emitting device that is free from the above identified problems of the prior art and can be used for producing a large and high quality electron source and an image forming apparatus using such an electron source as well as a method of manufacturing the same. The present invention is also intended to provide an electron source comprising a large number of such surface conduction electron-emitting devices and an image forming apparatus using such an electron source as well as a method of manufacturing the same. According to an aspect of the invention, there is provided an electron-emitting device comprising an electroconductive thin film including an electron-emitting region disposed between a pair of device electrodes arranged on a substrate, characterized in that said electron-emitting region is formed close to a step portion formed by one of said device electrodes and said substrate. According to another aspect of the invention, there is provided an electron source comprising a plurality of electron-emitting devices arranged on a substrate, characterised in that the electron-emitting devices are those as defined above. According to still another aspect of the invention, there is provided an image forming apparatus comprising an electron source and an image-forming member, characterised in that the electron source is the one as defined above. According to a further aspect of the invention, there is provided a method of manufacturing an electron-emitting device as described above comprising the steps of: providing a first and a second device electrode on a substrate; depositing an electroconductive thin film on said substrate (1) between said device electrodes; and energising said thin film to form an electron-emitting region between said device electrodes, adjacent to a step portion formed by one of said device electrodes and said substrate; characterised in that: in said step of depositing said electroconductive thin film is deposited so as to have its thinnest portion between said pair of electrodes at a position close to a step portion formed by one of said device electrodes and said substrate. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are schematic views of an embodiment of surface conduction electron-emitting device according to the invention, showing a first basic structure. Figs. 2A through 2C are schematic sectional views of the surface conduction electron-emitting device of Figs. 1A and 1B in different manufacturing steps. Figs. 3A and 3B are graphs schematically showing voltage waveforms that can be used for an energization forming process. Figs. 4A and 4B are schematic views of another embodiment of surface conduction electron-emitting device according to the invention, showing a second basic structure. Figs. 5A and 5B are schematic views of still another embodiment of surface conduction electron-emitting device according to the invention obtained by a first mode of manufacturing method according to the invention. Fig. 6A is a schematic view of a surface conduction electron-emitting device according to the invention, illustrating a first method of manufacturing the same. Fig. 6B is a schematic view of a surface conduction electron-emitting device according to the invention, illustrating a second method of manufacturing the same. Figs. 7A and 7B are schematic views of another embodiment of surface conduction electron-emitting device according to the invention, showing a third basic structure. Figs. 8A through 8D are schematic sectional views of the surface conduction electron-emitting device of Figs. 7A and 7B in different manufacturing steps. Figs. 9A and 9B are schematic views of another embodiment of surface conduction electron-emitting device according to the invention, showing a modified third basic structure. Figs. 10A to 10C are schematic sectional views of the surface conduction electron-emitting device of Figs. 9A and 9B in different manufacturing steps. Fig. 11 is a block diagram of a gauging system for determining the electron emitting performance of a surface conduction electron-emitting device having the first basic structure. Fig. 12 is a schematic sectional view of the surface conduction electron-emitting device of Example 12 in a manufacturing step. Fig. 13 is a graph showing a typical relationship between the device voltage Vf and the device current If and between the device voltage Vf and the emission current Ie of a surface conduction electron-emitting device or an electron source. Fig. 14 is a schematic plan view of the electron source having a simple matrix arrangement of Example 14. Fig. 15 is a schematic partial sectional view of the electron source of fig. 14. Figs. 16A through 16D are schematic sectional views of the electron source of Fig. 14 in different manufacturing steps. Fig. 17 is a partially cut away schematic perspective view of a display panel comprising an electron source having a simple matrix arrangement. Figs. 18A and 18B are schematic views, illustrating two possible configurations of fluorescent film of display panel of an image forming apparatus. Figs. 19E through 19H are also schematic sectional views of the electron source of Fig. 14 in different manufacturing steps. Fig. 20 is a partially cut away schematic perspective view of the display panel comprising the electron source having a simple matrix arrangement of Example 11. Fig. 21 is a partially cut away schematic perspective view of the display panel comprising the electron source having a simple matrix arrangement of Example 14. Figs. 22AA through 22AC and 22BA through 22BC are schematic sectional views of the electron-emitting device of Example 1 in different manufacturing steps. Figs. 23A and 23B are schematic plan views of the surface conduction electron-emitting device of Example 1, showing in particular its electron emitting region. Figs. 24AA through 24AC and 24BA through 24BC are schematic sectional views of the surface conduction electron-emitting device of Example 2 in different manufacturing steps. Figs. 25A and 25B are schematic plan views of the surface conduction electron-emitting device of Example 2, showing in particular its electron emitting region. Fig. 26 is a schematic plan view of the electron source having a simple matrix arrangement of Example 3. Fig. 27 is a schematic partial sectional view of the electron source of Fig. 26. Figs. 28A through 28D are schematic sectional views of the electron source of Fig. 26 in different manufacturing steps. Figs. 29E through 29H are also schematic sectional views of the electron source of Fig. 26 in different manufacturing steps. Fig. 30 is a block diagram of the image forming apparatus of Example 4. Figs. 31A through 31D are schematic sectional views of the surface conduction electron-emitting device of Example 5 having the second basic structure, the device being shown in different manufacturing steps. Figs. 32AA through 32AC and 32BA through 32BC are schematic sectional views of the surface conduction electron-emitting device of Example 6 in different manufacturing steps. Figs. 33A and 33B are schematic plan views of the surface conduction electron-emitting device of Example 6, showing in particular its electron emitting region. Figs. 34A through 34C are schematic sectional views of the surface conduction electron-emitting device of Example 7 in different manufacturing steps. Figs. 35AA through 35AC and 35BA through 35BC are schematic sectional views of the surface conduction electron-emitting device of Example 8 in different manufacturing steps. Figs. 36A and 36B are schematic plan views of the surface conduction electron-emitting device of Example 8, showing in particular its electron emitting region. Figs. 37AA through 37AD and 37BA through 37BD are schematic sectional views of the surface conduction electron-emitting device of Example 10 having the second basic structure, the device being shown in different manufacturing steps. Fig. 38 is a schematic plan view of the electron source having a simple matrix arrangement of Example 11. Fig. 39 is a schematic partial sectional view of the electron source of Fig. 38. Figs. 40A through 40D are schematic sectional views of the electron source of Fig. 38 in different manufacturing steps. Figs. 41E through 41H are also schematic sectional views of the electron source of Fig. 38 in different manufacturing steps. Figs. 42AA through 42AC and 42BA through 42BC are schematic sectional views of the surface conduction electron-emitting device of Example 12 in different manufacturing steps. Fig. 43 is a schematic view of a conventional surface conduction electron-emitting device, showing its basic structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention an electron-emitting device is made in which the electroconductive film has an area that poorly covers either one of the step portions formed by a pair of device electrodes at a location close to that step portion, preferably also close to the surface of the substrate so that fissures may be generated preferentially in that area to produce an electron-emitting region. Consequently, the electron-emitting region is located close to the device electrode of that step portion so that the electron beam emitted from the electron-emitting device is directly affected by the electronic potential of that device electrode until it gets to the target with improved convergence. The convergence of the electron beam emitted from the electron-emitting region is greatly improved if the device electrode located close to the electron-emitting region is held to a low electric potential. Additionally, since the electron-emitting region is formed along the related device electrode and hence can be well controlled for its location and profile, it is not swerved unlike its counterpart of a conventional device and the electron beam emitted therefrom is similarly convergent as the electron beam emitted from a conventional electron-emitting device having a short distance between the device electrodes. Still additionally, since an area that poorly covers the related step portion is arranged in the electroconductive thin film to preferentially generate fissures and produce an electron-emitting region there, the level of power required for energization forming is remarkably reduced as compared with a conventional device so that consequently the produced electron-emitting device operates much better than any comparable conventinal devices. The electron-emitting device can be operated better for electron emission and the electron beam emitted from the device can be controlled better if a control electrode for operating the electron-emitting device is arranged on the device electrodes or close to the device itself. If a control electrode is arranged on the substrate, the trajectory of the electron beam can be made free from distortions attributable to a charged-up state of the substrate. According to a method of manufacturing an electron-emitting device according to the invention, an electroconductive thin film is formed in an electron-emitting device by spraying a solution containing component elements of the electroconductive film. Such a method is safe and particularly suitable for producing a large display screen. It is preferable that the solution containing component elements of the electroconductive thin film is electrically charged or the device electrodes are held to different electric potentials during the step of spraying the solution in order to produce an area that poorly covers the related step portion so that fissures may be preferentially generated there to produce an electron-emitting region there because, with such an arrangement, the electron-emitting region may be formed along the related device electrode regardless of the profiles of the device electrodes and the electroconductive thin film and the electroconductive thin film may be strongly bonded to the substrate to produce a highly stable electron-emitting device. Thus, electron-emitting devices manufactured by a method according to the invention are highly uniform particularly in terms of the location and the profile of the electron-emitting region and hence operate uniformly. An electron source comprising a large number of electron-emitting devices according to the invention also operate uniformly and stably because the electron-emitting devices are manufactured by the above method. Additionally, since the power required for energization forming for the electron-emitting devices is not high, no siginificant voltage drop occurs in the process of energization forming so that consequently, the electron-emitting devices operate even more uniformly and stably. As the location and the profile of the electron-emitting region can be controlled well if the distance separating the device electrodes is greater than several µm or several hundred µm, the electron-emitting region is completely free from the problem of swerving and poor convergence of electron beam and hence electron-emitting devices according to the invention can be manufactured at a high yield. Consequently, an electron source that can generate highly convergent electron beams can be manufactured at low cost and a high yield. Additionally, in an image forming apparatus according to the present invention, electron beams are highly converged as they collide with the image-forming member of the apparatus so that it can produce fine and clear images that are free from blurs particularly in terms of color. Since the electron-emitting devices comprised in the apparatus operate uniformly and efficiently, it is suited for a large display screen. Now, the present invention will be described in greater detail by referring to preferred embodiments of electron-emitting device, of electron source comprising a large number of such electron-emitting devices and of image forming apparatus realized by using such an electron source. An electron-emitting device according to the invention may have one of three different basic structures and may be manufactured basically with one of two different methods. Embodiment 1 This embodiment is configured to show a first basic structure as schematically illustrated in Figs. 1A and 1B. Note that, in Figs. 1A and 1B, reference numerals 1, 2 and 3 respectively denote a substrate, an electron-emitting region and an electroconductive thin film including an electron-emitting region, whereas reference numerals 4 and 5 denote device electrodes. Materials that can be used for the substrate 1 include quartz glass, glass containing impurities such as Na to a reduced concentration level, soda lime glass, glass substrate realized by forming an SiO 2 layer on soda lime glass by means of sputtering, ceramic substances such as alumina as well as Si. While the oppositely arranged device electrodes 4 and 5 may be made of any highly conducting material, preferred candidate materials include metals such as Ni, Cr, Au, Mo, W, Pt, Ti, Al, Cu and Pd and their alloys, printable conducting materials made of a metal or a metal oxide selected from Pd, Ag, RuO 2 , Pd-Ag and glass, transparent conducting materials such as In 2 O 3 -SnO 2 and semiconductor materials such as polysilicon. The distance L separating the device electrodes, the length W1 of the device electrodes, the contour of the electroconductive film 3 and other factors for designing a surface conduction electron-emitting device according to the invention may be determined depending on the application of the device. The distance L separating the device electrodes 4 and 5 is normally between several hundred angstroms and several hundred micrometers, although it is determined as a function of the performance of the aligner and the specific etching technique used in the photolithography process for the purpose of the invention as well as the voltage to be applied to the device electrodes, although a distance between several to several hundred micrometers is preferable because such a distance matches the exposing technique and the printing technique to be used for preparing a large display screen. While the length W1 and the film thicknesses d1, d2 of the device electrodes 4 and 5 are typically determined as a function of the electric resistances of the electrodes and other factors that may be involved when a large number of such electron-emitting devices are used, the length W1 is preferably between several micrometers and hundreds of several micrometers and the film thicknesses d1, d2 of the device electrodes 2 and 3 are between several tens of nanometers and several micrometers. A surface conduction electron-emitting device according to the invention has an electron-emitting region 2 located close to one of the device electrodes (or the device electrode 5 in Figs. 1A and 1B). As will be described in greater detail hereinafter, such an electron-emitting region 2 can be formed by differentiating the heights of the step portions of the device electrodes. Such differentiation between the step portions can be achieved by using films having different thicknesses d1 and d2 for the device electrodes 5 and 4 respectively or, alternatively, by forming an insulation layer typically made of SiO 2 film under either one of the device electrodes. The height of the step portion of each of the device electrodes is selected, taking the method of preparing the electroconductive thin film 3 and the morphology of the film 3 into consideration, in such way that the electroconductive thin film 3 shows a relatively high electric resistance and therefore a relatively reduced thickness due to poor step coverage or, if the electroconductive thin film is made of fine particles as will be described hereinafter, a relatively low density of fine particles in an area located close to the step portion of the device electrode having a greater thickness (or the step portion of the device electrode 5 in Figs. 1A and 1B) if compared with the remaining area of the electroconductive thin film. The step portion of the higher device electrode has a height typically more than five times, preferably more than ten times, as large as the thickness of the electroconductive thin film 3. The electroconductive thin film 3 is preferably a fine particle film in order to provide excellent electron-emitting characteristics. The thickness of the electroconductive thin film 3 is determined as a function of the electric resistance between the device electrodes 4 and 5 and the parameters for the forming operation that will be described hereinafter as well as other factors and preferably between several and several hundred nanometers, preferably between 1 and 50 nanometers. The electroconductive thin film 4 normally shows a resistance per unit surface area between 10 2 and 10 7 Ω/cm 2 . The term a "fine particle film" as used herein refers to a thin film constituted of a large number of fine particles that may be loosely dispersed, tightly arranged or mutually and randomly overlapping (to form an island structure under certain conditions). If a fine particle film is used, the particle size is preferably between several tenths and several tens of nanometers, preferably between 1 and 20 nanometers. By forming device electrodes having respective step portions whose heights are different from each other, the electroconductive thin film 3 that is prepared in a subsequent step comes to show a good step coverage relative to the device electrode 4 having a low step portion and a poor step coverage relative to the device electrode 5 having a high step portion. Note that the area of the electroconductive thin film 3 that poorly covers the step portion is preferably located close to the surface of the substrate. The electroconductive thin film 3 is made of a material selected from metals such as Pd, Ru, Ag, Au, Ti, In, Cu, Cr, Fe, Zn, Sn, Ta, W and Pb, oxides such as PdO, SnO 2 , In 2 O 3 , PbO and Sb 2 O 3 , borides such as HfB 2 , ZrB 2 , LaB 6 , CeB 6 , YB 4 and GdB 4 , carbides such as TiC, ZrC, HfC, TaC, SiC and WC, nitrides such as TiN, ZrN and HfN, semiconductors such as Si and Ge and carbon. The electron-emitting region 2 contains fissures and electrons are emitted from these fissures. The electron-emitting region 2 containing such fissures and the fissures themselves are produced as a function of the thickness, the state and the material of the electroconductive thin film 3 and the parameters for carrying out an energization forming process for the electron-emitting region 2. As described above, an area of the electroconductive thin film 3 is made to poorly covers the step portion of one of the device electrodes having a greater thickness at a position located close to the surface of the substrate by selecting an appropriate technique for preparing the electroconductive thin film in a subsequent step. With this arrangement, fissures can be generated preferentially in that area in the process of energization forming, which will be described hereinafter, to produce an electron-emitting region. As shown in Figs. 1A and 1B, a substantially linear electron-emitting region 2 is formed along the straight step portion of the device electrode having a greater thickness at a position close to the surface of the substrate, although the location of the electron-emitting region 2 is not limited to that of Fig. 1A or 1B. The fissures may contain electroconductive fine particles having a diameter of several to hundreds of several angstroms. The fine particles are part of some or all of the elements constituting the electroconductive thin film 3. Additionally, the electron-emitting region 2 containing fissures and the neighboring areas of the electroconductive thin film 3 may contain carbon and carbon compounds. Now, a method of manufacturing a surface conduction electron-emitting device according to the invention and illustrated in Figs. 1A and 1B will be described by referring to Figs. 2A through 2C. 1) After thoroughly cleansing a substrate 1 with detergent and pure water, a material is deposited on the substrate 1 by means of vacuum deposition, sputtering or some other appropriate technique for a pair of device electrodes 4 and 5, which are then produced by photolithography. Then, the material of the electrodes is further deposited only on the device electrode 5, masking the other device electrode 4, to make the step portion of the device electrode 5 higher than that of the device electrode 4 (Fig. 2A). 2) An organic metal thin film is formed on the substrate 1 carrying thereon the pair of device electrodes 4 and 5 by applying an organic metal solution and leaving the applied solution for a given period of time. The organic metal solution may contain as a principal ingredient any of the metals listed above for the electroconductive thin film 3. Thereafter, the organic metal thin film is heated, baked and subsequently subjected to a patterning operation, using an appropriate technique such as lift-off or etching, to produce an electroconductive thin film 3 (Fig. 2B). While an organic metal solution is used to produce a thin film in the above description, an electroconductive thin film 3 may alternatively be formed by vacuum deposition, sputtering, chemical vapor phase deposition, dispersed application, dipping, spinner or some other technique. 3) Thereafter, the device electrodes 4 and 5 are subjected to a process referred to as "energization forming". More specifically, the device electrodes 4 and 5 are electrically energized by means of a power source (not shown) until a substantially linear electron emitting region 3 is produced at a position of the electroconductive thin film 3 near the step portion of the device electrode 5 (Fig. 2C) as an area where the electroconductive thin film is structurally modified. In other words, the electron-emitting region 2 is a portion of the electroconductive thin film 3 that is locally destructed, deformed or transformed as a result of energization forming to show a modified structure. Figs. 3A and 3B show two different pulse voltages that can be used for energization forming. The voltage to be used for energization forming preferably has a pulse waveform. A pulse voltage having a constant height or a constant peak voltage may be applied continuously as shown in Fig. 3A or, alternatively, a pulse voltage having an increasing height or an increasing peak voltage may be applied as shown in Fig. 3B. Firstly, a pulse voltage having a constant height will be described. In Fig. 3A, the pulse voltage has a pulse width T1 and a pulse interval T2, which are typically between 1 psec. and 10 msec. and between 10 µsec. and 100 msec. respectively. The height of the triangular wave (the peak voltage for the energization forming operation) may be appropriately selected depending on the profile of the surface conduction electron-emitting device. The voltage is typically applied for tens of several minutes in vacuum of an appropriate degree. Note, however, that the pulse waveform is not limited to triangular and a rectangular or some other waveform may alternatively be used. Now, a pulse voltage having an increasing height will be described. Fig. 3B shows a pulse voltage whose pulse height increases with time. In Fig. 3B, the pulse voltage has a width T1 and a pulse interval T2 that are substantially similar to those of Fig. 3A. The height of the triangular wave (the peak voltage for the energization forming operation) is increased at a rate of, for instance, 0.1V per step. Note again that the pulse waveform is not limited to triangular and a rectangular or some other waveform may alternatively be used. The energization forming operation will be terminated as appropriately judged by measuring the current running through the device electrodes when a voltage that is sufficiently low and cannot locally destroy or deform the electroconductive thin film 3 is applied to the device during an interval T2 of the pulse voltage. Typically the energization forming operation is terminated when a resistance greater than 1M ohms is observed for the device current running through the electroconductive thin film 3 while applying a voltage of approximately 0.1V to the device electrodes. 4) After the energization forming operation, the device is preferably subjected to an activation process. An activation process is a process to be carried out in order to dramatically change the device current (film current) If and the emission current Ie. In an activation process, a pulse voltage may be repeatedly applied to the device in a vacuum atmosphere. In this process, a pulse voltage is repeatedly applied as in the case of energization forming in an organic gas containing atmosphere. Such an atmosphere may be produced by utilizing the organic gas remaining in a vacuum chamber after evacuating the chamber by means of an oil diffusion pump or a rotary pump or by sufficiently evacuating a vacuum chamber by means of an ion pump and thereafter introducing the gas of an organic substance into the vacuum. The gas pressure of the organic substance is determined as a function of the profile of the electron-emitting device to be treated, the profile of the vacuum chamber, the type of the organic substance and other factors. The organic substances that can be suitably used for the purpose of the activation process include aliphatic hydrocarbons such as alkanes, alkenes and alkynes, aromatic hydrocarbons, alcohols, aldehydes, ketones, amines, organic acids such as, phenol, carbonic acids and sulfonic acids. Specific examples include saturated hydrocarbons expressed by general formula C n H 2n+2 such as methane, ethane and propane, unsaturated hydrocarbons expressed by general formula C n H 2n such as ethylene and propylene, benzene, toluene, methanol, ethanol, formaldehyde, acetaldehyde, acetone, methylethylketone, methylamine, ethylamine, phenol, formic acid, acetic acid and propionic acid. As a result of this process, carbon and carbon compounds contained in the atmosphere are deposited on the device to remarkably change the device current If and the emission current Ic. The activation process is terminated whenever appropriate, observing the device current If and the emission current Ie. The pulse width, the pulse interval and the pulse wave height are appropriately selected. For the purpose of the invention, carbon and carbon compounds typically refer to graphite (including so-called highly oriented pyrolytic graphite (HOPG), pyrolitic graphite (PG) and glassy carbon (GC), of which HOPG has a nearly perfect crystal structure of graphite and PG contains crystal grains having a size of about 200 angstroms and has a somewhat disturbed crystal structure, while GC contains crystal grains having a size as small as 2 nm and has a crystal structure that is remarkably in disarray) and non-crystalline carbon (including amorphous carbon and a mixture of amorphous carbon and fine crystals of graphite) and the thickness of film formed by deposition is preferably less than 50nm and more preferably less than 300 nm 5) A surface conduction electron-emitting device according to the invention and have gone through the above listed steps is preferably subjected to a stabilizing step. This step is designed to evacuate the vacuum container arranged for manufacturing the device to eliminate organic substances therefrom. Preferably, an oil free vacuum apparatus is used to evacuate the vacuum container so that it may not produce any oil that can adversely affect the performance of the electron-emitting device. Specific examples of oil free vacuum apparatus that can be used for the purpose of the invention include a sorption pump and an ion pump. If an oil diffusion pump of a rotary pump is used to evacuate the container to utilize the organic gas generated from one or more than one ingredients the oil of such a pump in the preceding activation step, the partial pressure of the oil ingredients has to be held as low as possible. The partial pressure of the organic gas within the vacuum container is preferably less than 1x10 -6 Pa (1x10 -8 Torr) and more preferably less than 1x10 -8 Pa (1x10 -10 Torr) under the condition where carbon and carbon compounds are no longer deposited on the electron-emitting device. For evacuating the vacuum container, it is preferable that the entire container is heated so that the molecules of the organic substances adsorbed to the inner walls of the container and the electron-emitting device may easily move away therefrom and become removed from the container. The heating operation may preferably be conducted at 80 to 200°C for more than five hours, although values for these parameters should be appropriately selected depending on the size and shape of the vacuum container, the configuration of the electron-emitting device and other considerations. High temperature is advantageous for causing the adsorbed molecules to move away. While the temperature range of 80 to 200°C is selected to minimize the possible damage by heat to the electron source to be prepared in the container, a higher temperature may be recommended if the electron source is resistant against heat. It is also necessary to keep the overall pressure in the vacuum container as low as possible. It is preferably less than 1 to 4x10 -5 Pa (1 to 3x10 -7 Torr) and more preferably less than 1x10 -6 Pa (1x10 -8 Torr). After completing the stabilising step, the electron-emitting device is preferably driven in an atmosphere same as that in which said stabilising process is terminated, although a different atmosphere may also be used. So long as the organic substances are satisfactorily removed, a lower degree of vacuum may be permissible for a stabilised operation of the device. With the use of such a vacuum condition, any additional deposition of carbon and carbon compounds is effectively prevented to stabilise both the device current If and the emission current Ie. A second manufacturing method according to the invention, will be now described. In this method 1) After thoroughly cleansing a substrate 1 with detergent and pure water, a material is deposited on the substrate 1 by means of vacuum deposition, sputtering or some other appropriate technique for a pair of device electrodes 4 and 5, which are then produced by photolithography. Then, the material of the electrodes is further deposited only on the device electrode 5, masking the other device electrode 4, to make the step portion of the device electrode 5 higher than that of the device electrode 4 (Fig. 2A) While the organic metal solution is sprayed with a mask member 32 interposed between the nozzle 33 and the substrate 1 in order to omit an independent patterning step in the above description, an electroconductive thin film 3 may alternatively be formed without such a mask member 32 by using an appropriate photolithography technique such as lift-off or etching. 2) An organic metal thin film is formed on the insulating substrate by spraying an organic metal solution through a nozzle 33 with a mask member 32 interposed therebetween as shown in Fig. 6A. The organic metal solution contains organic metal compounds of the metals that are principal components of the electroconductive thin film 3 to be formed there. Thereafter, the organic metal thin film is heated and baked to produce a patterned electroconductive thin film 3 (Fig. 2B). Note that the components in Fig. 6A that are same or similar to those of Figs. 1A and 1B are denoted by the same reference symbols. In Fig. 6A, reference numeral 31 denotes an area where organic metal solution fine particles are applied and reference numeral 34 denotes organic meal solution fine particles. Thereafter, the device electrodes 4 and 5 are subjected to a process referred to as "energisation forming". The steps of energisation and subsequent to energisation are the same as those previously described and will not be repeated here. Embodiment 2 Now, a second basic structure of surface conduction electron-emitting device according to the invention will be described. In a surface conduction electron-emitting device having this basic structure as shown in Figs. 4A and 4B, an electron-emitting region is formed close to either one of a pair of device electrodes 4 and 5 having respective step portions whose heights are equal to each other. As seen from figs. 4A and 4B, an electroconductive thin film 3 is formed on the device electrode 5 and under the other device electrode 4. Thus, a step is produced on the electroconductive thin film only on the device electrode 5 and, consequently, an electron-emitting region 2 is formed at a position close to the device electrode 5 as a result of energization forming. As described above by referring to the first embodiment, the relationship between the height of the device electrode 5 and the thickness of the electroconductive thin film 3 is preferably such that the device electrode 5 is more than five time, preferably more than ten times, greater than the thickness of the electroconductive thin film 3. The remaining requirements of the configuration of the first embodiment are mostly applicable to the second embodiment. While the device electrodes 4 and 5 may have different heights, they are preferably equal in the height from the manufacturing point of view. A method of manufacturing a surface conduction electron-emitting device having a configuration as illustrated in Figs. 4A and 4B will be described by referring to Figs. 31A through 31D. 1) After thoroughly cleansing an insulating substrate 1 with detergent and pure water, a material is deposited thereon by means of vacuum deposition, sputtering or some other appropriate technique for device electrodes, only a device electrode 5 is produced on the insulating substrate 1 by photolithography (Fig. 31A). 2) An organic metal thin film is formed on the substrate 1 carrying thereon the device electrode 5 by applying an organic metal solution and leaving the applied solution for a given period of time. The organic metal solution may contain as a principal ingredient any of the metals listed above for the electroconductive thin film 3. Thereafter, the organic metal thin film is heated, baked and subsequently subjected to a patterning operation, using an appropriate technique such as lift-off or etching, to produce an electroconductive thin film 3 (Fig. 31B). While an organic metal solution is used to produce a thin film in the above description, an electroconductive thin film 3 may alternatively be formed by vacuum deposition, sputtering, chemical vapour phase deposition, dispersed application, dipping, spinner or some other technique. 3) Another device electrode 4 is formed on the electroconductive thin film 3 at a position separated from the device electrode 5 (Fig. 31C). The height of the device electrode 4 may be same as or different from that of the device electrode 5. 4) Thereafter, the device electrodes 4 and 5 are subjected to a process referred to as "energization forming". More specifically, the device electrodes 4 and 5 are electrically energized by means of a power source (not shown) until a substantially linear electron-emitting region 3 is produced at a position of the electroconductive thin film 3 near the step portion of the device electrode 5 (Fig. 31D) as an area where the electroconductive thin film is structurally modified. In other words, the electron-emitting region 2 is a portion of the electroconductive thin film 3 that is locally destructed, deformed or transformed as a result of energization forming to show a modified structure. The subsequent steps are same as those of Embodiment 1 and therefore will not be described here any further. Embodiment 3 As described above with the second method of manufacturing an electron-emitting device according to the first embodiment of the invention, a pair of device electrodes 4 and 5 are so formed that their step portions show different heights and a solution containing component elements of the electroconductive thin film 3 is sprayed onto them through a nozzle. As the step portions of the device electrodes are formed to show different heights with the first manufacturing method, the electroconductive thin film 3 formed thereafter is made to show a good step coverage for the device electrode 4 having a low step portion and a poor step coverage for the device electrode 5 having a high step portion. Thus, in the above described energisation forming step, fissures can be preferentially generated in the poor step coverage area of the electroconductive thin film 3 to produce there an electron-emitting region 2, which is substantially linear and located close to the step portion of the device electrode 5 as shown in Figs. 1A and 1B. With the previously described manufacturing method of the invention, an electroconductive thin film may be formed so as to show a good step coverage for one of the device electrodes and a poor step coverage for the other device electrode in accordance with this embodiment by tilting the substrate 1 (or the nozzle 33) of Fig. 6A as shown in Fig. 12 without differentiating the heights of the step portions of the device electrodes 4 and 5 unlike those of the device electrodes 4 and 5 of the electron-emitting device of Figs. 1A and 1B. Note that the components in Fig. 12 that are similar to those of Fig. 6A are denoted by the same reference symbols. Thus, with such a manufacturing method, since the electron-emitting device is prepared by means of a process exactly same as that of preparing a device comprising device electrodes whose step portions have different heights, a substantially linear electron-emitting region is formed in the energization forming step at a position close to the step portion of one of the device electrodes without differentiating the heights of the step portions of the device electrodes to consequently reduce the number of steps necessary for preparing the device electrodes and make the method advantageous. Now, electrostatic spraying to be used for the purpose of the invention will be described by referring to Fig. 6B. Fig. 6B schematically illustrates the principle of electrostatic spraying. An electrostatic spraying system that can be used for the purpose of the invention comprises a nozzle 131 for spraying an organic metal solution, a generator for atomizing an organic metal solution 132, a tank 133 for storing an organic metal solution, a high voltage DC power source for electrically charging fine particles of organic metal atomized in the generator 134 to a level of -10 to -100kV and a table 135 for carrying a substrate 1. The nozzle 131 can be so operated as to two-dimensionally scan the upper surface of the substrate 1 at a constant rate. The substrate 1 is grounded. With the above arrangement, negatively charged fine organic metal solution particles are sprayed through the nozzle 131 and move with an accelerated speed until they collide with the grounded substrate 1 and become deposited there to produce an organic metal film that is more cohesive than a film produced by any other spray method. The electroconductive thin film can be subjected to a patterning operation by means of photolithography as described above by referring to Fig. 6A and, if a mask member 32 as shown in Fig. 6A is used with electrostatic spraying, a highly cohesive, tight and uniform film can be produced by applying a voltage between the nozzle 33 and the mask member 32 to electrically charge fine particles of organic metal solution 34 sprayed from the nozzle 33 to a level of 10 to 100kV to accelerate them before they collide with the substrate 1. A surface conduction electron-emitting device according to this embodiment of the invention can alternatively be prepared by a method of spraying a solution containing component elements of the electroconductive thin film through a nozzle, whilst applying a voltage to a pair of device electrode formed on a substrate. More specifically, with this method, unlike the first basic arrangement of forming a pair of device electrodes that are arranged asymmetrically (Embodiment 1), a pair of device electrodes appear identical physically appear identical as shown in Figs. 5A and 5B and differentiated only by the electric potentials of the electrodes so that the electroconductive thin film formed from an organic metal solution sprayed through a nozzle is made more cohesive and tight for the device electrode with a lower electric potential than for the device electrode with a higher electric potential and provides a poor step coverage for the device electrode with a higher electric potential. Consequently, a substantially linear electron-emitting region 2 is formed at a position close to the step portion of the device electrode with a lower electrode as shown in Figs. 5A and 5B. For spraying a solution containing component elements of the electroconductive thin film from a nozzle of these manufacturing methods, it is preferable to provide an electric potential difference between the nozzle and the substrate or enhance the adhesion between the substrate and the device electrodes and the electroconductive thin film to make the prepared surface conduction electron-emitting device operate more stably. As described above, with a manufacturing method according to this embodiment of the invention, a substantially linear electron-emitting region is formed along one of the device electrodes of a surface conduction electron-emitting device at a position close to the step portion of the electrode and the surface of the substrate if the device electrodes are separated by a large distance so that the electron-emitting region can be prepared uniformly in terms of position and profile and the surface conduction electron-emitting device operates excellently as will be described hereinafter. Additionally, since a nozzle is used to spray an organic metal solution onto a substrate to produce an electroconductive thin film with a manufacturing method according to the invention and hence the substrate is not rotated unlike the case where a spinner is used with a conventional manufacturing method, it is advantageous and effective when a large number of such surface conduction electron-emitting devices are arranged to produce an electron source because a large substrate carrying a number of surface conduction electron-emitting device is made to rotate with a risk of damaging itself and an electron source and an image forming apparatus incorporating such an electron source can be manufactured with relatively simple equipment. Embodiment 4 Now, a fourth embodiment of surface conduction electron-emitting device according to the invention will be described below. This embodiment of surface conduction electron-emitting device comprises a pair of device electrodes and an electroconductive thin film including an electron-emitting region arranged close to one of the device electrodes and additionally provided with a control electrode. In this embodiment, the control electrode may be arranged on one of the device electrodes or, alternatively, it may be arranged at a peripheral area of the device electrode or the electro-conductive thin film. Figs. 7A and 7B show a surface conduction electron-emitting device according to the invention where a control electrode is arranged on one of the device electrodes. Referring to Figs. 7A and 7B, the surface conduction electron-emitting device comprises a substrate 1, an electroconductive thin film 3 including an electron-emitting region 2, a pair of device electrodes 4 and 5, an insulation layer 6 and a control electrode 7. The control electrode is arranged on the device electrode 5 and the electroconductive thin film 3 with an insulation layer 6 interposed therebetween and made of a material popularly used for electrodes. Possible relations among the electric potentials of the components for driving the surface conduction electron-emitting device will be described below. The device electrode 5 is held to a potential lower than that of the device electrode 4 and the control electrode 7 is held to a potential higher than that of the device electrode 4. Under this condition, electrons emitted from the electron-emitting region 2 located close to the device electrode 5 move toward an anode (not shown), following a trajectory directed from the lower potential device electrode 5 to the higher potential device electrode 4 as described earlier and, since the control electrode 7 is located close to the electron-emitting region 2, the moving electrons are effectively effected by the electric potential of the control electrode 7. More specifically, since the electric potential of the control electrode 7 is higher than the device electrodes, the trajectory of electrons is modified so as to make the moving electrons to be less attracted by the electroconductive thin film 3 and the device electrode 4 and more effectively drawn toward the anode. As a result, the rate of electron emission increases as compared with that of electron emission when the control electrode 7 is not provided. If, on the other hand, the electric potential of the control electrode 7 is made lower than that of the device electrode 4 and equal to that of the device electrode 5, the net effect will be equivalent to the one obtained when the device electrode 5 is made tall to improve the convergence of electrons. If the electric potential of the device electrode 5 is made higher than that of the device electrode 4 and that of the control electrode 7 is made equal to that of the device electrode 4, electrons emitted from the electron-emitting region 2 located close to the device electrode 5 toward the device electrode 5 are effectively cut off by the control electrode 7. Since the electron-emitting region is located close to one of the device electrodes and the control electrode 7 is arranged on that device electrode with an insulation layer interposed therebetween, the trajectory of electrons emitted from the electron-emitting region 2 can be effectively controlled by means of the control electrode 7. While the control electrode has an end surface that agrees with those of the device electrode 5 and the insulation layer 6 in Fig. 7A, the profile of the control electrode 7 is not limited thereto and those of the insulation film 6 and the control electrode 7 may be shifted to the left from that of the device electrode 5 in Fig. 7A (Fig. 12). Embodiment 5 In this embodiment, the control electrode is formed on the substrate as shown in Figs. 9A and 9B. The components that are same or similar to those of the embodiment of Figs. 7A and 7B are denoted by the same reference symbols. In the following description, X denotes the direction of L1 and Y denotes a direction perpendicular to X. Referring to Figs. 9A and 9B, the control electrode 7 is formed on the substrate 1. The control electrode 7 may be placed between the device electrodes as shown or, alternatively, it may be so arranged as to surround the device electrodes and the electroconductive thin film. It may be electrically connected to either one of the device electrodes. Assume here that the control electrode is arranged in a manner as shown in Figs. 9A and 9B and the electric potential of the device electrode 5 is lower than that of the device electrode 4 while the electric potential of the control electrode 7 is equal to that of the device electrode 5. Then, electrons emitted from the electron-emitting region 2 move toward the higher potential device electrode 4 along the X-direction and, if no voltage is applied to the control electrode 7, spread in the Y-direction. However, since the control electrode 7 is held to a relatively low electric potential, the spread of electrons in the Y-direction is suppressed to improve the convergence. Additionally, if no voltage is applied to the control electrode 7 and the substrate is electrically insulated, the electric potential of the insulated substrate is unstable and emitted electrons are affected by the electric potential of the substrate to swerve the trajectory of emitted electrons so that, if the electron-emitting device is used in an image display apparatus, the light emitting spot of the display screen of the apparatus that provides the target of electrons from the electron-emitting device may change its profile to degrade the image displayed on the screen. Such a problem is eliminated by applying an appropriate voltage to the control electrode 7 to stabilize the electric potential of the substrate 1 and hence the trajectory of emitted electrons and consequently improve the quality of the image on the screen. Note that the control electrode 7 may alternatively be arranged on one of the device electrodes and around the device electrodes and the electroconductive thin film. Now, a method of manufacturing an surface conduction electron-emitting device comprising a control electrode 7 will be described below by referring to a case where the control electrode is formed on one of the device electrodes and another case where the control electrode is formed on the substrate. Case 1: The control electrode is formed on one of the device electrodes. A surface conduction electron-emitting device shown in figs. 7A and 7B is manufactured by a method as illustrated in Figs. 8A through 8D. (1) A substrate (1) having a pair of device electrodes (4,5) of differing heights is formed (Fig.8A) and (2) an organic thin film is then formed on the substrate (1) carrying the pair of device electrodes (4,5), (Fig.8B). These steps being performed in accordance with the methods previously described in relation to embodiment 1. 3) After depositing a material for an insulation layer on the substrate 1 that carries a pair of device electrodes 4 and 5 and an electroconductive thin film 3 by vacuum deposition or sputtering, a mask is formed only on the device electrode 5 having a step portion higher than that of the other device electrode 4 by photolithography and an insulation layer 6 having a desired profile is produced by etching, utilizing the mask. Note that the insulation layer 6 does not entirely cover the device electrode 5 and should have a profile that provides appropriate electric contact necessary for applying a voltage to the device electrode. Then, all the area other than the insulation layer 6 is masked and a control electrode 7 is formed on the insulation layer 6 by vacuum deposition or sputtering (Fig. 8C). 4) Thereafter, the device electrodes 4 and 5 are subjected to a process referred to as "energization forming". More specifically, the device electrodes 4 and 5 are electrically energized by means of a power source (not shown) until a substantially linear electron-emitting region 3 is produced at a position of the electroconductive thin film 3 near the step portion of the device electrode 5 (Fig. 8D) as an area where the electroconductive thin film is structurally modified. In other words, the electron-emitting region 2 is a portion of the electroconductive thin film 3 that is locally destructed, deformed or transformed as a result of energization forming to show a modified structure. The steps subsequent to the energization forming step are same as those of Embodiment 1 and therefore will not be described here any further. Case 2: The control electrode is formed on the substrate. A surface conduction electron-emitting device shown in Figs. 9A and 9B is manufactured by a method as illustrated in Figs. 10A through 10C. 1) After thoroughly cleansing a substrate 1 with detergent and pure water, a material is deposited on the substrate 1 by means of vacuum deposition, sputtering or some other appropriate technique for a pair of device electrodes 4 and 5, which are then produced by photolithography. Then, the material of the electrodes is further deposited only on the device electrode 5, masking the other device electrode 4, to make the step portion of the device electrode 5 higher than that of the device electrode 4. At the same time, a control electrode 7 is formed on the insulating substrate 1 by photolithography like the device electrodes 4 and 5 (Fig. 10A). 2) An organic metal thin film is formed on the substrate 1 carrying thereon the pair of device electrodes 4 and 5 by applying an organic metal solution and leaving the applied solution for a given period of time. The steps subsequent to the application of the organic metal solution to form a thin film (10B) and subsequent energisation to form an electron emissive portion (Fig.10C) are the same as those of Embodiment 1 and therefore will not be described here any further. The performance of a surface conduction electron-emitting device according to the invention and manufactured by a method as described above can be determined in a manner as described below. Fig. 11 is a schematic block diagram of a gauging system for determining the performance of an electron-emitting device of the type under consideration. Firstly, this gauging system will be described. Referring to Fig. 11, the components that are the same as those of Figs. 1A and 1B are denoted by the same reference symbols. Otherwise, the gauging system has a power source 51 for applying a device voltage Vf to the device, an ammeter 50 for metering the device current electrodes 4 and 5, an anode 54 for capturing the emission current Ie produced by electrons emitted from the electron-emitting region of the device, a high voltage source 53 for applying a voltage to the anode 54 of the gauging system and another ammeter 52 for metering the emission current Ie produced by electrons emitted from the electron-emitting region 2 of the device. Reference numerals 55 and 56 respectively denotes a vacuum apparatus and a vacuum pump. The surface conduction electron-emitting device to be tested, the anode 54 and other components are disposed within the vacuum apparatus 55, which is provided with instruments including a vacuum gauge and other pieces of equipment necessary for the gauging system so that the performance of the surface conduction electron-emitting device or the electron source in the chamber may be properly tested. The vacuum pump 56 is provided with an ordinary high vacuum system comprising a turbo pump or a rotary pump or an oil-free high vacuum system comprising an oil-free pump such as a magnetic levitation turbo pump or a dry pump and an ultra-high vacuum system comprising an ion pump. The entire vacuum apparatus 55 and the substrate of the electron source held therein can be heated to 250°C by means of a heater (not shown). Note that a display panel of an image forming apparatus according to the invention can be configured as such a gauging system. Thus, all the processes from the energization forming process on can be carried out with this gauging system. For determining the performance of a surface conduction electron-emitting device according to the invention, a voltage between 1 and 10kV may be applied to the anode 54 of the gauging system, which is spaced apart from the electron-emitting device by distance H which is between 2 and 8mm. Note that the performance of a surface conduction electron-emitting device as illustrated in Figs. 7A and 7B or Figs. 9A and 9B is determined by using a power source (not shown) for applying a voltage to the control electrode 7 (not shown). Fig. 13 shows a graph schematically illustrating the relationship between the device voltage Vf and the emission current Ie and the device current If typically observed by the gauging system. Note that different units are arbitrarily selected for Ie and If in Figs. 8A through 8D in view of the fact that Ie has a magnitude by far smaller than that of If. Note that both the vertical and transversal axes of the graph represent a linear scale. As seen in Fig. 13, an electron-emitting device according to the invention has three remarkable features in terms of emission current Ie, which will be described below. Firstly, an electron-emitting device according to the invention shows a sudden and sharp increase in the emission current Ie when the voltage applied thereto exceeds a certain level (which is referred to as a threshold voltage hereinafter and indicated by Vth in Fig. 13), whereas the emission current Ie is practically undetectable when the applied voltage is found lower than the threshold value Vth. Differently stated, an electron-emitting device according to the invention is a non-linear device having a clear threshold voltage Vth to the emission current Ie. Secondly, since the emission current Ie is highly dependent on the device voltage Vf, the former can be effectively controlled by way of the latter. Thirdly, the emitted electric charge captured by the anode 54 is a function of the duration of time of application of the device voltage Vf. In other words, the amount of electric charge captured by the anode 54 can be effectively controlled by way of the time during which the device voltage Vf is applied. The relationship indicated by the solid line in Fig. 13 represents that both the emission current Ie and the device current If show a monotonically-increasing characteristic (hereinafter referred to as MI characteristic) relative to the device voltage Vf but the device current If can show a voltage-controlled-negative-resistance characteristic (hereinafter referred to as VCNR characteristic) (not shown). The electron-emitting device shows either of the two characteristics depending on the method used for manufacturing it, the parameters of the gauging system and other factors. Note that, if the device current If shows a VCNR characteristic to the device voltage Vf, the emission current Ie shows an MI characteristic relative to the device voltage Vf. Because of the above remarkable characteristic features, it will be understood that the electron-emitting behavior of an electron source comprising a plurality of electron-emitting devices according to the invention and hence that of an image-forming apparatus incorporating such an electron source can easily be controlled in response to the input signal. Thus, such an electron source and an image-forming apparatus may find a variety of applications. An electron source according to the invention can be realized by arranging surface conduction electron-emitting devices· For instance, a number of electron-emitting devices may be arranged in a ladder-like arrangement to realize an electron source as described earlier by referring to the prior art. Alternatively, an electron source according to the invention may be realized by arranging n Y-directional wires on m X-directional wires with an interlayer insulation layer interposed therebetween and placing a surface conduction electron-emitting device close to each crossing of the wires, the pair of electrodes of device being connected to the corresponding X- and Y-directional wires respectively. This arrangement is referred to as simple matrix wiring arrangement · Because of the basic characteristics of a surface conduction electron-emitting device as described above, the rate at which the device emit electrons can be controlled for by controlling the wave height and the wave width of the pulse voltage applied to the opposite electrodes of the device above the threshold voltage level if the applied device voltage Vf exceeds the threshold voltage Vth. On the other hand, the device does not practically emit any electron below the threshold voltage Vth. Therefore, regardless of the number of electron-emitting devices arranged in an apparatus, desired surface conduction electron-emitting devices can be selected and controlled for electron emission in response to an input signal by applying a pulse voltage to each of the selected devices if a simple matrix wiring arrangement is employed. As described above, an electron-emitting device, to which the present invention is applicable, is characterized by the following features in terms of emission current Ie. Firstly, there exists a clear threshold voltage Vth and the device emit electrons only a voltage exceeding Vth is applied thereto. Secondly, the level of emission current Ie changes as a function of the change in the applied voltage above the threshold level Vth, although the value of Vth and the relationship between the applied voltage and the emission current may vary depending on the materials, the configuration and the manufacturing method of the electron-emitting device. More specifically, when a pulse-shaped voltage is applied to an electron-emitting device according to the invention, practically no emission current is generated so far as the applied voltage remains under the threshold level, whereas an electron beam is emitted once the applied voltage rises above the threshold level. It should be noted here that the intensity of an output electron beam can be controlled by changing the peak level of the pulse-shaped voltage. Additionally, the total amount of electric charge of an electron beam can be controlled by varying the pulse width. Thus, either modulation method or pulse width modulation may be used for modulating an electron-emitting device in response to an input signal. Now, the present invention will be described by way of examples. [Example 1] In this example, a number of surface conduction electron-emitting devices having a configuration illustrated in Figs. 1A and 1B were prepared along with a number of surface conduction electron-emitting devices for the purpose of comparison and they were tested for performance. Fig. 1A is a plan view and Fig. 1B is a cross sectional side view of a surface conduction electron-emitting device according to the invention and used in this example. Referring to Figs. 1A and 1B, W1 denotes the width of the device electrodes 4 and 5 and W2 denotes the width of the electroconductive thin film 3, while L denotes the distance separating the device electrodes 4 and 5 and d1 and d2 respectively denotes the height of the device electrode 4 and that of the device electrode 5. Figs. 22AA through 22AC show a surface conduction electron-emitting device arranged on substrate A in different manufacturing steps whereas Figs. 22BA through 22BC show another surface conduction electron-emitting device also in different manufacturing steps, the latter being prepared for the purpose of comparison and arranged on substrate B. Four identical electron-emitting devices were produced on each of the substrates A and B. 1) After thoroughly cleansing a quartz glass plate with a detergent, pure water and an organic solvent for each of the substrates A and B, a Pt film was formed thereon by sputtering to a thickness of 30nm for a pair of device electrodes for each device, using a mask. For the substrate A, Pt was deposited further to a thickness of 80 nm for the device electrode 4 (Figs. 22AA and 22BA). Both of the device electrodes 4 and 5 on the substrate B had a thickness of 30 nm whereas the device electrodes 4 and 5 on the substrate A had respective thicknesses of 30nm and 110nm The device electrodes were separated by a distance L of 100µm for both the substrate A and the substrate B. Thereafter, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 100nm on each of the substrates A and B for the purpose of patterning the electroconductive thin film 3. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film. The subsequent steps were identical to both the substrate A and the substrate B. 2) Thereafter, a solution of organize palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the Cr film by means of a spinner and left there to produce an organic Pd thin film. Thereafter, the organic Pd thin film was heated and baked at 300°C for 10 minutes in the atmosphere to produce an electroconductive thin film 3 mainly constituted by fine PdO particles. The film had a thickness of about 10nm and an electric resistance of Rs=5x10 4 Ω/□. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductive thin film 3 having a desired pattern by means of an acidic wet etchant (Figs. 22AB and 22BB). 3) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging system as illustrated in Fig. 11 and heated in vacuum to chemically reduce the PdO to Pd in the electroconductive thin film 3 of each sample device. Then, the sample devices were subjected to an energization forming process to produce an electron-emitting region 2 by applying a device voltage Vf between the device electrodes 4 and 5 of each device (Figs. 22AC and 22BC). The applied voltage was a pulse voltage as shown in Fig. 3B (which was, however, not triangular but rectangularly parallelepipedic). The peak value of the wave height of the pulse voltage was gradually increased with time as shown in Fig. 3B. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. During the energization forming process, an extra pulse voltage of 0.1V (not shown) was inserted into intervals of the forming pulse voltage in order to determine the resistance of the electron emitting region, constantly monitoring the resistance, and the energization forming process was terminated when the resistance exceeded 1MΩ. If the product of the pulse wave height and the device voltage If at the end of the energization forming process is defined as forming power (P form ), the forming power P form of the substrate A (10mW) was five times as small as the forming power P form of the substrate B (50mW). 4) Subsequently, the substrates A and B were subjected to an activation process, maintaining the inside pressure of the vacuum apparatus 55 to about 10 -5 Torr. A pulse voltage (which was, however, not triangular but rectangularly parallelepipedic) was applied to each sample device to drive it. The pulse width of T1=1msec and the pulse interval of T2=10msec were used and the drive voltage (wave height) was 15V. 5) Then, each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 of about 10 -4 Pa (10 -6 Torr) in order to see the device current If and the emission current Ie. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. As for the parameters of the measurement, the distance H between the anode 54 and the electron-emitting device was 5mm and the anode voltage and the device voltage Vf were respective 1kV and 18V. The electric potential of the device electrode 5 was made lower than that of the device electrode 6. As a result of the measurement, the device current If and the emission current of each device on the substrate B were 1.2mA±25% and 1.0µA±30% respectively. On the other hand, the device current If and the emission current of each device on the substrate A were 1.0mA±5% and 1.95µA±4.5% to show a remarkably reduced deviation among the devices. It is assumed as a result of this observation that the above described magnitude of forming power P form will more or less affect the deviation in the performance of electron emission. At the same time, a fluorescent member was arranged on the anode 54 to see the bright spot on the fluorescent member produced by an electron beam emitted from each sample electron-emitting device surface and it was observed that the bright spot produced by a device on the substrate A was smaller than its counterpart produced by a device on the substrate B by about 30µm. Figs. 23A and 23B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrates A and B. As seen from Figs. 23A and 23B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion in each of the four devices on the substrate A, whereas a swerved electron-emitting region 2 was observed in the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. The electron-emitting region 2 was swerved by about 50µm at the middle point. As described above, a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes operates remarkably well to emit highly convergent electron beams without showing any substantial deviation in the performance. It was also found that a surface conduction electron-emitting device according to the invention produces a relatively large bright spot on the fluorescent member if the electric potential of the device electrode 5 is made higher than that of the device electrode 4. [Example 2] In this example, surface conduction electron-emitting devices according to the invention and surface conduction electron-emitting devices were prepared for comparison respectively on substrates A and B and tested for the electron-emitting performance as in the case of Example 1. This example will be described by referring to Figs. 24AA through 24AC (for substrate A) and Figs. 24BA through 24BC (for substrate B). Four identical surface conduction electron-emitting devices according to the invention were prepared on the substrate A. Likewise, four identical conventional surface conduction electron-emitting devices were prepared on the substrate B for comparison. 1) After thoroughly cleansing a quartz glass plate with a detergent, pure water and an organic solvent for each of the substrates A and B, an SiO x film was formed to a thickness of 150nm only on the substrate A, to which resist was subsequently applied and patterned. Thereafter, the SiO x film was removed by reactive ion etching except an area for producing device electrode 5 in each device so that a control member 21 of SiO x was formed in the area of the device electrode 5. Subsequently, Pt was deposited by sputtering to a thickness of 30nm for device electrodes on the substrates A and B, using masks (Figs. 24AA and 24BA). The stepped portions of the device electrodes 4 and 5 were 30nm high on the substrate B, whereas those of the device electrodes 5 were 180nm high and those of the device electrodes 4 were 30nm on the substrate A. The distance L separating the device electrodes of each device was 50µm on the substrate A, whereas the corresponding value was 2µm on the substrate B. Thereafter, a Cr film (not shown) to be used for lift-off was formed by vacuum deposition to a thickness of 100nm on each of the substrates A and B for the purpose of patterning the electroconductive thin film 3. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film. The subsequent steps were identical to both the substrate A and the substrate B. 2) Thereafter, Pd was deposited on the substrate carrying the device electrodes 4 and 5 by sputtering to produce an electroconductive thin film 3 for each device. The film had a thickness of about 3nm and an electric resistance per unit area of 5xl0 2 Ω/□. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductuctive thin film 3 having a desired pattern by means of an acidic wet etchant (Figs. 24AB and 24BB). 3) Then, the devices on the substrates A and B were subjected to an energization forming process as in the case of Example 1 (Figs. 24AC and 24BC). In this example, the forming power P form of the substrate A (6mW) was about ten times as small as the forming power P form of the substrate B (55mW). 4) Subsequently, the substrates A and B were subjected to an activation process as in case of Example 1. 5) Then, each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 of about 10 -4 Pa (10 -6 Torr) in order to see the device current If and the emission current Ie. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. As for the parameters of the measurement, the distance H between the anode 54 and the electron-emitting device was 5mm and the anode voltage and the device voltage Vf were respective 1kV and 15V. The electric potential of the device electrode 5 was made lower than that of the device electrode 6. As a result of the measurement, the device current If and the emission current of each device on the substrate B were 1.0mA±5% and 1.0µA±5% respectively. On the other hand, the device current If and the emission current of each device on the substrate A were 0.95mA±4.5% and 1.92µA±5.0% to show a substantially even deviation among the devices and the emission current of each device on the substrate A was large emission current. At the same time, a fluorescent member was arranged on the anode 54 to see the bright spot on the fluorescent member produced by an electron beam emitted from each sample electron-emitting device surface and it was observed that the bright spot produced by a device on the substrate A was substantially equal to its counterpart produced by a device on the substrate B. Figs. 25A and 25B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrates A and B. As seen from Figs. 25A and 25B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion in each of the four devices on the substrate A, whereas a substantially linear electron-emitting region 2 was observed at the center of the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. As described above, with a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes, the distance between the device electrodes can be made as long as 50µm, or 25 times as large as the comparable distance of a conventional electron-emitting device, while the both devices operate almost identically in terms of deviation in the performance of electron emission and spread of the bright spot on the fluorescent member. [Example 3] In this example, an image forming apparatus was prepared by using an electron source comprising a plurality of surface conduction electron-emitting devices of Figs. 1A and 1B on a substrate and wiring them to form a simple matrix arrangement. Fig. 17 schematically illustrates the image forming apparatus. Fig. 26 shows a schematic partial plan view of the electron source. Fig. 27 is a schematic sectional view taken along line 27-27 of Fig. 26. Throughout Figs. 17, 26 and 27, same reference symbols denote same or similar components. The electron source had a substrate 1, X-directional wires 102 (also referred to as lower wires) and Y-directional wires 103 (also referred to as upper wires). Each of the devices of the electron source comprised a pair of device electrodes 4 and 5 and an electroconductive thin film 3 including an electron-emitting region. Otherwise, the electron source was provided with an interlayer insulation layer 401 and contact holes 402, each of which electrically connected a corresponding device electrode 4 and a corresponding lower wire 102. The steps of manufacturing the electron source will be described by referring to Figs. 28A through 28D and 29E through 29H, which respectively correspond to the manufacturing steps as will be described hereinafter. Step a: After thoroughly cleansing a soda lime glass plate a silicon oxide film was formed thereon to a thickness of 0.5µm by sputtering to produce a substrate 1, on which Cr and Au were sequentially laid to thicknesses of 5nm and 600nm respectively and then a photoresist (AZ1370: available from Hoechst Corporation) was formed thereon by means of a spinner, while rotating the film, and baked. Thereafter, a photo-mask image was exposed to light and developed to produce a resist pattern for lower wires 102 and then the deposited Au/Cr film was wet-etched to produce lower wires 102. Step b: A silicon oxide film was formed as an interlayer insulation layer 401 to a thickness of 1.0µm by RF sputtering. Step c: A photoresist pattern was prepared for producing a contact hole 402 for each device in the silicon oxide film deposited in Step b, which contact hole 102 was then actually formed by etching the interlayer insulation layer 401, using the photoresist pattern for a mask. A technique of RIE (Reactive Ion Etching) using CF 4 and H 2 gas was employed for the etching operation. Step d: Thereafter, a pattern of photoresist (RD-2000N-41: available from Hitachi Chemical Co., Ltd.) was formed for a pair of device electrodes 4 and 5 of each device and a gap L separating the electrodes and then Ti and Ni were sequentially deposited thereon respectively to thicknesses of 5nm and 40nm by vacuum deposition. The photoresist pattern was dissolved by an organic solvent and the Ni/Ti deposit film was treated by using a lift-off technique to produce a pair of device electrodes 4 and 5 having a width W1 of 200µm and separated from each other by a distance L of 80µm. The device electrode 5 had a thickness of 140nm Step e: After forming a photoresist pattern on the device electrodes 4 and 5 for an upper wire 103, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5nm and 500nm and then unnecessary areas were removed by means of a lift-off technique to produce an upper wire 103 having a desired profile. Step f: Then, a Cr film 404 was formed to a film thickness of 100nm by vacuum deposition, using a mask having an opening on and around the gap L between the device electrodes, which Cr film 404 was then subjected to a patterning operation. Thereafter, an organic Pd compound (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the Cr film by means of a spinner, while rotating the film, and baked at 300°C for 12 minutes. The formed electroconductive thin film 3 was made of fine particles containing PdO as a principal ingredient and had a film thickness of 7nm and an electric resistance per unit area of 2x10 4 Ω/□. Step g: The Cr film 404 and the baked electroconductive thin film 3 were wet-etched by using an acidic etchant to provide the electroconductive thin film 4 with a desired pattern. Step h: Then, resist was applied to the entire surface of the substrate, which was then exposed to light and developed, using a mask, to remove it only on the contact holes 402. Thereafter, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5nm and 500nm Any unnecessary areas were removed by means of a lift-off technique to consequently bury the contact holes. With the above steps, there was prepared an electron source comprising an insulating substrate 1, lower wires 102, an interlayer insulation layer 401, upper wires 103, device electrodes 4, 5 and electroconductive thin film 3, although the electron source had not been subjected to energization forming. Then, an image forming apparatus was prepared by using the electron source that had not been subjected to energization forming in a manner as described below by referring to Figs. 17 and 18A. After rigidly securing an electron source substrate 1 onto a rear plate 111, a face plate 116 (carrying a fluorescent film 114 and a metal back 115 on the inner surface of a glass substrate 113) was arranged 5mm above the substrate 1 with a support frame 112 disposed therebetween and, subsequently, frit glass was applied to the contact areas of the face plate 116, the support frame 112 and rear plate 111 and baked at 400°C for 10 minutes in ambient air to hermetically seal the inside of the assembled components. The substrate 1 was also secured to the rear plate 111 by means of frit glass. The fluorescent film 114 of this example was prepared by forming black stripes (as shown in Fig. 18A) and filling the gaps with stripe-shaped fluorescent members of red, green and blue. The black stripes were made of a popular material containing graphite as a principal ingredient. A slurry technique was used for applying fluorescent bodies 122 of three primary colors onto the glass substrate to produce the fluorescent film 114. A metal back 115 is arranged on the inner surface of the fluorescent film 114. After preparing the fluorescent film 114, the metal back 115 was prepared by carrying out a smoothing operation (normally referred to as "filming") on the inner surface of the fluorescent film 114 and thereafter forming thereon an aluminum layer by vacuum deposition. A transparent electrode (not shown) was be arranged on the face plate 116 in order to enhance the electroconductivity of the fluorescent film 114. For the above bonding operation, the components were carefully aligned in order to ensure an accurate positional correspondence between the color fluorescent bodies 122 and the electron-emitting devices 104. The inside of the prepared glass envelope 118 (airtightly sealed container) was then evacuated by way of an exhaust pipe (not shown) and a vacuum pump to a sufficient degree of vacuum and, thereafter, a forming process was carried out on the devices to produce respective electron-emitting regions 2 by applying a voltage to the device electrodes 4, 5 of the surface conduction electron-emitting devices 104 by way of the external terminals Dx1 through Dxm and Dy1 through Dyn. For the energization forming process, a pulse voltage as shown in Fig. 3A (which was, however, not triangular but rectangularly parallelepipedic) was applied to each device in vacuum of about 1x10 -3 Pa (1x10 -5 Torr) The pulse width of T1=1msec and the pulse interval of T2=10msec were used. The electron-emitting region 2 of each surface conduction electron-emitting device produced in this manner is constituted by fine particles containing palladium as a principal ingredient and dispersed appropriately. The average particle size of the fine particles was 5.0 nm (50Å). Then, the apparatus was subjected to an activation process by applying a pulse voltage as shown in Fig. 3A (which was, however, not triangular but rectangularly parallelepipedic) in vacuum of about 3x10 -3 Pa (2x10 -5 Torr), while observing the device current If and the emission current Ie. The pulse width T1, the pulse interval T2 and the wave height were lmsec, 10msec and 14V respectively. Subsequently, the envelope 118 was evacuated via an exhaust pipe (not shown) to achieve a degree of vacuum of about 10 -5 Pa (10 -7 Torr). Then, the ion pump used for evacuation was switched to an oil-free pump to produce an ultrahigh vacuum condition and the electron source was baked at 200°C for 24 hours. After the baking operation, the inside of the envelope was held to a degree of vacuum of 1x10 -7 Pa (1x10 -9 Torr), when the exhaust pipe was sealed by heating and melting it with a gas burner to hermetically seal the envelope 118. Finally, the display panel was subjected to a getter operation by means of high frequency heating in order to maintain the inside to a high degree of vacuum. In order to drive the display panel 201 (Fig. 17) of the image-forming apparatus, scan signals and modulation signals were applied to the electron-emitting devices 104 to emit electrons from respective signal generation means (not shown) by way of the external terminals Dx1 through Dxm and Dy1 through Dyn, while a high voltage of greater than 5kV was applied to the metal back 115 or a transparent electrode (not shown) by way of the high voltage terminal Hv so that electrons emitted from the surface conduction electron-emitting devices were accelerated by the high voltage and collided with the fluorescent film 54 to cause the fluorescent members to excite and emit light to produce fine images of the quality of television. Separately, an image-forming apparatus comprising the surface conduction electron-emitting devices (Fig. 23B) fabricated for the purpose of comparison in Example 1 was manufactured. This image-forming apparatus exhibited a low luminosity with larger deviation. Thus, not only an effectively lowered forming power was observed, but also the lowered forming power improved the deviation of emission current of plural surface conduction electron-emitting devices simultaneously subjected to forming operation, which is assumingly due to the deviation of forming voltages applied to the respective devices. [Example 4] Fig. 30 is a block diagram of a display apparatus realized by using an image forming apparatus (display panel) 201 of Example 3 and arranged to provide visual information coming from a variety of sources of information including television transmission and other image sources. In Fig. 30, there are shown a display panel 201, a display panel drive circuit 1001, a display panel controller 1002, a multiplexer 1003, a decoder 1004, an input/output interface circuit 1005, a CPU 1006, an image generator 1007, image input memory interface circuits 1008, 1009 and 1010, an image input interface circuit 1011, TV signal reception circuits 1012 and 1013 and an input unit 1014. Thus, a display apparatus according to the invention and having a configuration as described above can have a wide variety of industrial and commercial applications because it can operate as a display apparatus for television broadcasting, as a terminal apparatus for video teleconferencing, as an editing apparatus for still and movie pictures, as a terminal apparatus for a computer system, as an OA apparatus such as a word processor, as a game machine and in many other ways. It may be needless to say that Fig. 30 shows only an example of possible configuration of a display apparatus comprising a display panel provided with an electron source prepared by arranging a number of surface conduction electron-emitting devices and the present invention is not limited thereto. For example, some of the circuit components of Fig. 30 may be omitted or additional components may be arranged there depending on the application. To the contrary, if a display apparatus according to the invention is used for visual telephone, it may be appropriately made to comprise additional components such as a television camera, a microphone, lighting equipment and transmission/reception circuits including a modem. Since the display panel 201 of the image forming apparatus of this example can be realized with a remarkably reduced depth, the entire apparatus can be made very flat. Additionally, since the display panel can provide very bright images and a wide viewing angle, it produces very exciting sensations in the viewer to make him or her feel as if he or she were really present in the scene. [Advantages of the Invention] As described above in detail, since a surface conduction electron-emitting device according to the invention comprises a substrate and a pair of device electrodes having respective step portions with different heights and an electroconductive thin film is formed after the device electrodes to show an area of poor step coverage located for the step portion of the device electrode having a larger height, fissures can be preferentially generated by energization forming to produce an electron-emitting region along the corresponding edge of the device electrode in the area of poor step coverage of the electroconductive thin film at a position close to the surface of the substrate even if the device electrodes are separated from each other by a long distance. So, the electron-emitting region is made substantially linear without showing any swerve as in the case of conventional surface conduction electron-emitting devices. Thus, even a large number of surface conduction electron-emitting devices according to the invention are formed on a common substrate, they are made uniform in terms of the relative position and the profile of the electron-emitting region so that the devices operate uniformly for electron emission. Since a large number of surface conduction electron-emitting devices according to the invention arranged in an electron source having a large surface area operate uniformly for electron emission, an image forming apparatus comprising such an electron source is free from the problem of uneven brightness, degraded images and spreading electron beams attributable to swerved electron-emitting regions so that high quality images can always be produced on the display screen. The convergence of electron beams emitted from the electron-emitting region of a surface conduction electron-emitting device according to the invention can be improved if the electric potential of the device electrode located close to the electron-emitting region is made lower than that of the other device electrode. The boundaries of the light emitting spots on the image forming member of an image forming apparatus according to the invention can be made remarkably sharp and clear by applying this electric potential relationship to the entire electron source and the image forming apparatus. [Example 5] In this example, surface conduction electron-emitting devices according to the invention and having a configuration illustrated in Figs. 4A and 4B were prepared along with surface conduction electron-emitting devices for the purpose of comparison and they were tested for performance. They will be described by referring to Figs. 1, 24AA to 24BC and 25A and 25B, where same reference symbols denote same or similar components. Since the devices for comparison were same as those of Example 2, they will not be described here any further. The devices according to the invention were prepared in manner as described below by referring to Figs. 31A through 31D. These devices were arranged on substrate A, whereas the devices for comparison were formed on substrate B. Four identical devices were prepared on each substrate. 1) The substrate A was made of quartz glass. After thoroughly cleansing it with a detergent, pure water and an organic solvent, a Pt film was formed thereon by sputtering to a thickness of 160nm for device electrode 5 for each device (Figs. 31A to 31D). Subsequently, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 200nm. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film. 2) Thereafter, a solution of organize palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the substrate A carrying device electrodes 5 by means of a spinner and left there to produce an organic Pd thin film. Then, the organic Pd thin film was heated and baked at 300°C for 10 minutes in the atmosphere to produce an electroconductive thin film 3 mainly constituted by fine Pd particles. The film had a thickness of about 12.onm (120Å) and an electric resistance of 1x10 4 Ω/□. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductive thin film 3 having a desired pattern by means of an acidic wet etchant (Fig. 3B). 3) Thereafter, Pt was deposited on the substrate A to a thickness of 160.0 nm (1,600Å) by sputtering, using a mask for device electrode 4 for each device (Fig. 31C). Note that the device electrodes 4 and 5 of each device was separated by 50µm on the substrate A, while by 2µm on the substrate B. 4) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging system as illustrated in Fig. 11 and used in Example 2 and the inside of the vacuum apparatus was evacuated by means of a vacuum pump 56 to a degree of vacuum of 3x10 -4 Pa (2x10 -6 Torr). Thereafter, the sample devices were subjected to an energization forming process to produce an electron-emitting region 2 for each device by applying a voltage Vf between the device electrodes 4 and 5 of each device from a power source 51 (Fig. 31D). The applied voltage was a pulse voltage as shown in Fig. 3B. The peak value of the wave height of the pulse voltage was increased stepwise by 0.1V each time as shown in Fig. 3B. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. During the energization forming process, an extra pulse voltage of 0.1V (not shown) was inserted into intervals of the forming pulse voltage in order to determine the resistance of the electron emitting region, constantly monitoring the resistance, and the energization forming process was terminated when the resistance exceeded 1MΩ. 5) Subsequently, the inside of the vacuum apparatus 55 of the qauqing system of Fig. 11 was further evacuated to about 10 -3 Pa (10 -5 Torr) and then acetone was introduced into the vacuum apparatus 55 as an organic substance. The partial pressure of acetone was set to 1x10 -2 Pa (1x10 -4 Torr). A pulse voltage was applied to each sample device on the substrates A and B to drive it for an activation process. Referring to Fig. 3A, the pulse width of T1=1msec and the pulse interval of T2=10msec were used and the drive voltage (wave height) was 15V. A voltage of 1kV was also applied to the anode 54 of the vacuum apparatus, while observing the emission current (Ie) of each electron-emitting device. The activation process was terminated when Ie got to a saturated state. The time required for the activation process was about 20 minutes. 6) Then, after further evacuating the inside of the vacuum apparatus to about 1x10 -4 (1x10 -6 Torr), each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 of about 10 -4 Pa (10 -6 Torr) in order to see the device current If and the emission current Ie. The voltage applied to the anode 54 was 1kV and the device voltage (Vf) was 15V. The electric potential of the device electrode 4 was held higher than of the device electrode 5 for each device. As a result of the measurement, the device current (If) and the emission current (Ie) of each device on the substrate B were 1.0mA±5% and 0.9µA±4% respectively. On the other hand, the device current (If) and the emission current (Ie) of each device on the substrate A were 0.9mA±5% and 0.85µA±4% respectively to show a level of deviation substantially equal to all the devices. At the same time, a fluorescent member was arranged on the anode 54 to observe bright spots produced on the fluorescent member as electron beams emitted from the electron-emitting devices collide with it. The size and profile of the bright spots were substantially same for all the devices. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. Figs. 25A and 25B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrates A and B. As seen from Figs. 25A and 25B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion in each of the four devices on the substrate A, whereas a substantially linear electron-emitting region 2 like the devices on the substrate A was observed in the generally central portion between the device electrodes in each device. As described above, a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes operates to emit highly convergent electron beams without showing any substantial deviation in the performance like a conventional surface conduction electron-emitting device wherein the device electrodes are separated by only 2µm. Thus, the distance separating the device electrodes of a surface conduction electron-emitting device according to the invention can be made as large as 50µm or 25 times larger than that of a conventional surface conduction electron-emitting device. While the device electrodes 4 and 5 of each device was prepared by aputtering in this example, the technique that can be used for producing device electrodes is not limited thereto and a surface conduction electron-emitting device according to the invention may be prepared in a more simple way by utilizing a printing technique. [Example 6] In this example, a number of surface conduction electron-emitting devices having a configuration illustrated in Figs. 1A and 1B were prepared along with a number of surface conduction electron-emitting devices for the purpose of comparison and they were tested for performance. Fig. 1A is a plan view and Fig. 1B is a cross sectional side view of a surface conduction electron-emitting device according to the invention and used in this example. Referring to Figs. 1A and 1B, W1 denotes the width of the device electrodes 4 and 5 and W2 denotes the width of the electroconductive thin film 3, while L denotes the distance separating the device electrodes 4 and 5 and d1 and d2 respectively denotes the height of the device electrode 4 and that of the device electrode 5. Figs. 32AA through 32AC show a surface conduction electron-emitting device arranged on substrate A in different manufacturing steps whereas Figs. 32BA through 32BC shows another surface conduction electron-emitting device also in different manufacturing steps, the latter being prepared for the purpose of comparison and arranged on substrate B. Four identical electron-emitting devices were produced on each of the substrates A and B. 1) After thoroughly cleansing a quartz glass plate with a detergent, pure water and an organic solvent for each of the substrates A and B, a Pt film was formed thereon by sputtering to a thickness of 30 nm for a pair of device electrodes for each device, using a mask. For the substrate A, Pt was deposited further to a thickness of 80 nm for the device electrode 4 (Figs. 32AA and 32BA). Both of the device electrodes 4 and 5 on the substrate B had a thickness of 30nm whereas the device electrodes 4 and 5 on the substrate A had respective thicknesses of 30nm and 110nm The device electrodes were separated by a distance L of 100µm for both the substrate A and the substrate B. Thereafter, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 100nm on each of the substrates A and B for the purpose of patterning the electroconductive thin film 3. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film. The subsequent steps were identical to both the substrate A and the substrate B. 2) Thereafter, a solution of organize palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was sprayed onto the substrate 1 with the device electrodes 4 and 5 formed thereon. In the course of this operation, a voltage of 5kV was applied to between the nozzle and the device electrodes to charge and accelerate the fine liquid particles of organic palladium solution. Thereafter, the organic Pd thin film was heated and baked at 300°C for 10 minutes in the atmosphere to produce an electroconductive thin film 3 mainly constituted by fine PdO particles. The film had a thickness of about 10nm and an electric resistance of Rs=5x10 3 Ω/□. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductive thin film 3 having a desired pattern by means of an acidic wet etchant. (Figs. 32AB and 32BB) 3) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging system as illustrated in Fig. 11 and heated in vacuum to chemically reduce the PdO to Pd in the electroconductive thin film 3 of each sample device. Then, the sample devices were subjected to an energization forming process to produce an electron-emitting region 2 by applying a device voltage Vf between the device electrodes 4 and 5 of each device (Figs. 32AC and 32BC). The applied voltage was a pulse voltage as shown in Fig. 3B (which was, however, not triangular but rectangularly parallelepipedic). Referring to Fig. 3B, the pulse width of Tl=lmsec and the pulse interval of T2=10msec were used. The wave height of the rectangularly parallelepipedic wave was increased gradually. 4) Subsequently, the substrates A and B were subjected to an activation process, maintaining the inside pressure of the vacuum apparatus 55 to about 10 -3 Pa (10 -5 Torr). A pulse voltage (which was, however, not triangular but rectangularly parallelepipedic) was applied to each sample device to drive it. The pulse width of T1=1msec and the pulse interval of T2=10msec were used and the drive voltage (wave height) was 15V. The activation process was terminated in 30 minutes. 5) Then, each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 of about 10 -6 Torr in order to see the device current If and the emission current Ie. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. As for the parameters of the measurement, the distance H between the anode 54 and the electron-emitting device was 5mm and the anode voltage and the device voltage Vf were respective 1kV and 18V. The electric potential of the device electrode 5 was made lower than that of the device electrode 6. As a result of the measurement, the device current If and the emission current of each device on the substrate B were 1.2mA±25% and 1.0µA±30% respectively. On the other hand, the device current If and the emission current of each device on the substrate A were 1.0mA±5% and 0.95µA±4.5% to show a remarkably reduced deviation among the devices. At the same time, a fluorescent member was arranged on the anode 54 to see the bright spot on the fluorescent member produced by an electron beam emitted from each sample electron-emitting device surface and it was observed that the bright spot produced by a device on the substrate A was smaller than its counterpart produced by a device on the substrate B by about 30µm. Figs. 33A and 33B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrate A and B. As seen from Figs. 33A and 33B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion (having a larger thickness) in each of the four devices on the substrate A, whereas a swerved electron-emitting region 2 was observed in the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. The electron-emitting region 2 was swerved by about 50µm at the middle point. As described above, a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes operates remarkably well to emit highly convergent electron beams without showing any substantial deviation in the performance. It was also found that a surface conduction electron-emitting device according to the invention produces a relatively large bright spot on the fluorescent member if the electric potential of the device electrode 5 is made higher than that of the device electrode 4. [Example 7] In this example, the second method of manufacturing a surface conduction electron-emitting device according to the invention was used as will be described below by referring to Figs. 34A through 34C. 1) After thoroughly cleansing a quartz glass plate with a detergent, pure water and an organic solvent for a substrates 1, a Pt film was formed thereon by sputtering to a thickness of 30nm for a pair of device electrodes (Fig. 34A). The device electrodes were separated by a distance L of 100µm. 2) Thereafter, a solution of organic palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was sprayed onto the substrate 1 from a nozzle, while applying a voltage of 5kV to the device electrodes 4 and 5 from a power source 11. As in the case of Example 6, a voltage of 5kV was also applied between the device electrodes and the nozzle in order to charge the fine drops of the sprayed organic palladium solution with electricity and accelerate their speed before they got to the substrate 1. As a result, a dense film was formed on the device electrode 4 having a lower electric potential, whereas a less dense film was formed on the other device electrode 5 having a higher electric potential to produce a poorly covered area on the step portion of the device electrode 5. Thereafter, the organic Pd thin film was heated and baked at 300°C for 10 minutes in the atmosphere to produce an electroconductive thin film 3 mainly constituted by fine PdO particles. The film had a thickness of about 10nm and an electric resistance of Rs=5x10 3 Ω/□. Subsequently, any unnecessary areas of the Cr film were removed by patterning to prouce an electroconductive thin film 3 having a desired profile (Fig. 34B). 3) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging systemtem as illustrated in Fig. 11 and heated in vacuum to chemically reduce the PdO to Pd in the electroconductive thin film 3 of each sample device. Then, the sample device was subjected to an energization forming process to produce an electron-emitting region 2 by applying a device voltage Vf between the device electrodes 4 and 5 of each device (Fig. 34C). The applied voltage was a pulse voltage as shown in Fig. 3B (which was, however, not triangular but rectangularly parallelepipedic). The peak value of the wave height of the rectangularly parallelepipedic pulse voltage was gradually increased with time as shown in Fig. 3B. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. Thereafter, as in case of Example 6, the sample device was subjected to an activation process and then tested for performance. It was found that the device performed well for electron emission like the devices of Example 6. When viewed through a microspcope, a substantially linear electron-emitting region 2 was observed along and near the device electrode 5 that had been held to a higher electric potential for spraying an organic palladium solution through a nozzle. [Example 8] In this example, surface conduction electron-emitting devices according to the invention and surface conduction electron-emitting devices were prepared for comparison respectively on substrates A and B and tested for the electron-emitting performance as in the case of Example 6. This example will be described by referring to Figs. 35AA through 35AC (for substrate A) and Figs. 35BA through 35BC (for substrate B). Four identical surface conduction electron-emitting devices according to the invention were prepared on the substrate A. Likewise, four identical surface conduction electron-emitting devices were prepared on the substrate B for comparison. 1) After thoroughly cleansing a quartz glass plate with a detergent, pure water and an organic solvent for each of the substrates A and B, an SiO x film was formed to a thickness of 150nm. only on the substrate A, to which resist was subsequently applied and patterned. Thereafter, the SiO x film was removed by reactive ion etching except an area for producing device electrode 5 in each device so that a control member 21 of SiO x was formed in the area of the device electrode 5. Subsequently, Pt was deposited by sputtering to a thickness of 30nm for device electrodes on the substrates A and B, using masks (Figs. 35AA and 35BA). The stepped portions of the device electrodes 4 and 5 were 30nm high on the substrate B, whereas those of the device electrodes 5 were 180nm high and those of the device electrodes 4 were 30nm on the substrate A. The distance L separating the device electrodes of each device was 50µm on the substrate A, whereas the corresponding value was 2µm on the substrate B. Thereafter, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 100nm on each of the substrates A and B for the purpose of patterning the electroconductive thin film 3. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film. The subsequent steps were identical to both the substrate A and the substrate B. 2) Thereafter, an organic metal solution obtained by dissolving an organic complex of Pt into solvent was sprayed through a nozzle to form an organic Pt thin film on the substrates that carried the device electrodes thereon, which organic Pt thin film was heated and baked in vacuum to produce an electroconductive thin film 3 of Pt for each device. The thin film had a thickness of about 3nm and an electric resistance per unit area of 5x10 2 Ω/□. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductive thin film 3 having a desired pattern by means of an acidic wet etchant (Figs. 35AB and 35BB). 3) Then, the devices on the substrates A and B were subjected to an energization forming process as in the case of Example 6 (Figs. 35AC and 35BC). 4) Subsequently, the substrates A and B were subjected to an activation process as in case of Example 6. 5) Then, each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 of about 10 -6 Torr in order to see the device current If and the emission current Ie. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. As for the parameters of the measurement, the distance H between the anode 54 and the electron-emitting device was 5mm and the anode voltage and the device voltage Vf were respective 1kV and 15V. The electric potential of the device electrode 5 was made lower than that of the device electrode 6. As a result of the measurement, the device current If and the emission current of each device on the substrate B were 1.0mA±5% and 1.0µA±5% respectively. On the other hand, the device current If and the emission current of each device on the substrate A were 0.95mA±4.5% and 0.9µA±5.0% to show a substantially equal deviation among the devices. At the same time, a fluorescent member was arranged on the anode 54 to see the bright spot on the fluorescent member produced by an electron beam emitted from each sample electron-emitting device surface and it was observed that the bright spot produced by a device on the substrate A was substantially equal to its counterpart produced by a device on the substrate B. Figs. 36A and 36B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrates A and B. As seen from Figs. 36A and 36B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion in each of the four devices on the substrate A, whereas a substantially linear electron-emitting region 2 was observed at the center of the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. As described above, with a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes, the distance between the device electrodes can be made as long as 50µm, or 25 times as large as the comparable distance of a conventional electron-emitting device, while the both devices operate almost identically in terms of deviation in the performance of electron emission and spread of the bright spot on the fluorescent member. [Example 9] In this example, an image forming apparatus was prepared by using an electron source comprising a plurality of surface conduction electron-emitting devices of Figs. 1A and 1B on a substrate and wiring them to form a simple matrix arrangement as shown in Fig. 14. Fig. 17 schematically illustrates the image forming apparatus. Fig. 26 shows a schematic partial plan view of the electron source. Fig. 27 is a schematic sectional view taken along line 27-27 of Fig. 26. Throughout Figs. 14, 17, 26 and 27, same reference symbols denote same or similar components. The steps of manufacturing the electron source will be described by referring to Figs. 28A through 28D and 29E through 29H, which respectively correspond to the manufacturing steps as will be described hereinafter. Step a: After thoroughly cleansing a soda lime glass plate a silicon oxide film was formed thereon to a thickness of 0.5µm by sputtering to produce a substrate 1, on which Cr and Au were sequentially laid to thicknesses of 5nm and 600nm respectively and then a photoresist (AZ1370: available from Hoechst Corporation) was formed thereon by means of a spinner, while rotating the film, and baked. Thereafter, a photo-mask image was exposed to light and developed to produce a resist pattern for lower wires 102 and then the deposited Au/Cr film was wet-etched to produce lower wires 102. Step b: A silicon oxide film was formed as an interlayer insulation layer 401 to a thickness of 1.0µm by RF sputtering. Step c: A photoresist pattern was prepared for producing a contact hole 402 for each device in the silicon oxide film deposited in Step b, which contact hole 102 was then actually formed by etching the interlayer insulation layer 401, using the photoresist pattern for a mask. A technique of RIE (Reactive Ion Etching) using CF 4 and H 2 gas was employed for the etching operation. Step d: Thereafter, a pattern of photoresist (RD-2000N-41: available from Hitachi Chemical Co., Ltd.) was formed for a pair of device electrodes 4 and 5 of each device and a gap L separating the electrodes and then Ti and Ni were sequentially deposited thereon respectively to thicknesses of 5nm and 40nm by vacuum deposition. The photoresist pattern was dissolved by an organic solvent and the Ni/Ti deposit film was treated by using a lift-off technique to produce a pair of device electrodes 4 and 5 having a width W1 of 200µm and separated from each other by a distance L of 80µm. The device electrode 5 had a thickness of 140nm Step e: After forming a photoresist pattern on the device electrodes 4 and 5 for an upper wire 103, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5nm and 500nm and then unnecessary areas were removed by means of a lift-off technique to produce an upper wire 103 having a desired profile. Step f: Then, a Cr film 404 was formed to a film thickness of 100nm by vacuum deposition, using a mask having an opening on and around the gap L between the device electrodes, which Cr film 404 was then subjected to a patterning operation. Thereafter, an organic Pd compound (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was sprayed onto the Cr film and baked at 300°C for 12 minutes. The formed electroconductive thin film 3 was made of fine particles containing PdO as a principal ingredient and had a film thickness of 7nm and an electric resistance per unit area of 2x10 4 Ω/□. Step g: The Cr film 404 and the baked electroconductive thin film 3 were wet-etched by using an acidic etchant to provide the electroconductive thin film 4 with a desired pattern. Step h: Then, resist was applied to the entire surface of the resist on the substrate, which was then exposed to light and developed to remove it only on the contact hole 404. Thereafter, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5nm and 500nm. Any unnecessary areas were removed by means of a lift-off technique to consequently bury the contact hole 402. With the above steps, there was prepared an electron source comprising an insulating substrate 1, lower wires 102, an interlayer insulation layer 401, upper wires 103, device electrodes 4, 5 and electroconductive thin films 3, although the electron source had not been subjected to energization forming. Then, an image forming apparatus was prepared by using the electron source that had not been subjected to energization forming in a manner as described below by referring to Figs. 17 and 18A. After rigidly securing an electron source substrate 1 onto a rear plate 111, a face plate 116 (carrying a fluorescent film 114 and a metal back 115 on the inner surface of a glass substrate 113) was arranged 5mm above the substrate 1 with a support frame 112 disposed therebetween and, subsequently, frit glass was applied to the contact areas of the face plate 116, the support frame 112 and rear plate 111 and baked at 400°C for 10 minutes in ambient air to hermetically seal the inside of the assembled components. The substrate 1 was also secured to the rear plate 111 by means of frit glass. The fluorescent film 114 of this example was prepared by forming black stripes (as shown in Fig. 18A) and filling the gaps with stripe-shaped fluorescent members of red, green and blue. The black stripes were made of a popular material containing graphite as a principal ingredient. A slurry technique was used for applying fluorescent bodies 122 of three primary colors onto the glass substrate to produce the fluorescent film 114. A metal back 115 is arranged on the inner surface of the fluorescent film 114. After preparing the fluorescent film 114, the metal back 115 was prepared by carrying out a smoothing operation (normally referred to as "filming") on the inner surface of the fluorescent film 114 and thereafter forming thereon an aluminum layer by vacuum deposition. A transparent electrode (not shown) was be arranged on the face plate 116 in order to enhance the electroconductivity of the fluorescent film 114. For the above bonding operation, the components were carefully aligned in order to ensure an accurate positional correspondence between the color fluorescent bodies 122 and the electron-emitting devices 104. The inside of the prepared glass envelope 118 (airtightly sealed container) was then evacuated by way of an exhaust pipe (not shown) and a vacuum pump to a sufficient degree of vacuum and, thereafter, a forming process was carried out on the devices to produce respective electron-emitting regions 2 by applying a voltage to the device electrodes 4, 5 of the surface conduction electron-emitting devices 104 by way of the external terminals Dx1 through Dxm and Dy1 through Dyn. For the energization forming process, a pulse voltage as shown in Fig. 3B (which was, however, not triangular but rectangularly parallelepipedic) was applied to each device in vacuum of about 1x10 -3 Pa (1x10 -5 Torr). The pulse width of T1=1msec and the pulse interval of T2=10msec were used. The electron-emitting region 2 of each surface conduction electron-emitting device produced in this manner is constituted by fine particles containing palladium as a principal ingredient and dispersed appropriately. The average particle size of the fine particles was 5.0 nm (50Å). Then, the apparatus was subjected to an activation process by applying a pulse voltage as shown in Fig. 3A (which was, however, not triangular but rectangularly parallelepipedic) was applied to each device in vacuum of about 3x10 -3 Pa (2x10 -5 Torr). The pulse width T1, the pulse interval T2 and the wave height were 1msec, 10msec and 14V respectively. Subsequently, the envelop 118 was evacuated via an exhaust pipe (not shown) to achieve a degree of vacuum of about 3x10 -3 Pa (2x10 -7 Torr). Then, the ion pump used for evacuation was switched to an oil-free pump to produce an ultrahigh vacuum condition and the electron source was baked at 180°C for 10 hours. After the baking operation, the inside of the envelope was held to a degree of vacuum of 1X10 -6 (1x10 -8 Torr), when the exhaust pipe was sealed by heating and melting it with a gas burner to hermetically seal the envelope 118. Finally, the display panel was subjected to a getter operation by means of high frequency heating in order to maintain the inside to a high degree of vacuum. In order to drive the display panel 201 (Fig. 17) of the image-forming apparatus, scan signals and modulation signals were applied to the electron-emitting devices 104 to emit electrons from respective signal generation means (not shown) by way of the external terminals Dx1 through Dxm and Dy1 through Dyn, while a high voltage of greater than 5kV was applied to the metal back 115 or a transparent electrode (not shown) by way of the high voltage terminal Hv so that electrons emitted from the cold cathode devices were accelerated by the high voltage and collided with the fluorescent film 54 to cause the fluorescent members to excite and emit light to produce fine images of the quality of high definition television, which were free from the problem of uneven brightness. [Example 10] In this example, surface conduction electron-emitting devices according to the invention and conventional surface conduction electron-emitting devices were prepared for comparison respectively on substrates A and B and tested for the electron-emitting performance. This example will be described by referring to Figs. 37AA through 37AD (for substrate A) and Figs. 37BA through 37BD (for substrate B). Four identical surface conduction electron-emitting devices according to the invention were prepared on the substrate A. Likewise, four identical conventional surface conduction electron-emitting devices were prepared on the substrate B for comparison. 1) After thoroughly cleansing the substrates with a detergent, pure water and an organic solvent, Pt was deposited by sputtering on them to a thickness of 30nm for device electrodes 4 and 5, using a mask on the both substrate A and B and, thereafter, Pt was further deposited only on the substrate A to a thickness of 80nm masking the device electrodes 4. Thus, the device electrodes 5 had a thickness of 30nm on the substrate B but a greater thickness of 110nm on the substrate A. All the device electrodes 4 had an equal thickness of 30nm on the both substrate A and B. 2) Thereafter, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 100nm on each of the substrates A and B for the purpose of patterning the electroconductive thin film 3. The distance L between the device electrodes of each device and the width W of the electroconductive thin film of each device for producing an electron-emitting region were equally 100µm. Thereafter, an organic Pd compound (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the substrates between the device electrodes 4 and 5 of each device by means of a spinner and left there until an electroconductive thin film was produced. The electroconductive thin film was then heated and baked at 300°C for 10 minutes in ambient air. The formed electroconductive thin film 3 was made of fine particles containing PdO as a principal ingredient and had a film thickness of 10nm and an electric resistance per unit area of 5x10 4 Ω/□. Thereafter, the Cr film and the baked electroconductive thin film 3 were wet-etched by means of an acidic etchant to produce a desired pattern for the films (Figs. 37AB and 37BB). 3) An SiO x insulation layer was formed to a thickness of 0.5µm by RF sputtering only on the substrate A carrying thereon device electrodes 4 and 5. Then, masks were formed only on the device electrodes 5 to exactly cover them by photolithography and the deposited insulating material was removed from the remaining areas to produce an insulation layer 6 for each device by means of RIE (Reactive Ion Etching), using CF 4 and H 2 gases. Note that not the entire device electrodes 5 were covered by the insulation layer but a boundary was defined for the insulation layer 6 on each device electrode 5 so as to ensure electric contact between the device electrode 5 and the power source for applying a voltage thereto. Thereafter, all the surface area of each device was masked except the insulation layer and Pt was deposited on the insulation layer to a thickness of 30nm by sputtering to form a control electrode 7 (Fig. 37AC). The subsequent steps were identical to both the substrate A and the substrate B. 4) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging system as illustrated in Fig. 11 (power source for control electrodes being unshown) and heated in vacuum to chemically reduce the PdO to Pd in the electroconductive thin film 3 of each sample device. Then, the sample devices were subjected to an energization forming process to produce an electron-emitting region 2 by applying a device voltage Vf between the device electrodes 4 and 5 of each device (Figs. 37AD and 37BD). The applied voltage was a pulse voltage as shown in Fig. 3B which was, however, not triangular but rectangularly parallelepipedic. The peak value of the wave height of the pulse voltage was gradually increased with time as shown in Fig. 3B in vacuum. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. 5) Then, both the substrate A and the substrate B were subjected to activation operation, where a driving voltage of 15V, a rectangular wave pulse with T1=1ms and T2=10ms of Fig. 3A, and a vacuum degree of 10 -3 Pa (10 -5 Torr) were employed. To the devices on the substrate A, 0V was applied to the device electrodes 5, while +15V was applied to the device electrodes 4 and the control electrodes 7. 6) Subsequently, the inside of the vacuum apparatus of Fig. 11 was further reduced to 10 -5 Pa (10 -7 Torr) and the device current If and the emission current Ie were measured for all the surface conduction electron-emitting devices on the substrates A and B. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. As for the parameters of the measurement, the distance H between the anode 54 and the electron-emitting device was 5mm and the anode voltage and the device voltage Vf were respective 1kV and 18V. As a result of the measurement, the device current If and the emission current of each device on the substrate B were 1.2mA±25% and 1.0µA±30% respectively to give rise to an electron emission efficiency (100×Ie/If) of 0.08%. On the other hand, the device current If and the emission current of each device on the substrate A were 1.0mA±5% and 1.3µA±4.5% to show a remarkably improved electron emission efficiency of 0.13% and a significantly reduced deviation among the devices. The electric potential of the device electrode 5 was made higher than that of the device electrode 4 and the electric potential of the control electrode was made equal to that of the device electrode 4. As the same time, a fluorescent member was arranged on the anode 54 to see the bright spot on the fluorescent member produced by an electron beam emitted from each sample electron-emitting device surface and it was observed that the bright spot produced by a device on the substrate A was smaller than its counterpart produced by a device on the substrate B by about 20µm. When the electroconductive thin film 3 of each device was observed through a microscope for both the substrate A and the substrate B, a substantially linear electron-emitting region 2 produced as a result of structural modification of the electroconductive thin film 3 was found near the device electrode 5 having a higher step portion in each of the four devices on the substrate A and no carbon nor carbides were found on the electroconductive thin film 3 and the device electrode 4 except in an area near the electron-emitting region. On the other hand, a swerved electron-emitting region 2 was observed at the center of the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. The electron-emitting region 2 was swerved by about 50µm at the middle point. Additionally, a relatively large amount of carbon and carbides was found on the electroconductive thin film and the device electrode with a higher electric potential within 30 to 60µm from the electron-emitting region 2. Since a substantially linear electron-emitting region was formed close to one of a pair of device electrodes and a control electrode was arranged on the device electrode with an insulation layer interposed therebetween, each of the electron-emitting devices according to the invention operated highly efficiently for electric emission. [Example 11] In this example, an image forming apparatus was prepared by using an electron source comprising a plurality of surface conduction electron-emitting devices as those of Example 10 on a substrate and wiring them to form a simple matrix arrangement with 40 rows and 120 columns (inclusive of those for three primary colors). Fig. 38 shows a schematic partial plan view of the electron source. Fig. 39 is a schematic sectional view taken along line 39-39 of Fig. 38. Throughout Figs. 38, 39, 40A through 40D and 41E through 41H, same reference symbols denote same or similar components. The electron source had a substrate 1, X-directional wires 102 (also referred to as lower wires) that correspond to Dx1 through Dxm of Fig. 15, Y-directional wires 103 (also referred to as upper wires) that correspond to Dy1 through Dyn of Fig. 15 and wires 106 for control electrodes that correspond to G1 through Gm of Fig. 15. Each of the devices of the electron source comprised a pair of device electrodes 4 and 5 and an electroconductive thin film 3 including an electron-emitting region. Otherwise, the electron source was provided with an interlayer insulation layer 401, a set of contact holes 402, each of which electrically connected a corresponding device electrode 4 and a corresponding lower wire 102 and another set of contact holes 403, each of which electrically connected a corresponding control electrode 7 and a corresponding wire 106 for the control electrode 7. The steps of manufacturing the electron source will be described below by referring to Figs. 40A through 40D and 41E through 41H. Step a: After thoroughly cleansing a soda lime glass plate a silicon oxide film was formed thereon to a thickness of 0.5µm by sputtering to produce a substrate 1, on which Cr and Au were sequentially laid to thicknesses of 5nm and 600nm respectively and then a photoresist (AZ1370: available from Hoechst Corporation) was formed thereon by means of a spinner, while rotating the film, and baked. Thereafter, a photo-mask image was exposed to light and developed to produce a resist pattern for lower wires 102 and wires for control electrodes 106 then the deposited Au/Cr film was wet-etched to produce lower wires 102 and wires for control electrodes 106 (Fig. 40A). Step b: A silicon oxide film was formed as an interlayer insulation layer 401 to a thickness of 1.0µm by RF sputtering (Fig. 40B). Step c: A photoresist pattern was prepared for producing contact holes 402 and 403 for each device in the silicon oxide film deposited in Step b, which contact holes 402 and 403 were then actually formed by etching the interlayer insulation layer 401, using the photoresist pattern for a mask. A technique of RIE (Reactive Ion Etching) using CF 4 and H 2 gas was employed for the etching operation (Fig. 40C). Step d: Thereafter, a pattern of photoresist was formed for a pair of device electrodes 4 and 5 of each device and a gap L separating the electrodes and then Ti and Ni were sequentially deposited thereon respectively to thicknesses of 5nm and 40nm by vacuum deposition. The photoresist pattern was dissolved by an organic solvent and the Ni/Ti deposit film was treated by using a lift-off technique. Thereafter, the device was covered by photoresist except the device electrode 5 and Ni was deposited to a thickness of 100nm so that the device electrode 5 showed an overall height of 140nm The produced device electrodes 4 and 5 of each device had a width W1 of 200µm and were separated from each other by a distance L of 80µm (Fig. 40D). Step e: After forming a photoresist pattern on the device electrode 5 for an upper wire 103, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5nm and 500nm and then unnecessary areas were removed by means of a lift-off technique to produce an upper wire 103 having a desired profile (Fig. 41E). Step f: Then, a Cr film 404 was formed to a film thickness of 100nm by vacuum deposition, using a mask having an opening on and around the gap L between the device electrodes, which Cr film 404 was then subjected to a patterning operation. Thereafter, an organic Pd compound (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the Cr film by means of a spinner, while rotating the film, and baked at 300°C for 12 minutes. The formed electroconductive thin film 3 was made of fine particles containing PdO as a principal ingredient and had a film thickness of 7nm and an electric resistance per unit area of 2x10 4 Ω/□. The Cr film and the baked electroconductive thin film 3 were etched by using an acidic etchant until it showed a desired pattern (Fig. 41F). Step g: Then, an insulation layer of silicon oxide film was deposited on the substrate 1 prepared in Step e to a thickness of 0.5µm. Then, the device electrode 5 having a higher step portion was covered by a mask showing a profile similar to that of the device electrode 5 by means of a photolithography technique and the insulating material deposited in this step was etched out except the area on the device electrode 5 to produce an insulation layer 6. An RIE technique, using CF 4 gas and H 2 gas, was used for the etching operation. Note that not the entire device electrode 5 was covered by the insulation layer but a boundary was defined for the insulation layer 6 on each device electrode 5 so as to ensure electric contact between the device electrode 5 and the power source for applying a voltage thereto. Thereafter, all the surface area of each device was masked except the insulation layer and Ni was deposited on the insulation layer 6 to a thickness of 50nm to form a control electrode 7 (Fig. 41G). Step h: Then, resist was applied to the entire surface of the substrate except the contact holes 402 and 403, which was then exposed to light and developed to remove it only on the contact holes 402 and 403. Thereafter, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 50Å and 500nm Any unnecessary areas were removed by means of a lift-off technique to consequently bury the contact holes 402 and 403 (Fig. 41H). With the above steps, there was prepared an electron source comprising an insulating substrate 1, lower wires 102, wires for control electrodes 106, an interlayer insulation layer 401, upper wires 103, device electrodes 4, 5 and electroconductive thin films 3, although the electron source had not been subjected to energization forming. Then, an image forming apparatus was prepared by using the electron source that had not been subjected to energization forming in a manner as described below by referring to Figs. 20 and 18A. After rigidly securing an electron source substrate 1 carrying thereon a large number of surface conduction electron-emitting devices onto a rear plate 111, a face plate 116 (carrying a fluorescent film 114 and a metal back 115 on the inner surface of a glass substrate 113) was arranged 5mm above the substrate 1 with a support frame 112 disposed therebetween and, subsequently, frit glass was applied to the contact areas of the face plate 116, the support frame 112 and rear plate 111 and baked at 400°C for 10 minutes in ambient air to hermetically seal the inside of the assembled components. In Fig. 20, reference symbols 104 denote an electron-emitting device and reference symbols 102 and 103 respectively denote an X-directional wire and a Y-directional wire, while reference numeral 106 denotes a wire for a control electrode. The fluorescent film 114 of this example was prepared by forming black stripes (as shown in Fig. 18A) and filling the gaps with stripe-shaped fluorescent members of red, green and blue. The black stripes were made of a popular material containing graphite as a principal ingredient. A slurry technique was used for applying fluorescent bodies 122 of three primary colors onto the glass substrate 103 to produce the fluorescent film 114. A metal back 115 is arranged on the inner surface of the fluorescent film 114. After preparing the fluorescent film 114, the metal back 115 was prepared by carrying out a smoothing operation (normally referred to as "filming") on the inner surface of the fluorescent film 114 and thereafter forming thereon an aluminum layer by vacuum deposition. A transparent electrode (not shown) was be arranged on the face plate 116 in order to enhance the electroconductivity of the fluorescent film 114. For the above bonding operation, the components were carefully aligned in order to ensure an accurate positional correspondence between the color fluorescent bodies 122 and the electron-emitting devices 104. The inside of the prepared glass envelope 118 (airtightly sealed container) was then evacuated by way of an exhaust pipe (not shown) and a vacuum pump to a sufficient degree of vacuum and, thereafter, a forming process was carried out on the devices to produce respective electron-emitting regions 2 by applying a voltage to the device electrodes 4, 5 of the surface conduction electron-emitting devices 104 by way of the external terminals Dx1 through Dxm and Dy1 through Dyn. For the energization forming process, a pulse voltage as shown in Fig. 3B which was, however, not triangular but rectangularly parallelepipedic was applied to each device in vacuum of about 1x10 -3 (1x10 -5 Torr). The pulse width of T1=1msec and the pulse interval of T2=10msec were used. Then, the apparatus was subjected to an activation process by applying a pulse voltage same as the one used for the energization forming operation in vacuum of about 3x10 -3 Pa (2x10 -5 Torr), while observing the device current If and the emission current Ie. The pulse width T1, the pulse interval T2 and the wave height were 1msec, 10msec and 14V respectively. As a result of the above energization forming and activation steps, electron-emitting regions 2 were formed in the electron-emitting devices 104. Subsequently, the envelope 118 was evacuated via an exhaust pipe (not shown) to achieve a degree of vacuum of about 1X10 -5 Pa (10 -7 Torr). Then, the ion pump used for evacuation was switched to an oil-free pump to produce an ultrahigh vacuum condition and the electron source was baked at 180°C for 10 hours. After the baking operation, the inside of the envelope was held to a degree of vacuum of 1x10 -6 Pa (1x10 -8 Torr), when the exhaust pipe was sealed by heating and melting it with a gas burner to hermetically seal the envelope 118. Finally, the display panel was subjected to a getter operation on by means of high frequency heating in order to maintain the inside to a high degree of vacuum. This was an operation where a getter (not shown) arranged within the image forming apparatus was heated by high frequency heating to produce a film by vapor deposition immediately before the apparatus was hermetically sealed. The getter contained Ba as a principal ingredient. In order to drive the display panel 201 (Fig. 17) of the image-forming apparatus, scan signals and modulation signals were applied to the electron-emitting devices 104 to emit electrons from respective signal generation means (not shown) by way of the external terminals Dx1 through Dxm and Dy1 through Dyn, while a voltage of 5kV was applied to the metal back 115 or a transparent electrode (not shown) by way of the high voltage terminal Hv so that electrons emitted from the surface conduction electron-emitting devices were accelerated by the high voltage and collided with the fluorescent film 114 to cause the fluorescent members to excite and emit light to produce fine images of the quality of television, which were free from the problem of uneven brightness. [Example 12] In this example, surface conduction electron-emitting devices according to the invention and having a configuration illustrated in Figs. 5A and 5B were prepared along with surface conduction electron-emitting devices for the purpose of comparison and they were tested for performance. The electron emission performance of these devices will be described below. Fig. 5A is a plan view of a surface conduction electron-emitting device according to the invention and used in this example and Fig. 5B is a cross sectional view thereof. Figs. 42AA through 42AC show a surface conduction electron-emitting device arranged on substrate A in different manufacturing steps, whereas Figs. 42BA through 42BC show another surface conduction electron-emitting device also in different manufacturing steps, the latter being prepared for the purpose of comparison and arranged on substrate B. Four identical electron-emitting devices were produced on each of the substrates A and B. 1) The both substrates A and B were made of quartz glass. After thoroughly cleansing them with a detergent, pure water and an organic solvent, a Pt film was formed thereon by sputtering for device electrodes 4 and 5 to a thickness of 600Å for the substrate A and 30nm for the substrate B (Figs. 42AA and 42BA). The device electrodes 4 and 5 had a thickness of 50nm on the substrate A and 30nm on the substrate B. The device electrodes of each device were separated by a distance of 60µm on the substrate A, whereas they were separated by 2µm on the substrate B. 2) Subsequently, a Cr film (not shown) to be used for lift-off is formed by vacuum deposition to a thickness of 60nm for the purpose of patterning the electroconductive thin film 3 on both the substrate A and the substrate B. At the same time, an opening of 100µm corresponding to the width W2 of the electroconductive thin film 3 was formed in the Cr film for each device on both the substrate A and substrate B. Thereafter, a solution of organize palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was sprayed onto the substrate A by means of an apparatus as shown in Fig. 6B to form an organic palladium thin film. At this time, unlike the case of Example 6, the substrate A carrying device electrodes was tilted by 30° relative to the normal line of Example 6 (Fig. 43). As a result of using the arrangement of tiling the substrate by 30° relative to the normal line of Example 6 for spraying the solution, a dense film was formed on and securely held to the device electrode 4 of each device, whereas a less dense film was formed on the device electrode 5 of each device and the device electrode 5 showed an area in the step portion that is poorly covered by the film. On the other hand, the solution of organized palladium (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied to the substrate B carrying device electrodes 4 and 5 by means of a spinner and left there to produce an organic Pd thin film. Thereafter, the organic Pd thin film was heated and baked at 300°C for 10 minutes in the atmosphere to produce an electroconductive thin film 3 mainly constituted by fine PdO particles for both the substrate A and the substrate B. The film had a thickness of about 12nm and an electric resistance of 5x10 4 Ω/□ for both the substrate A and the substrate B. Subsequently, the Cr film and the electroconductive thin film 3 were wet etched to produce an electroconductive thin film 3 having a desired pattern by means of an acidic wet etchant (Figs. 42AB and 42BB). 3) Then, the substrates A and B were moved into the vacuum apparatus 55 of a gauging system as illustrated in Fig. 11. Thereafter, the sample devices were subjected to an energization forming process to produce an electron-emitting region 2 for each device by applying a voltage between the device electrodes 4 and 5 of each device from a power source 51 (Figs. 42AC and 42BC). The applied voltage was a pulse voltage as shown in Fig. 3B (although it was not triangular but rectangularly parallelepipedic). The peak value of the wave height of the pulse voltage was increased stepwise. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. During the energization forming process, an extra pulse voltage of 0.1V (not shown) was inserted into intervals of the forming pulse voltage in order to determine the resistance of the electron emitting region, constantly monitoring the resistance, and the energization forming process was terminated when the resistance exceeded 1MΩ. If the product of the pulse wave height and the device current If at the end of the energization forming process is defined as forming power (P form ), the forming power P form of the substrate A was seven times as small as the forming power P form of the substrate B. 4) Subsequently, the inside of the vacuum apparatus 55 of the gauging system of Fig. 11 was further evacuated to about 10 -5 Pa (10 -7 Torr), leaving the substrates A and B within the vacuum apparatus 55 and then acetone was introduced into the vacuum apparatus 55 as an organic substance. The partial pressure of acetone was set to 3x10 -2 Pa (2x10 -4 Torr). A pulse voltage was applied to each sample device on the substrates A and B to drive it for an activation process. Referring to Fig. 3A (although the pulse was not triangular but rectangularly parallelepipedic), the pulse width of T1=1msec and the pulse interval of T2=10msec were used and the drive voltage (wave height) was 15V. A voltage of 1kV was also applied to the anode 54 of the vacuum apparatus, while observing the emission current (Ie) of each electron-emitting device. The activation process was terminated when Ie got to a saturated state. 5) Then, after further evacuating the inside of the vacuum apparatus to about 1x10 -5 Pa (1x10 -7 ) Torr, the ion pump used for evacuation was switched to an oil-free pump to produce an ultrahigh vacuum condition and the electron source was baked at 150°C for 2 hours. After the baking operation, the inside of the vacuum apparatus was held to a degree of vacuum of 1x10 -5 Pa (1x10 -7 Torr). Subsequently, each sample surface conduction electron-emitting device on the substrates A and B was driven to operate within the vacuum apparatus 55 in order to see the device current (If) and the emission current (Ie). The voltage applied to the anode 54 was 1kV and the device voltage (Vf) was 15V. The electric potential of the device electrode 4 was held higher than of the device electrode 5 for each device. As a result of the measurement, the device current (If) and the emission current (Ie) of each device on the substrate B were 0.90mA±6% and 0.7µA±5% respectively. On the other hand, the device current (If) and the emission current (Ie) of each device on the substrate A were 0.8mA±5% and 0.7µA±4% respectively to show a level of deviation substantially equal to all the devices. At the same time, a fluorescent member was arranged on the anode 54 to observe bright spots produced on the fluorescent member as electron beams emitted from the electron-emitting devices collide with it. The size and profile of the bright spots were substantially same for all the devices. After the measurement, the electron-emitting regions 2 of the devices on the substrates A and B were microscopically observed. Figs. 25A and 25B schematically illustrate what was observed for the electron-emitting region 2 of the electroconductive thin film 3 of each device on the substrates A and B. As seen from Figs. 25A and 25B, a substantially linear electron-emitting region 2 was observed near the device electrode 5 having a higher step portion in each of the four devices on the substrate A, while a similarly linear electron-emitting region 2 was observed at the middle point of the device electrodes in the electroconductive thin film 3 of each of the four devices on the substrate B prepared for comparison. As described above, a surface conduction electron-emitting device according to the invention and comprising a substantially linear electron-emitting region 2 located close to one of the device electrodes operates to emit highly convergent electron beams without showing any substantial deviation in the performance like a surface conduction electron-emitting device for comparison wherein the device electrodes are separated by only 2µm. Thus, the distance separating the device electrodes of a surface conduction electron-emitting device according to the invention can be made as large as 60µm or 30 times larger than that of a surface conduction electron-emitting device for comparison. [Example 13] In this example, a surface conduction electron-emitting device according to the invention and having a configuration as illustrated in Figs. 9A and 9B was prepared. Fig. 9A is a plan view and Fig. 9B is a cross sectional view of the device. Figs. 10A through 10C also show the surface conduction electron-emitting device of this example in different manufacturing steps. Referring to Figs. 9A and 9B, the device comprises a substrate 1, a pair of device electrodes 4 and 5, an electroconductive thin film 3 including an electron-emitting region 2 and a control electrode 7. The steps followed to prepare the device will be described below by referring to Figs. 9A and 9B and 10A through 10C. Step-a: After thoroughly cleansing a substrate of soda lime glass, an SiO x film was formed to a thickness of 0.5µm by sputtering and then Pt was deposited also by sputtering to form a pair of device electrodes 4 and 5 and a control electrode 7, using a mask. The device electrodes 4 and 5 and the control electrode 7 were differentiated by film thickness. The device electrode 5 and the control electrode 7 were 150 nm thick, whereas the device electrode 4 had a film thickness of 30 nm. The distance L separating the device electrodes was 50 micrometers and the device electrodes had a width W1 of 300 micrometers. As shown in Fig. 9A, the control electrode 7 was arranged near the electroconductive thin film 3 and electrically isolated from the device electrodes 4 and 5 and the electroconductive thin film 3. Step-b: A Cr film was formed by vacuum deposition to a thickness of 50nm over the entire surface of the substrate including the device electrodes formed in Step-a and then photoresist was applied also to the entire surface of the substrate. Then, the Cr film was etched by patterning and photochemically developing a pattern, using a mask (not shown) having an opening with a length greater than the distance between the device electrodes and a width equal to W2, on the gap between the device electrodes and its vicinity, to produce a Cr mask that exposed part of the device electrodes and the gap between the electrodes and had a width equal to W2, which was 100µm. Thereafter, an organic palladium solution (ccp-4230: available from Okuno Pharmaceutical Co., Ltd.) was applied thereon by means of a spinner and the applied solution was heated and baked at 300°C for 10 minutes. Subsequently, the Cr film was etched by means of an acidic etchant and lifted off to produce an electroconductive thin film 3, which was constituted by fine particles of Pd and had a film thickness of 100 angstroms. The electric resistance per unit area of the film was 2x10 4 Ω/□. Thus, a pair of device electrodes 4 and 5, an electroconductive thin film 3 and a control electrode 7 were formed on the substrate 1. Step-d: A gauging system as illustrated in Fig. 11 was prepared and the inside was evacuated by means of a vacuum pump to a degree of vacuum of 3x10 -4 Pa (2x10 -6 Torr). Thereafter, the sample was subjected to an energization forming process by applying a device voltage Vf between the device electrodes 4 and 5 from a power source 51. The applied voltage was a pulse voltage as shown in Fig. 3B. The peak value of the wave height of the pulse voltage as shown in Fig. 3B was increased stepwise by 0.1V. The pulse width of T1=1msec and the pulse interval of T2=10msec were used. During the energization forming process, an extra pulse voltage of 0.1V (not shown) was inserted into intervals of T2s of the forming pulse voltage in order to determine the resistance of the device, and the energization forming process was terminated when the resistance exceeded 1MΩ. The energization forming voltage was about 11V. Thus, an electron-emitting region 2 was produced to finish the operation of preparing the electron-emitting device. The performance of the prepared surface conduction electron-emitting device was examined by means of the above gauging system. The electron-emitting device was separated from the anode by 4mm and an voltage of 1kV was applied to the anode. The inside of the vacuum apparatus was held to 1x10 -5 Pa (1x10 -7 Torr) during the test. The anode was constituted by a transparent electrode arranged on a glass substrate, on which a fluorescent substance was deposited so that the bright spot formed by the profile of the electron beam emitted from the electron-emitting device might be closely observed. Fig. 13 schematically illustrates the relationship between the emission current Ie and the device voltage Vf and between the device current If and the device voltage Vf of the device observed in the gauging system of Fig. 11. Note that the units of the graph of Fig. 13 are arbitrarily selected because the emission current Ie is very small relative to the device current If. Additionally, a voltage lower than the electric potential of the high potential device electrode 4, or typically 0V, was applied to the control electrode 7, while the electron-emitting device was driven to operate. With such an arrangement, a highly convergent bright spot was observed on the fluorescent film arranged on the anode 54. [Example 14] In this example, an image forming apparatus was prepared by arranging an electron source comprising a plurality of surface conduction electron-emitting devices of Example 13 to form a simple matrix arrangement. Fig. 14 shows a schematic partial plan view of the electron source. Fig. 15 is a schematic sectional view taken along line 45-45 of Fig. 14. Throughout Figs. 14, 15, 16A through 16D and 19E through 19H, same reference symbols denote same or similar components. The electron source had a substrate 1, X-directional wires 102 corresponding to Dmx of Fig. 21 (also referred to as lower wires) and Y-directional wires 103 corresponding to Dyn of Fig. 21 (also referred to as upper wires). Each of the devices of the electron source comprised a pair of device electrodes 4 and 5 and an electroconductive thin film 3 including an electron-emitting region. Otherwise, the electron source was provided with an interlayer insulation layer 401, contact holes 402, each of which electrically connected a corresponding device electrode 4 and a corresponding lower wire 102 and wires for control electrodes 106. Reference numerals 104 and 105 respectively denote a surface conduction electron-emitting device and a device electrode including a connecting wire. Step-a: After thoroughly cleansing a soda lime glass plate a silicon oxide film was formed thereon to a thickness of 0.5µm by sputtering to produce a substrate 1, on which Cr and Au were sequentially laid to thicknesses of 5nm and 60nm respectively and then a photoresist (AZ1370: available from Hoechst Corporation) was formed thereon by means of a spinner, while rotating the film, and baked. Thereafter, a photo-mask image was exposed to light and developed to produce a resist pattern for lower wires 102 and then the deposited Au/Cr film was wet-etched to produce lower wires 102. Step-b: A silicon nitride film was formed as an interlayer insulation layer 401 to a thickness of 1.0µm by means of a plasma CVD technique. Step-c: A photoresist pattern was prepared for producing a contact hole 402 for each device in the silicon oxide film deposited in Step b, which contact hole 102 was then actually formed by etching the interlayer insulation layer 401, using the photoresist pattern for a mask. A technique of RIE (Reactive Ion Etching) using CF 4 and H 2 gas was employed for the etching operation. Step-d: Thereafter, a pattern of photoresist (RD-2000N-41: available from Hitachi Chemical Co., Ltd.) was formed for a device electrode 4 of each device and then Ti and Ni were sequentially deposited thereon respectively to thicknesses of 5.0 nm and 40 nm by vacuum deposition. The photoresist pattern was dissolved by an organic solvent and the Ni/Ti deposit film was treated by using a lift-off technique to produce a device electrode 4. In a similar manner, another device electrode 5, a coupling wire and a control electrode 106 were formed to a thickness of 200 nm. Thus, a pair of device electrodes 4 and 5 separated by a gap L1 of 50 micrometers and having a width W1 of 300 micrometers and a control electrode 106 were formed for each device. Step-e: After forming a photoresist pattern on the device electrodes 4 and 5 of each device for an upper wire 103, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5.0 nm and 500 nm and then unnecessary areas were removed by means of a lift-off technique to produce an upper wire 103 having a desired profile. Step-f: A Cr film 404 was formed to a film thickness of 100 nm by vacuum deposition, using a mask for forming an electroconductive thin film having an opening on and around the gap L between the device electrodes of each device, which Cr film 404 was then subjected to a patterning operation. Thereafter, an organic Pt compound was applied to the Cr film by means of a spinner, while rotating the film, and baked at 300°C for 10 minutes. The formed electroconductive thin film 3 was made of fine particles containing Pt as a principal ingredient and had a film thickness of 5 nm and an electric resistance per unit area of 2x10 3 Ω/□. Step-g: The Cr film 404 and the baked electroconductive thin film 3 of each device were wet-etched by using an acidic etchant to provide the electroconductive thin film 4 with a desired pattern. Step-h: Resist was applied to the entire surface of the substrate of each device, which was then exposed to light and developed, using a mask, to remove it only on the contact holes 402. Thereafter, Ti and Au were sequentially deposited by vacuum deposition to respective thicknesses of 5.0 nm and 500 nm. Any unnecessary areas were removed by means of a lift-off technique to consequently bury the contact hole 402. With the above steps, there was prepared an electron source comprising surface conduction electron-emitting devices, each being provided with an insulating substrate 1, a lower wire 102, an interlayer insulation layer 401, an upper wire 103, a pair of device electrodes 4, 5 and an electroconductive thin film 3, although the devices had not been subjected to energization forming. Then, an image forming apparatus was prepared by using the electron source that had not been subjected to energization forming in a manner as described below by referring to Figs. 21 and 18A. After rigidly securing an electron source substrate 1 carrying the surface conduction electron-emitting devices onto a rear plate 111, a face plate 116 (carrying a fluorescent film 114 and a metal back 115 on the inner surface of a glass substrate 113) was arranged 5mm above the substrate 1 with a support frame 112 disposed therebetween and, subsequently, frit glass was applied to the contact areas of the face plate 116, the support frame 112 and rear plate 111 and baked at 500°C for more than 5 minutes in a nitrogen atmosphere to hermetically seal the inside of the assembled components. The substrate 1 was also secured to the rear plate 111 by means of frit glass. In Fig. 21, 104 denotes an electron-emitting device and 102 and 103 respectively denote an X-directional wire and a Y-directional wire. While the fluorescent film 114 is consisted only of a fluorescent body if the apparatus is for black and white images, the fluorescent film 114 of this example was prepared by forming black stripes and filling the gaps with stripe-shaped fluorescent members of red, green and blue. The black stripes were made of a popular material containing graphite as a principal ingredient. A slurry technique was used for applying fluorescent materials onto the glass substrate 113. A metal back 115 is arranged on the inner surface of the fluorescent film 114. After preparing the fluorescent film, the metal back was prepared by carrying out a smoothing operation (normally referred to as "filming") on the inner surface of the fluorescent film and thereafter forming thereon an Al layer by vacuum deposition. While a transparent electrode (not shown) might be arranged on the outer surface of the fluorescent film 114 on the face plate 116 in order to enhance its electroconductivity, it was not used in this example because the fluorescent film 114 showed a sufficient degree of electroconductivity by using only a metal back. For the above bonding operation, the components were carefully aligned in order to ensure an accurate positional correspondence between the color fluorescent members and the electron-emitting devices. The inside of the prepared glass envelope (airtightly sealed container) was then evacuated by way of an exhaust pipe (not shown) and a vacuum pump to a sufficient degree of vacuum and, thereafter, an energization forming process was carried out on the devices to produce electron-emitting regions 2 in the electroconductive thin films 3 by applying an voltage to between the device electrodes 4 and 5 of the electron-emitting devices 114 by way of external terminals Dx1 through Dxm and Dy1 through Dyn. The pulse voltage used for the energization forming is shown in Fig. 3B. In this example, T1 and T2 were respectively equal to 1 ms and 10 ms. The energization forming operation was carried out in vacuum of about 1x10 -4 Pa (1x10 -6 Torr). As a result of energization forming, the electron-emitting regions 2 came to be constituted by dispersed fine particles containing Pt as a principal ingredient, the average diameter of the particles being about 3.0 nm. Subsequently, the inside of the envelope was evacuated through an exhaust pipe (not shown) to a degree of vacuum of about 3x10 -5 Pa (2x10 -7 Torr) and acetone as an organic substance was introduced into the envelope to a partial pressure of acetone of 3x10 -2 Pa (2x10 -4 Torr). Then, a voltage pulse was applied to each surface conduction electron-emitting device for activation. The voltage pulse applied was of the type shown in Fig. 3A with T1=1ms, T2=10ms and a wave height of 15V. The activation operation was carried out with measuring the device current If and the emission current Ie. The operation of preparing electron-emitting devices was completed as the electron-emitting regions 2 were formed. Then, the inside of the image forming apparatus was evacuated to a degree of 10 -6 Pa (10 -8 Torr) and subsequently, the ion pump used for evacuation was switched to an oil-free pump to produce an ultrahigh vacuum condition and the electron source was baked at 180°C for 7 hours. After the baking operation, the inside of the image-forming apparatus was held to a degree of vacuum of 1X10 -5 Pa (1x10 -7 Torr), when the exhaust pipe (not shown) was molten by means of a gas burner to completely seal the envelop of the image forming apparatus. Finally, the apparatus was subjected to a getter process, using a high frequency heating method to maintain the obtained high degree of vacuum. In order to drive the prepared image-forming apparatus comprising a display panel, scan signals and modulation signals were applied to the electron-emitting devices to emit electrons from respective signal generation means by way of the external terminals Dx1 through Dxm and Dy1 through Dyn, while a high voltage was applied to the metal back 115 or a transparent electrode (not shown) by way of the high voltage terminal Hv so that electrons emitted from the surface conduction electron-emitting devices were accelerated by the high voltage and collided with the fluorescent film 114 to cause the fluorescent members to excite to emit light and produce images. The above described image forming apparatus operated excellently to stably produce highly defined clear images. This example deals with an image-forming apparatus comprising a large number of surface conduction electron-emitting devices and modulation electrodes (grids). Since the surface conduction electron-emitting devices used in this example were prepared in a way as described above by referring to Example 1, the method of manufacturing the same will not be described any further. Now, the electron source realized by arranging the surface conduction electron-emitting devices on a substrate and the image forming apparatus prepared by using the electron source will be described hereinafter. Figs. 49 and 50 schematically illustrate two possible arrangements of surface conduction electron-emitting devices on a substrate to realized an electron source. Referring firstly to Fig. 49, S denotes an insulating substrate typically made of glass and ES surrounded by a dotted circle denotes a surface conduction electron-emitting device arranged on the substrate S. The electron source comprises wire electrodes E1 through E10 for wiring the surface conduction electron-emitting devices of the corresponding rows. The surface conduction electron-emitting devices were arranged in rows along the X-direction (hereinafter referred to as device rows). The surface conduction electron-emitting devices of each row are connected in parallel by a pair of common wire electrodes running along the rows. (For example, the first row is wired by the wire electrodes E1 and E2 arranged along the lateral sides.) In the electron source having the above described configuration, each of the device rows can be driven independently by applying an appropriate drive voltage to the related wire electrodes. More specifically, a voltage exceeding the threshold voltage level for electron emission is applied to the device rows to be driven to emit electrons, whereas a voltage not exceeding the threshold voltage level for electron emission (e.g., 0V) is applied to the remaining device rows. (A voltage exceeding the threshold voltage level and used for the purpose of the invention is expressed by drive voltage VE[V] hereinafter.) Fig. 50 illustrates the other possible arrangement of surface conduction electron-emitting devices for the electron source. In Fig. 50, S denotes an insulating substrate typically made of glass and ES surrounded by a dotted circle denotes a surface conduction electron-emitting device arranged on the substrate S. The electron source comprises wire electrodes E'1 through E'6 for wiring the surface conduction electron-emitting devices of the corresponding rows. The surface conduction electron-emitting devices were arranged in rows along the X-direction (hereinafter referred to as device rows). The surface conduction electron-emitting devices of each row are connected in parallel by a pair of common wire electrodes running along the rows. Note that a single common wire electrode is arranged between any two adjacent device rows to serve for the both rows as a wire electrode. For instance, common wire electrode E'2 serves for both the first device row and the second device row. This arrangement of wire electrodes is advantageous in that, if compared with the arrangement of Fig. 49, the space separating any two adjacent rows of surface conduction electron-emitting devices can be significantly reduced in Y-direction. Each of the device rows can be driven independently by applying an appropriate drive voltage to the selected wire electrodes. More specifically, a voltage VE[V] exceeding the threshold voltage level for electron emission is applied to the device rows to be driven to emit electrons, whereas a voltage not exceeding the threshold voltage level for electron emission, e.g. 0[V], is applied to the remaining device rows. For instance, only the devices of the third row can be driven to operate by applying 0[V] to the wire electrodes E'1 through E'3 and VE[V] to the wire electrodes E'4 through E'6. Consequently, VE-0=VE[V] is applied to the devices of the third row, whereas 0[V], 0-0=0[V] or VE-VE=0[V], is applied to all the devices of the remaining rows. Likewise, the devices of the second and the fifth rows can be driven to operate simultaneously by applying 0[V] to the wire electrodes E'1, E'2 and E'6 and VE[V] to the wire electrodes E'3, E'4 and E'5. In this way, the devices of any device row of this electron source can be driven selectively. While each device row has twelve (12) surface conduction electron-emitting devices arranged along the X-direction in the electron sources of Figs. 49 and 50, the number of devices to be arranged in a device row is not limited thereto and a greater number of devices may alternatively be arranged. Additionally, while there are five (5) device rows in the electron source, the number of device rows is not limited thereto and a greater number of device rows may alternatively be arranged. Now, a panel type CRT incorporating an electron source of the above described type will be described. Fig. 51 is a schematic perspective view of a panel type CRT incorporating an electron source as illustrated in Fig. 49. In Fig. 51, VC denote a glass vacuum container provided with a face plate for displaying images as a component thereof. A transparent electrode made of ITO is arranged on the inner surface of the face plate and red, green and blue fluorescent members are applied onto the transparent electrode in the form of a mosaic or stripes without interfering with each other. To simplify the illustration, the transparent electrodes and the fluorescent members are collectively indicated by reference symbol PH in Fig. 51. Black stripes known in the field of CRT may be arranged to fill the blank areas of the transparent electrode that are not occupied by the fluorescent stripes. Similarly, a metal back layer of any known type may be arranged on the fluorescent members. The transparent electrode is electrically connected to the outside of the vacuum container by way of a terminal Hv so that an voltage may be applied thereto in order to accelerate electron beams. In Fig. 51, S denotes the substrate of the electron source rigidly fitted to the bottom of the vacuum container VC, on which a number of surface conduction electron-emitting devices are arranged in a manner as described above by referring to Fig. 49. In this example, a total of 200 device rows are arranged, each comprising 200 devices. Thus, the wire electrodes of the device rows are electrically connected to respective external terminals Dp1 through Dp200 and intersecting respective external terminals Dm1 through Dm200 arranged on the lateral panels of the apparatus so that electric drive signals may be applied thereto from outside of the vacuum enclosure. The surface conduction electron-emitting devices of this example differ from those of Example 1 in the manufacturing steps from the energization forming process on. Therefore, these steps will be described for the current example hereinafter. The inside of the vacuum container VC (Fig. 51) was evacuated through an exhaust pipe (not shown) by means of a vacuum pump. When a sufficient degree of vacuum was reached, a voltage was applied to the surface conduction electron-emitting devices by way of the external terminals Dp1 through Dp200 and Dm1 through Dm200 for carrying out an energization forming operation. Fig. 3B shows the wave form of the pulse voltage used for the energization forming operation. In this example, T1 was equal to 2 ms and T2 was equal to 10 ms. The operation was conducted in vacuum of a degree of about 1x10 -4 Pa (1x10 -6 Torr). Thereafter, acetone was introduced into the vacuum container VC until it showed a partial pressure of 1x10 -4 Torr and an activation process was carried out, applying a voltage to the surface conduction electron-emitting devices ES by way of the external terminals Dp1 through Dp200 and Dm1 through Dm200. After the activation process, the acetone was removed from the inside to produce finished surface conduction electron-emitting devices. The electron-emitting region of each device was constituted by dispersed fine particles containing palladium as a principal ingredient. The average diameter of the fine particles was 3.0 nm (30 angstroms). Thereafter, the ion pump used for evacuation was switched to an oil-free pump to produce an ultra-high vacuum condition and the electron source was baked at 120°C for a sufficient period of time. After the baking operation the inside of the container was held to a degree of vacuum of 1x10 -5 Pa (1x10 -7 Torr). Then, the exhaust pipe was heated and molten by means of a gas burner to hermetically seal the vacuum container VC. Finally, the electron source was subjected to a getter process, using a high frequency heating technique, in order to maintain the high degree of vacuum after the container was sealed. In the image forming apparatus of this example, stripe-shaped grid electrodes GR are arranged in the middle between the substrate S and the face plate FP. There are provided a total of 200 grid electrodes GR arranged in a direction perpendicular to that of the device rows (or in the Y-direction) and each grid electrode has a given number of openings Gh for allowing electron beams to pass therethrough. More specifically, a circular opening Gh is provided for each surface conduction electron-emitting device. The grid electrodes are electrically connected to the outside of the vacuum container via respective electric terminals G1 through G200 for the apparatus of this example. Note that the shape and the locations of the grid electrodes are not limited to those illustrated in Fig. 51 so long as they can appropriate modulate electron beams emitted from the surface conduction electron-emitting devices. For instance, they may be arranged close to the surface conduction electron-emitting devices. The above described display panel comprises surface conduction electron-emitting devices arranged in 200 device rows and 200 grid electrodes to form an X-Y matrix of 200x200. With such an arrangement, an image can be displayed on the screen on a line by line basis by applying a modulation signal to the grid electrodes for a single line of an image in synchronism with the operation of driving (scanning) the surface conduction electron-emitting devices on a row by row basis to control the irradiation of electron beams onto the fluorescent film. Fig. 52 is a block diagram of an electric circuit to be used for driving the display panel of Fig. 51. In Fig. 52, the circuit comprises the display panel 1000 of Fig. 24, a decode circuit 1001 for decoding composite image signals transmitted from outside, a serial/parallel conversion circuit 1002, a line memory 1003, a modulation signal generation circuit 1004, a timing control circuit 1005 and a scan signal generating circuit 1006. The electric terminals of the display panel 1000 are connected to the related circuits. Specifically, the terminal EV is connected to a voltage source HV for generating an acceleration voltage of 10[kV] and the terminals G1 through G200 are connected to the modulation signal generation circuit 1004 while the terminals Dp1 through Dp200 are connected to the scan signal generation circuit 1006 and the terminals Dm1 through Dm200 are grounded. Now, how each component of the circuit operates will be described. The decode circuit 1001 is a circuit for decoding incoming composite image signals such as NTSC television signals and separating brightness signals and synchronizing signals from the received composite signals. The former are sent to the serial/parallel conversion circuit 1002 as data signals and the latter are forwarded to the timing control circuit 1005 as Tsync signals. In other words, the decode circuit 1001 rearranges the values of brightness of the primary colors of RGB corresponding to the arrangement of color pixels of the display panel 1000 and serially transmits them to the serial/parallel conversion circuit 1002. It also extracts vertical and horizontal synchronizing signals and transmits them to the timing control circuits 1005. The timing control circuit 1005 generates various timing control signals in order to coordinate the operational timings of different components by referring to said synchronizing signal Tsync. More specifically, it transmits Tsp signals to the serial/parallel conversion circuit 1002, Tmry signals to the line memory 1003, Tmod signals to the modulation signal generation circuit 1004 and Tscan signals to the scan signal generation circuit 1005. The serial/parallel conversion circuit 1002 samples brightness signals Data it receives from the decode circuit 1001 on the basis of timing signals Tsp and transmits them as 200 parallel signals I1 through I200 to the line memory 1003. When the serial/parallel conversion circuit 1002 completes an operation of serial/parallel conversion on a set of data for a single line of an image, the timing control circuit 1005 a write timing control signal Tmry to the line memory 1003. Upon receiving the signal Tmry, it stores the contents of the signals I1 through I200 and transmits them to the modulation signal generation circuit 1004 as signals I'1 through I'200 and holds them until it receives the next timing control signal Tmry. The modulation signal generation circuit 1004 generates modulation signals to be applied to the grid electrodes of the display panel 1000 on the basis of the data on the brightness of a single line of an image it receives from the line memory 1003. The generated modulation signals are simultaneously applied to the modulation signal terminals G1 through G200 in correspondence to a timing control signal Tmod generated by the timing control circuit 1005. While modulation signals typically operate in a voltage modulation mode where the voltage to be applied to a device is modulated according to the data on the brightness of an image, they may alternatively operate in a pulse width modulation mode where the length of the pulse voltage to be applied to a device is modulated according to the data on the brightness of an image. The scan signal generation circuit 1006 generates voltage pulses for driving the device columns of the surface conduction electron-emitting devices of the display panel 1000. It operates to turn on and off the switching circuits it comprises according to timing control signals Tscan generated by the timing control circuit 1005 to apply either a drive voltage VE[V] generated by a constant voltage source DV and exceeding the threshold level for the surface conduction electron-emitting devices or the ground potential level (or 0[V]) to each of the terminals Dp1 through Dp200. As a result of coordinated operations of the above described circuits, drive signals are applied to the display panel 1000 with the timings as illustrated in the graphs of Fig. 53. In Fig. 53, graphs (a) through (d) show part of signals to be applied to the terminals Dp1 through Dp200 of the display panel from the scan signal generation circuit 1006. It is seen that a voltage pulse having an amplitude of VE[V] is applied sequentially to Dp1, Dp2, Dp3, ... within a period of time for display a single line of an image. On the other hand, since the terminals Dm1 through Dm200 are constantly grounded and held to 0[V], the device columns are sequentially driven by the voltage pulse to emit electron beams from the first column. In synchronism of this operation, the modulation signal generation circuit 1004 applies modulation signals to the terminals G1 through G200 for each line of an image with the timing as shown by the dotted line in graph (f) of Fig. 53. Modulation signals are sequentially selected in synchronism with the selection of scan signals until an entire image is displayed. By continuously repeating the above operation, moving images are displayed on the display screen for television. A flat panel type CRT comprising an electron source of Fig. 49 has been described above. Now, a panel type CRT comprising an electron source of Fig. 50 will be described below by referring to Fig. 54. The panel type CRT of Fig. 54 is realized by replacing the electron source of the CRT of Fig. 51 with the one illustrated in Fig. 60, which comprises an X-Y matrix of 200 columns of electron-emitting devices and 200 grid electrodes. Note that the 200 columns of surface conduction electron-emitting devices are respectively connected to 201 wiring electrodes E1 through E201 and, therefore, the vacuum container is provided with a total of 201 electrode terminals Ex1 through Ex201. Since the electron source of Fig. 54 differs from that of Fig. 51 in terms of wirings, the manufacturing steps from the energization forming process on for the former also differs from those for the latter. The steps from the energization forming step on for the electron source of Fig. 54 will be described below. The inside of the vacuum container VC (Fig. 54) was evacuated through an exhaust pipe (not shown) by means of a vacuum pump. When a sufficient degree of vacuum was reached, a voltage was applied to the surface conduction electron-emitting devices ES by way of the external terminals Ex1 through Ex201 for carrying out an energization forming operation. Fig. 3B shows the wave form of the pulse voltage used for the energization forming operation. In this example, T1 was equal to 1 ms and T2 was equal to 10 ms. The operation was conducted in vacuum of a degree of about 1x10 -3 Pa (1x10 -5 Torr). Thereafter, acetone was introduced into the vacuum container VC until it showed a partial pressure of 1x10 -2 Pa (1x10 -4 Torr) and an activation process was carried out, applying a voltage to the surface conduction electron-emitting devices ES by way of the external terminals Dp1 through Dp200 and Dm1 through Dm200. After the activation process, the acetone was removed from the inside to produce finished surface conduction electron-emitting devices. The electron-emitting region of each device was constituted by dispersed fine particles containing palladium as a principal ingredient. The average diameter of the fine particles was 3.5 nm (35 angstroms). Thereafter, the ion pump used for evacuation was switched to an oil-free pump to produce an ultra-high vacuum condition and the electron source was baked at 120°C for a sufficient period of time. After the baking operation the inside of the container was held a degree of vacuum of 1x10 -5 Pa (1x10 -7 Torr). Then, the exhaust pipe was heated and molten by means of a gas burner to hermetically seal the vacuum container VC. Finally, the electron source was subjected to a getter process, using a high frequency heating technique, in order to maintain the high degree of vacuum after the container was sealed. Fig. 55 shows a block diagram of a drive circuit for driving the display panel 1008. This circuit has a configuration basically same as that of Fig. 52 except the scan signal generation circuit 1007. The scan signal generation circuit 1007 applies either a drive voltage VE[V] generated by a constant voltage source DV and exceeding the threshold level for the surface conduction electron-emitting devices or the ground potential level (0[V]) to each of the terminals of the display panel. Fig. 56 shows charts of the timings with which certain signals are applied to the display panel. The display panel operates to display an image with the timing as illustrated in graph (a) of Fig. 56 as drive signals shown in graphs (b) through (e) of Fig. 56 are applied to the electrode terminals Ex1 through Ex4 from the scan signal generation circuit 1007 and, consequently, voltages as shown in graphs (f) through (h) of Fig. 56 are sequentially applied to the corresponding columns of surface conduction electron-emitting devices to drive the latter. In synchronism with this operation, modulation signals are generated by the modulation signal generation circuit 1004 with the timing as shown in graph (i) of Fig. 56 to display images on the display screen. An image-forming apparatus of the type realized in this example operates very stably, showing full color images with excellent gradation and contrast. As described above in detail, since a surface conduction electron-emitting device according to the invention is provided with a electroconductive thin film having an area that poorly cover the step portion of one of the device electrodes located close to the substrate, fissures can be produced preferentially in that area in the energization forming operation to produce an electron-emitting region. Therefore, the electron-emitting region is located very close to the device electrode and the electron beam emitted from the electron-emitting region is easily affected by the electric potential of the device electrode to become highly convergent before it gets to the target. Additionally, if the device electrode close to the electron-emitting region is held to a relatively low voltage, the convergence of the electron beam emitted from the electron-emitting region can be further improved. Thus, if the device electrodes are separated from each other by a large distance, the electron-emitting region can always be formed along the related device electrode and therefore can be controlled in terms of location and profile so that it may not swerved like those of conventional electron-emitting devices. In other words, an surface conduction electron-emitting device according to the invention operates excellently in terms of convergence of electron beam like a conventional electron-emitting device having a narrow gap between the device electrodes even if the device electrodes of the device are separated from each other by a large distance. Since an area that poorly cover the step portion of the related device electrode is formed in the electroconductive thin film in order to preferentially generate fissures there, the power required for the energization forming operation can be significantly reduced and the electron-emitting region operates excellently for electron emission if compared with a conventional electron-emitting device. Additionally, the electron beam emitted from the electron-emitting region of the device can be controlled very well by arranging a control electrode on or close to the related device electrode. If the control electrode is arranged on the substrate, the deviation in the course of the electron beam caused by an electrically charged up condition of the substrate can be effectively corrected. In a preferably mode of carrying out a method of manufacturing a surface conduction electron-emitting device according to the invention, a solution containing the component elements of electroconductive thin film is sprayed through a nozzle to produce an electroconductive thin film on the substrate. Such an arrangement is particularly safe and suited to produce a large display screen. The operation of spraying the solution and producing an area in the electroconductive thin film that poorly cover the step portion of the related device electrode can be effectively and efficiently carried out if the nozzle is electrically charged and the device electrodes are differentiated in terms of their electric potentials so that fissures may be preferentially generated in the area of poor step coverage. Thus, an electron-emitting region is always formed along the related device electrode regardless of the profile of the device electrode and that of the electroconductive thin film. Additionally, the electroconductive thin film is made to firmly adhere to the substrate to produce a highly reliable electron-emitting device if the spraying technique is used. Therefore, a large number of surface conduction electron-emitting devices according to the invention can be manufactured uniformly particularly in terms of the electron-emitting regions and, therefore, such devices operate stably and uniformly for electron emission. Thus, an electron source realized by arranging a large number of surface conduction electron-emitting devices according to the invention, operates also stably and uniformly. Since the power required for the energization forming operation for each device is small, the operation can be conducted with a relatively low voltage to further improve the performance of the devices. The electron-emitting region of each electron-emitting device according to the invention can be controlled accurately in terms of location and profile if the device electrodes are separated from each other by several to several hundred micrometers. So, the problem of a swerved electron-emitting region is eliminated to improve the manufacturing yield. If a nozzle is used to spray a solution containing the component elements of the electroconductive thin film, an electron source comprising a large number of surface conduction electron-emitting devices can be prepared in a relatively simple manner and therefore at reduced cost without rotating a large substrate for carrying the surface conduction electron-emitting devices. Thus, according to the invention, an electron source that emits highly convergent electron beams and hence operate stably can be manufactured at low cost. Finally, an image forming apparatus according to the invention uses highly convergent electron beams on an image forming member and therefore, a high precision display apparatus with good separation between adjacent pixels and free from blurs in case of color display can be provided. In addition, a large display apparatus giving bright, high quality images can be provided due to the high uniformity and efficiency.

Claims: An electron-emitting device comprising an electroconductive thin film (3) including an electron-emitting region (2) disposed between a pair of device electrodes (4,5) arranged on a substrate (1), characterized in that said electron-emitting region (2) is formed close to a step portion formed by one of said device electrodes (4,5) and said substrate (1). An electron-emitting device according to claim 1, wherein the step portion formed by one of the device electrodes (4) and the substrate (1) has a height different from that of a step portion formed by the other device electrode (5) and the substrate. An electron-emitting device according to claim 2, wherein the electron-emitting region (2) is arranged close to the higher step portion. An electron-emitting device according to any preceding claim, wherein the electroconductive thin film (3) extends from the top of one of the device electrodes (5) to a position between the other electrode (4) and the substrate (1) to cover the substrate (1) between and connect the device electrodes (4,5). An electron-emitting device according to claim 4, wherein the electron-emitting region (2) is arranged close to the step portion of the device electrode (5) onto the top of which the electroconductive thin film extends (3). An electron-emitting device according to any preceding claim, wherein the electron-emitting region (2) is arranged within 1µm from the device electrode (4,5) having the step portion close to which the electron-emitting region is formed toward the other device electrode. An electron-emitting device according to any preceding claim, wherein the heights of the step portions are defined by the thicknesses of the device electrodes (4,5) themselves. An electron-emitting device according to any of claims 1 to 6, wherein the heights of the step portions are defined by the thicknesses of the device electrodes (4,5) and the thickness of a control member (21) beneath one of the device electrodes (5). An electron-emitting device according to claim 2 or any of claims 3 to 8 depending from claim 2, wherein the higher step portion has a height at least five times greater than the thickness of the electroconductive film (3). An electron-emitting device according to any preceding claim further comprising a control electrode (7) arranged on one of the device electrodes (5). An electron-emitting device according to claim 10, wherein the control electrode (7) is arranged on the device electrode (5) having the step portion close to which the electron-emitting region (2) is arranged. An electron-emitting device according to any of claims 1-9 further comprising a pair of control electrodes (7) arranged on the substrate either side of the electroconductive thin film (3). An electron-emitting device according to claim 12, wherein said control electrodes (7) are electrically connected to the device electrode. Use of an electron-emitting device according to any preceding claim, wherein the device electrode (5) having the step portion close to which the electron-emitting region (2) is formed is held at an electric potential lower than that of the other device electrode (4). An electron source comprising an assembly of electron-emitting devices each as claimed in any preceding claim. An electron source according to claim 15, wherein the electron-emitting devices are arranged in rows and are connected by wires. An electron source according to claim 15, wherein the electron-emitting devices are arranged as a matrix array. An image forming apparatus comprising an electron source and an image forming member, wherein the electron source is that which is defined in any of claims 15 to 17. An image forming apparatus according to claim 18, wherein the image forming member is a fluorescent body. A method of forming the electron-emitting device of claim 1, comprising the steps of: providing a first and a second device electrode (4,5) on a substrate (1); depositing an electroconductive thin film (3) on said substrate (1) between said device electrodes (4,5); and energising said thin film (3) to form an electron-emitting region (2) between said device electrodes (4,5), close to a step portion formed by one of said device electrodes and said substrate; characterised in that: in said step of depositing said electroconductive thin film (3) is deposited so as to have its thinnest portion between said pair of electrodes (4,5) at a position close to a step portion formed by one of said device electrodes and said substrate. A method according to claim 20, wherein said step of providing said pair of device electrodes (4,5) comprises: depositing a first device electrode (5) to a first height and depositing a second device electrode (4) to a second height wherein said electro-conductive thin film (3) is deposited on said substrate (1) after said device electrodes (4,5), said electroconductive thin film (3) being deposited so as to have its thinnest portion between said pair of electrodes (4,5) at a position close to said device electrode (5) having the greatest height. A method according to claim 20, wherein said step of providing said pair of device electrodes comprises: depositing a control member (21) on said substrate; depositing a first device electrode (5) on said control member (21) and a second device electrode (4) on said substrate (1) wherein the combined height of said control member (21) and first device electrode (5) is greater than the height of said second device electrode (4); wherein said electro-conductive thin film (3) is deposited on said substrate (1) after said device electrodes (4,5) have been deposited, said electroconductive thin film (3) being deposited so as to have its thinnest portion between said pair of electrodes (4,5) at a position close to said device electrode (5) on said control member (21). A method according to claim 20, wherein said electroconductive thin film is deposited on said substrate after a first device electrode (5) has been deposited on said substrate but prior to a second device electrode (4) being deposited on said substrate (1), wherein said thin film (3) extends from the top of said first device (5) electrode to a position between said second device (4) electrode and said substrate (1). A method according to any of claims 20-23 wherein said thin film (3) is formed to have a thickness less than one fifth of the greatest distance between the surface of said substrate (1) and a surface of one of said device electrodes (4;5).

Inventors: HAMAMOTO YASUHIRO, TSUKAMOTO TAKEO, YAMAMOTO KEISUKE, YAMANOBE MASATO, HAMAMOTO, YASUHIRO, TSUKAMOTO, TAKEO, YAMAMOTO, KEISUKE, YAMANOBE, MASATO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: H01J1
IPC codes: G09G3, H01J1, H01J9