Solid-state electron beam generator

Abstract: A solid-state electron beam generator has a hetero bi­ polar structure comprising an emitter region having a first band gap, a base region having a second band gap narrow­ er than the first band gap, and a collector region having an electron-emitting surface. Electrons are injected from the emitter region into the base region while a backward bias voltage being applied between the base region and the col­ lector region. In consequence, electrons are emitted from the electron-emitting surface of the collector region. The emitter region is constituted by an N-type Al x Ga (1-x) As layer (0 < X ≦ 1) having the first band gap and formed on an n-type or n⁺-type GaAs substrate or a semi-insulating GaAs substrate, the base region is constituted by a P-type Al z Ga (1-z) As layer (0 ≦ z < X) having the second band gap,and the collector region is constituted by an n-type Al t Ga (1-t) As layer (0≦t≦1) formed on the n-type or n⁺-type GaAs sub­ strate or a semi-insulating GaAs substrate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a solid-state electron beam generator. Related Background Art A known solid-state electron beam generator is shown in, for example, the specification of United States Patent No. 4,259,678. This known electron beam generator has a pn junction formed on a Si semiconductor substrate. A reverse voltage is applied to the pn junction so as to produce an avalanche effect thereby generating electrons (referred to as "hot electrons" hereinunder) having an energy level higher than that in a thermal equilibrium state. An electron beam is then emitted into vacuum by the kinetic energy of the hot electrons. In this known electron beam generator, the proportion of the number of hot electrons having energy levels higher than the vacuum energy level to the total number of hot electrons produced by the avalanche effect is rather small, so that only a small electric current is obtained. Another type of known solid-state electron beam generator has, as disclosed in Japanese Patent Publication No. 30274/1979, a pn junction composed of a Al x Ga (1-x) P layer (0 ≦ x ≦ 1) which is formed on a GaP semiconductor substrate and a forward voltage is applied to the pn junction region thereby causing emission of electrons which have been injected from the n-type region into the p-type region. This solid-state electron beam generator can provide a greater number of carriers than in the first-mentioned known electron beam generator disclosed in United States Patent No. 4,259,678, but the efficiency of emission of electrons into vacuum is impractically low because of a lack of any region for forming hot electrons. In addition, the GaP substrate in general tends to have crystalline defects so that it has been rather difficult to form a good pn junction region. In advance of the above-mentioned two types of known solid-state electron beam generator, a solid-state electron beam generator has been proposed in the specification of United States Patent No. 3,119,947 in which an npn region is formed on a Si semiconductor substrate and a voltage is applied between both n-type regions thereby causing electrons to be emitted. This known electron beam generator employing an npn junction can increase the emission efficiency to an order of 10⁻⁴ which is much higher than 10⁻⁶ which is obtained in the first-mentioned known electron beam generator which employs pn junction. This solid-state electron beam generator, however, is generally difficult to produce because the p-type region and the n-type region on the emission side have to be formed in an extremely small thickness in the order of several tens of nanometer (several hundreds of angstrom) and, in addition, with a high degree of uniformity in thickness. Thus, the solid-state electron beam generator of the third type cannot easily be put into practical use. Additionally, document EP-A-0 041 119 discloses an electrbn emitting device which comprises a GaAs substrate on which a n-type GaAlAs layer and a p-type GaAs layer are formed. At the bottom surface of the substrate there is provided an electrode. Another electrode is provided above the p-type GaAs layer without coming into contact with the layer for providing an electric field between the two electrodes. Due to the electric field, electrons are emitted through an opening which is provided in a barrier being formed on the p-type GaAs layer. The exposed part of the surface of the p-type GaAs layer is covered with a layer for reducing the work function. Furthermore, from JOURNAL OF APPLIED PHYSICS, vol. 58, no. 3, 1st August 1985, a heterojunction bipolar transistor with a quantum well structure is known. The emitter is a Al x Ga 1-x As layer and the base consists of a GaAs well and a double barrier of Al x Ga 1-x As. The well and the double barrier are preferably undoped. An electron of the emitter region can only pass through the base region when it is in an energetical resonance with the hole inside the double barrier. Moreover, document EP-A-0 106 724 discloses a heterojunction bipolar transistor wherein the emitter, the base and the collector are subsequently formed on a substrate of GaAs. The emitter is a n-type Al x Ga 1-x As layer, the base is a p-type GaAs layer and the collector a n-type GaAs layer. On the emitter and collector there are provided electrodes. Electrons of the emitter region are basically launched into the base region. Furthermore, a gradient region formed of Ga 1-y Al y As is provided between the emitter and base regions. It is an object of the present invention to provide a solid-state electron beam generator with a construction which is so simple that the production process is remarkably simplified and yet can operate at much higher electron emission efficiency than known solid-state electron beam generators. According to the invention this object is accomplished by a solid-state electron beam generator having: a hetero bipolar transistor comprising an emitter region having a first band gap on a substrate, a base region having a second band gap narrower than said first band gap on said emitter region, a base region electrode electrically connected to said base region, a collector region on said base region with an electron emission surface on said collector region, and a collector region electrode electrically connected to said collector region, wherein electrons are to be injected from said emitter region into said base region, a reverse bias source is connected between said base region electrode and said collector region electrode, and said electrons are to be injected from said base region to said collector region by use of said reverse bias, thereby being injected into said electron emission surface to emit said electrons from said electron emission surface as the electron beam; and wherein a material is included in said electron emission surface on said collector region for reducing the workfunction of said electron emission surface with respect to the work function of said collector region. Furthermore, according to the invention this object is accomplished by a method of emitting and using an electron beam from a hetero bipolar transistor, using as said hetero bipolar transistor a hetero bipolar transistor comprising an emitter region having a first band gap on a substrate, a base region having a second band gap narrower than said first band gap on said emitter region, a base region electrode electrically connected to said base region, a collector region on said base region with an electron emission surface on said collector region, and a collector region electrode electrically connected to said collector region, said method comprising the steps of: injecting electrons from said emitter region into said base region; applying a reverse bias voltage between said base region electrode and said collector region electrode; and injecting electrons from said base region to said collector region by use of said reverse bias, thereby injecting electrons into said electron emission surface and emitting said electrons from said electron emission surface as the electron beam. Moreover, this object is accomplished by a solid-state electron beam generator comprising: a first region with a first band gap; a second region with a second band gap narrower than said first band gap forming a heterojunction with said first region; and a gradient region wherein a mixed crystal ratio of predetermined material is gradually changed between said first and second regions, and electrons are to be injected from said first region into said second region, thereby emitting said electrons from an electron emission surface of said second region as the electron beam directly to the outside. Additionally, this object is accomplished by a solid-state electron beam generator comprising: a first region with a first band gap; and a second region with a second band gap narrower than said first band gap forming a heterojunction with said first region on a GaAs epitaxial film on a Si substrate; wherein electrons are to be injected from said first region into said second region, thereby emitting said electrons from an electron emission surface of said second region as the electron beam directly to the outside. Thus, in the solid-state electron beam generator according to the invention the electrons are emitted directly to the outside. Furthermore, the solid-state electron beam generator can produce electric current of a high density because a Si substrate exhibits a small heat resistance and the use of a Si substrate facilitates a connection of this solid-state electron beam generator to any integrated circuit having a Si substrate. Further objects, features and advantages of the invention will become apparent from the following description when taken into common consideration with the accompanying drawings, of which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a first embodiment of the present invention; Fig. 2 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the first embodiment; Fig. 3 is an energy band diagram showing the electron energy level when a bias voltage is applied in the first embodiment; Fig. 4 is a sectional view of a second embodiment of the present invention; Fig. 5 is a sectional view of a third embodiment of the present invention; Fig. 6 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the third embodiment; Fig. 7 is an energy band diagram showing the electron energy level when a bias voltage is applied in the third embodiment; Fig. 8 is a sectional view of a fourth embodiment of the present invention; Fig. 9 is a sectional view of a fifth embodiment of the present invention; Fig. 10 is a sectional view of a first reference example of the present invention; Fig. 11 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the first reference example; Fig. 12 is an energy band diagram showing the electron energy level when a bias voltage is applied in the first reference example; Fig. 13 is a sectional view of a second reference example of the present invention; Fig. 14 is a sectional view of an third reference example of the present invention; Fig. 15 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the third reference example; Fig. 16 is an energy band diagram showing the electron energy level when a bias voltage is applied in the third reference example; Fig. 17 is a sectional view of a fourth reference example of the present invention; Fig. 18 is a sectional view of a fifth reference example of the present invention; Fig. 19 is a sectional view of a sixth embodiment of the present invention; Fig. 20 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the sixth embodiment; Fig. 21 is an energy band diagram showing the electron energy level when a bias voltage is applied in the sixth embodiment; Fig. 22 is a sectional view of a seventh embodiment of the present invention; Fig. 23 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the seventh embodiment; Fig. 24 is an energy band diagram showing the electron energy level when a bias voltage is applied in the seventh embodiment; Fig. 25 is a sectional view of an eighth embodiment of the present invention; Fig. 26 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the eighth embodiment; Fig. 27 is an energy band diagram showing the electron energy level when a bias voltage is applied in the eighth embodiment; Fig. 28 is a sectional view of a ninth embodiment of the present invention; Fig. 29 is an energy band diagram showing the electron energy level in the thermal equilibrium state of the ninth embodiment; Fig. 30 is an energy band diagram showing the electron energy level when a bias boltage is applied in the ninth embodiment; Fig. 31 is a sectional view of tenth embodiment of the present invention; Fig. 32 is an energy band diagram showing the electron energy level in the tenth embodiment; Fig. 33 is a sectional view of a eleventh embodiment of the present invention; Fig. 34 is a sectional view of a twelfth embodiment of the present invention; Fig. 35 is an energy band diagram showing the electron energy level in the twelfth embodiment; Fig. 36 is a sectional view of an thirteenth embodiment of the present invention; Fig. 37 is a sectional view of a fourteenth embodiment of the present invention; Fig. 38 is an energy band diagram showing the electron energy level in the fourteenth embodiment; Fig. 39 is a sectional view of a fifteenth embodiment of the present invention; Fig. 40 is a sectional view of a sixteenth embodiment of the present invention; Fig. 41 is an energy band diagram showing the electron energy level in the sixteenth embodiment; and Fig. 42 is a sectional view of a seventeenth embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described hereinunder with reference to the accompanying drawings. Fig. 1 is a sectional view of an embodiment of a solid-state electron beam generator of the present invention which employs an n-type or n⁺-type GaAs substrate. This embodiment has an N-type Al x Ga (1-x) As layer 2 serving as an emitter. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. The embodiment further has an inert layer 4 which is formed by injecting oxygen into the N-type Al x Ga (1-x) As layer 2. The embodiment further has a p-type GaAs layer 6 which serves as a base. The small-letter symbol "p" is used to designate a p-type region with narrow band gap. In this embodiment, it is possible to add Al such that the p-type GaAs layer is substituted by a p-type Al z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap of the layer 6. The embodiment further has an n-type GaAs layer 8 serving as a collector. The small-letter "n" is used here to designate an n-type region of a narrow band gap. The n-type GaAs layer 8 may be substituted by an n-type Al t Ga (1-t) As layer (0 < t ≦ 1). Thus, this embodiment of the solid-state electron beam generator in accordance with the present invention has a layered structure similar to that of a hetero-bipolar transistor. A reference numeral 10 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the collector layer 8. This Cs-O layer serves as an electron-emission surface. The Cs-O layer 10 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The solid-state electron beam generator further has an SiO₂ insulating layer 12, an emitter electrode 14, a base electrode 16, a collector electrode 18, an acceleration electrode 20 and an n-type or n⁺ type GaAs substrate 22. Electrodes for n- or N-type semi-conductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while an electrode for the p-type semiconductor may be formed of Au-sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode 16 of the p-type GaAs is formed directly on the surface of the p-type GaAs layer 6. This, however, is not exclusive and the electrode may be formed after doping the surface of this GaAs layer with Be ions so as to form a p⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer surface. The operation of this embodiment will be described with reference to Figs. 2 and 3 which are energy band diagrams showing the energy level of electrons as observed when the electron beam generator is in a thermally equilibrium state and when a bias voltage is applied, respectively. As explained before, the emitter layer 2 is formed of, for example, a Al x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the base layer 6. In the diagrams shown in Figs. 2 and 3, the crystal mixing ratio x of Al is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 2 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base region. It is, however, to be noted that the regions other than the electron beam generating region have been rendered inert by, for example, oxygen ion implantation. This high level of doping causes the state of the layer to be changed into degenerating state and the Fermi level is set above the conductive band. Although the thickness of the emitter layer 2 is selected to be 150 nm (1500 Å) in Fig. 2, the thickness of this layer may be varied as desired insofar as it ensures a large rate of injection of carriers into the base layer 6. Referring now to the base layer 6, this layer 6 is formed of a p-type GaAs layer having a narrow band gap, in order to ensure a high efficiency of injection of current into the base layer 6. The amount of dope in this p-type GaAs layer is selected to be in the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the base layer 6 is selected to be about 30 nm (300 Å) so as to reduce scattering in this layer. Since the emitter layer 2 and the base layer 6 have different band gap widths, a spike is formed at the boundary of these layers as shown in Fig. 2. When Al 0.3 Ga 0.7 As is used as the material of the emitter layer 2 while GaAs is used as the material of the base layer 6, the height ΔE c of the spike is about 0.318 eV. The work function at the collector surface is as small as 1.4 eV because the Cs-O layer is diffused in the surface of the collector layer 8. In order to realize an ohmic contact of a low resistance between the collector layer 8 and the collector electrode 18, a dope amount which is as large as 1 x 10¹⁸ cm⁻³ is applied to the collector layer 8. Although in this embodiment the collector layer 8 has a thickness of 100 nm (1000 Å), this thickness value is only illustrative. More specifically, the collector layer 8 has a smaller thickness provided that a good ohmic contact is attained between the collector electrode 18 and the collector layer 8. A high quality and uniformity of the collector layer 8 are obtainable by the use of a molecular beam epitaxy (MBE) device or a metalorganic chemical vapour deposition (MOCVD) device. Fig. 3 shows the state of the electron beam generator under application of a bias voltage. More specifically, Fig. 3 shows the energy band as obtained when a forward bias voltage V EB is applied between the emitter and the base while a backward bias voltage V BC is applied between the base and the collector in the device which is in thermally equilibrium state as shown in Fig. 1. When a voltage of 1.45 V is applied between the emitter and the base, the quasi Fermi level E F in the emitter layer 2 approaches the conduction band of the base layer 6. Due to the presence of the spike as shown in Fig. 3, the carriers injected into the base layer 6 are changed into hot electrons due to thermal jumping or tunnel effect. The thus generated hot electrons are accelerated by the bias voltage V BC applied between the base and the collector, so as to have a high level of kinetic energy. The level of the energy possessed by the electrons passing through the base layer 6 is about 0.7 eV higher than the vacuum level. Therefore, a large proportion of electrons is emitted into vacuum through a considerable part of energy is lost due to scattering in the collector layer 8. It is also to be noted that, in the described embodiment, the regions of the collector layer surface with diffusion of Cs-O other than the electron emitting region 10 are provided with the SiO₂ insulating layer 12 and the external acceleration electrode 20. Therefore, the vacuum level is lowered by A B as shown by broken line in Fig. 3, as a result of application of an external electric field, whereby the electron emission efficiency is further increased. Fig. 4 is a sectional view of a second embodiment of the solid-state electron beam generator of the invention, which makes use of a semi-insulating GaAs substrate 26. In this embodiment, therefore, the emitter electrode 14 is formed on an n-type or n⁺-type GaAs layer 24. Other portions of the structure are materially the same as those of the embodiment shown in Fig. 1. Thus, the same reference numerals are used in Fig. 4 to denote the same parts as those in Fig. 1. Thus, the arrangement of layers of the compounds constituting hetero junction, as well as the principle of operation, is the same as that explained in connection with Figs. 1, 2 and 3. Fig. 5 shows a third embodiment of the solid-state electron beam generator in accordance with the present invention. Figs. 6 and 7 are energy band diagrams showing the energy levels of electrons as obtained when the electron beam generator is in the thermally equilibrium state and when a bias voltage is applied, respectively. The third embodiment shown in Fig. 5 is discriminated from the first embodiment shown in Fig. 1 in that the base region composed of the p-type GaAs layer 6 is provided with a resonance tunnel section 30 composed of a non-doped Al y Ga 1-y As(y=0,3) layer, serving as a barrier layer, a non-doped Al s Ga (1-s) As layer serving as a well layer, and a non-doped Al y Ga 1-y As(y=0,3) layer such as to meet the condition of 0 ≦ s < y ≦ 1, thereby forming a resonance tunnel level. Other portions are materially the same as those of the first embodiment. The principle and operation also are the same as those in the first embodiment shown in Figs. 1 to 3 so that description of principle and operation is omitted. When the thicknesses of the barrier layer and the well layer in the resonance tunnel section 30 are 3 nm (30 Å) and 2 nm (20 Å), respectively, the first resonance level appears at a point which is 0.11 eV above the conduction band in the base region. Therefore, as a forward voltage V EB is applied between the emitter and the base as shown in Fig. 7 so as to make the quasi Fermi level of the emitter region coincide with the resonance tunnel level, the hot electrons are made to pass through the base layer past the resonance tunnel. When the amount of dope of the emitter layer 2 is on the order of 1 x 10¹⁸ cm⁻³, the difference between the quasi Fermi level of the emitter layer and the energy level E C of the conduction band is given as follows. ΔE = E F - E C ≃ 0.01 (eV) This level difference coincides with the energy band width ΔE of the resonance tunnel level. In addition, since the p-type GaAs layer 6 constituting the base has a high rate of dope which is 1 x 10¹⁹ cm⁻³, the energy bands in the barrier layer and the well-layer are flattened, thus realizing a symmetrical double barrier structure. In consequence, the proportion of the electrons passing through the resonance tunnel 30 is increased. In the described embodiment of the present invention, the energy band width of the hot electrons is limited by the energy band width ΔE of the resonance tunnel level, so that carriers of low energy levels cannot flow into the base layer 6 and the collector layer. In consequence, the proportion of the carriers which fall to the level of the collector region surface, i.e., the proportion of electrons of low energy levels, is decreased, so that deterioration of the device can be suppressed advantageously. In the third embodiment explained in connection with Figs. 5 and 7, the hetero junction between the emitter region and the base region has a steep gradient so as to form a spike therebetween. This spike, however, is not essential because hot electrons can be formed also in the double-barrier structure which forms the resonance tunnel 30. When the spike is eliminated, the composition of the boundary between the emitter region and the base region is progressively changed so as to provide a graded layer. Fig. 8 shows a fourth embodiment of the present invention. This embodiment is basically the same as the third embodiment shown in Fig. 5 except that a semiinsulating GaAs substrate 26 is used as the substrate. In this embodiment, therefore, the emitter electrode 14 is provided on the n-type GaAs layer 24. Other structural features, as well as operation, are materially the same as those in the third embodiment so that detailed description thereof is omitted. Fig. 9 is a sectional view of a fifth embodiment of the present invention. Unlike the preceding embodiments, the fifth embodiment proposes a planar type device. This fifth embodiment is constituted by the following portions: an emitter electrode 40; n⁺-type GaAs layers 52, 63 (⁺ means high doping density); n-type GaAs layers 60, 64; an N-type Al x Ga (1-X) As layer (0 < x ≦ 1) 32 having a wide band gap; a p-type GaAs layer 35; a p⁺ layer 53 doped with Be; and a surface layer 38 doped with an agent (Cs-O) for reducing the work function. A numeral 39 denotes a B-injected layer for isolating adjacent regions. Numerals 41 and 42 denote base electrode and collector electrode, respectively. A numeral 51 denotes a substrate consisting of Si or semiinsulating GaAs. A numeral 62 denotes an inactive region. Numerals 66 and 68 denote an external acceleration electrode and an insulating region, respectively. It will be seen that this planar structure is suitable for production of a multiple-type device in which a multiplicity of devices are arranged on a common plane. Although the described first to fifth embodiments make use of GaAs which is one of semiconductors of compounds of elements belonging to groups III to V, such a material is not exclusive and various other materials such as InGaAsP/InP type materials and SiC/Si type materials can be used equally well. Examples of construction of the solid-state electron beam generator of the invention which incorporate such materials are shown in Table 1 below. InGaAsP/Inp type SiC/Si type substrate InP Si or SiC Growth method liquid phase growth Gaseous or liquid phase growth Emitter N⁺ type InP N⁺ type SiC N type InP N type SiC Base p-type InGaAsP p-type Si Collector n-type InGaAsP n-type Si n-type dopant Te (≃ 2 x 10¹⁹ cm⁻³) N(≃ 10²⁰ cm⁻³) p-type dopant Cd or Mg (≃ 5 x 10¹⁸ cm⁻³) Al (10¹⁸ cm⁻³) n-type electrode Au, Au-Ti, Pt, Sn Au, Au-Ta (99:1) p-type electrode For InP Au, Ni, Cu Al-Si (89:11) For InGaAsP Au, Ag As will be understood from the foregoing description, the first to fifth embodiments of the present invention offer the following advantages. (1) Since the emitter and the base have different band gap widths, the rate of injection of carrier is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers changed into hot electrons are directly emitted to the outside without propagating through the semiconductor. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometer several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Since the electron beam generator is produced from semiconductor materials, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. Furthermore, the electrons are changed into hot electrons by virtue of the spike caused by the hetero junction between the emitter region and the base region or a resonance tunnel in the base region, so that the efficiency of emission of electrons is further increased. Fig. 10 shows a first reference example of the solid-state electron beam generator of the present invention. This first reference example has the following portions: an n-type or n⁺-type GaAs substrate 101; an N-type Al x Ga (1-x) As layer 102 (0 < x ≦ 1); an inert layer 103 formed by, for instance, injection of ions of oxygen into the layer 102; a p-type GaAs layer 104; an insulating layer 105 such as of SiO₂; electrodes 106 and 107; an external accelerating electrode 108; external acceleration voltage 109; an electric source for the forward bias voltage 110; and a surface layer 111 of reduced work function through diffusion or deposition of, for example, caesium oxide (Cs-O). The Cs-O layer 111 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The capital-letter symbol N represents an n-type region having a wide band gap. The small-letter symbols "p" and "n" are used to designate a p-type region and a n-type region with narrow band gaps, respectively. In this embodiment, it is possible to add Al such that the p-type GaAs layer is substituted by a p-type Al z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap of the layer 104. Electrodes for n- or N-type semiconductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while the electrode for the p-type semiconductor may be formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode of the p-type GaAs is formed directly on the surface of the p-type GaAs layer. This, however, is not exclusive and the electrode may be formed after doping the surface of this GaAs layer with Be ions so as to form a p⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer surface. The operation of this first reference example will be described with reference to Figs. 11 and 12 which are energy band diagrams showing the energy level of electrons as observed when the electron beam generator is in a thermally equilibrium state and when a bias voltage is applied, respectively. As explained before, the layer 102 is formed of, for example, a Al x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the layer 104. In the diagrams shown in Figs. 11 and 12, the crystal mixing ratio x of Al is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the layer 102 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the layer 104. It is, however, to be noted that the regions other than the electron beam generating region have been rendered inert by, for example, oxygen ion implantation. This high level of doping causes the state of the layer to be changed into degenerating state and the Fermi level is set above the conductive band. Although the layer 102 is formed by an MBE device or an MOCVD device in a thickness selected to be 150 nm (1500 Å) in Fig. 11, the thickness of this layer 102 may be varied as desired insofar as it ensures a large rate of injection of carriers into the layer 104. The electrode of the layer 102 is provided on the reverse side of the n-type or n⁺-type GaAs substrate 101. It is, therefore, preferred that the substrate 101 has a high rate of doping, so as to minimize the voltage drop across this substrate 101. Referring now to the layer 104, this layer 104 is grown on the layer 102 by an MBE device or an MOCVD device from a p-type GaAs layer having a narrow band gap, in order to ensure a high efficiency of injection of current into the layer 104. The amount of dope in this p-type GaAs layer is selected to be on the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the layer 104 is selected to be about 30 nm (300 Å) for the purpose of suppressing scattering in the above-mentioned region. Since the layer 102 and the layer 104 have different band gap widths, a spike is formed at the boundary of these layers as shown in Fig. 11. When Al 0.3 Ga 0.7 AS is used as the material of the layer 102 while p-type GaAs is used as the material of the layer 104, the height Δ E C of the spike is about 0.318 eV. The work function at the base surface is as small as 1.4 eV because the Cs-O layer is diffused in this surface. As stated before, the surface layer for reducing the work function may be formed from a composite material containing another alkali metal, oxygen and at least one element selected from a group consisting of Sb, Bi, As, Ag, P, Te, Cu, Au and Si. The state of energy band in this first reference example under application of a bias voltage will be explained with reference to Fig. 12. Using a first power supply 110 shown in Fig. 10, a forward bias voltage V EB is applied between the layers 102 and 104. Meanwhile, a voltage Va is applied between the external acceleration electrode 108 and the layer 104 by a second power supply 109 such that the external electrode 108 constitutes the plus side. When the voltage V EB is 1.45 V, the quasi Fermi level E F in the layer 102 approaches the conduction band of the layer 104. The carriers injected into the layer 104 are those which have thermally skipped over the spike shown in Fig. 12 or permeated by a tunnel effect and, hence, have been changed into hot electrons. The work function of the p-type GaAs layer 104 with diffused Cs-O is 1.4 eV, while the electronic affinity of the p-type GaAs layer is 4.07 eV. Therefore, the band of the p-type GaAs is deflected downward at a region in the vicinity of the surface. However, the carriers injected into the layer 104 have been changed into hot electrons so that they are emitted into vacuum without dropping into the valley near the surface, as shown in Fig. 12. This is because the vacuum level is 1.4 eV which is lower than the band gap (1.42 eV) of the p-type GaAs. The vacuum level is deflected downward as shown in Fig. 12, because of application of the voltage Va between the external acceleration electrode 108 and the layer 104, so that an electric field is formed which acts to accelerate the emitted electrons. The first reference example shown in Fig. 10 makes use of an n-type or an n⁺-type GaAs substrate 101. This, however, is not exclusive and the solid-state electron beam generator of the invention may be realized with the use of a semi-insulating GaAs substrate, by forming the electrode for the layer 102 on the obverse side by making use of, for example, a technique called "viahole" (Mitsui et al., refer to "VIAHOLE STRUCTURE GAAS LARGE OUTPUT MONOLITHIC/ AMPLIFIER", All Japan Conference of Electro-Communication, 1983, Semi-conductor and Material Section, No. 122). A further reference example which makes use of such a substrate will be explained hereinunder. Fig. 13 is a sectional view of a second reference example of the solid-state electron beam generator of the invention, which makes use of a semi-insulating GaAs substrate 26. In this second reference example, therefore, the electrode 14 for the layer 102 is formed on an n-type or n⁺-type GaAs layer 124. Other portions of the structure are materially the same as those of the first reference example shown in Fig. 10. Thus, the same reference numerals are used in Fig. 13 to denote the same parts as those in Fig. 10. Thus, the arrangement of layers of the compounds constituting hetero junction, as well as the principle of operation, is the same as that explained in connection with Figs. 11 and 12. Fig. 14 shows a third reference example of the solid-state electron beam generator of the present invention. Figs. 15 and 16 are energy band diagrams showing the energy levels of electrons as obtained when the electron beam generator is in the thermally equilibrium state and when a bias voltage is applied, respectively. The third reference example shown in Fig. 14 is discriminated from the first reference example shown in Fig. 10 in that the region composed of the p-type GaAs layer 104 is provided with a resonance tunnel section 130 composed of a non-doped Al 0.3 Ga 0.7 As layer, serving as a barrier layer, a non-doped Al s Ga (1-s) As layer serving as a well layer, and a non-doped Al 0.3 Ga 0.7 As layer. Other portions are materially the same as those of the first reference example shown in Fig. 10. The principle and operation also are the same as those in the first reference example shown in Figs. 10 to 12 so that description of principle and operation is omitted. When the thicknesses of the barrier layer and the well layer in the resonance tunnel section 130 are 3 nm (30 Å) and 2 nm (20 Å), respectively, the first resonance level appears at a point which is 0.11 eV above the conduction band in the layer 104. Therefore, as a forward voltage V EB is applied between the layers 102 and 104 as shown in Fig. 14 so as to make the quasi Fermi level of the layer 102 coincide with the resonance tunnel level, the hot electrons are made to pass through the layer 104 past the resonance tunnel. When the amount of dope of the emitter layer 102 is on the order of 1 x 10¹⁸ cm⁻³, the difference between the quasi Fermi level of the layer 102 and the energy level E C of the conduction band is given as follows. ΔE = E F - E C ≃ 0.01 (eV) This level difference coincides with the energy band width ΔE of the resonance tunnel level. In addition, since the p-type GaAs 104 has a high rate of dope which is 1 x 10¹⁹ cm⁻³, the energy bands in the barrier layer and the well layer are flattened, thus realizing a symmetrical double barrier structure. In consequence, the proportion of the electrons passing through the resonance tunnel 130 is increased. In the described reference example of the present invention, the energy band width of the hot electrons is limited by the energy band width ΔE of the resonance tunnel level, so that carriers of low energy levels cannot flow into the layer 104 and the collector layer. In consequence, the proportion of the carriers which fall to the level of the surface of the layer 104, i.e., the proportion of electrons of low energy levels, is decreased, so that deterioration of the device can be suppressed advantageously. In the third reference example, the hetero junction between the layers 102 and 104 has a steep gradient so as to form a spike therebetween. This spike, however, is not essential because hot electrons can be formed also in the double-barrier structure which forms the resonance tunnel. When the spike is eliminated, the composition of the boundary between the layers 102 and 104 is progressively changed so as to provide a graded layer. Fig. 17 shows a fourth reference example of the present invention. This fourth reference example is basically the same as the third reference example shown in Fig. 14 except that a semi-insulating GaAs substrate 126 is used as the substrate. In this fourth reference example, therefore, the electrode 14 for the layer 102 is provided on the n-type GaAs layer 124. Other structural features, as well as operation, are materially the same as those in the third reference example so that detailed description thereof is omitted. Fig. 18 is a sectional view of a fifth reference example of the present invention. Unlike the preceding reference examples, the fifth reference example proposes a planar type device. This fifth reference example is constituted by the following portions: an electrode 140 for N-type AlGaAs layer; n⁺-type GaAs layer 152 (⁺ means high doping density), an N-type Al x Ga (1-x) As layer (0 < x ≦ 1) 132 having a wide band gap, a p-type GaAs layer 135; a p⁺ layer 153 doped with Be, and a surface layer 138 doped with an agent (Cs-O) for reducing the work function. A numeral 160 denotes a B-injected layer for isolating adjacent regions. Numerals 139, 141 and 143 denote an insulating region, an electrode for the second region and an external acceleration electrode, respectively. It will be seen that this planar structure is suitable for production of a multiple-type device in which a multiplicity of devices are arranged on a common plane. Although the described first to fifth reference examples make use of GaAs which is one of semiconductors of compounds of elements belonging to groups III to V, such a material is not exclusive and various other materials such as InGaAsP/InP type materials and SiC/Si type materials can be used equally well. Examples of construction of the solid-state electron beam generator of the invention which incorporate such materials are shown in Table 2 below. InGaAsP/Inp type SiC/Si type substrate InP Si or SiC Growth method liquid phase growth Gaseous or liquid phase growth N region N⁺ type InP N⁺ type SiC N type InP N type SiC p region p-type InGaAsP p-type Si n-type dopant Te (≃ 2 x 10¹⁹ cm⁻³) N(≃ 10²⁰ cm⁻³) p-type dopant Cd or Mg (≃ 5 x 10¹⁸ cm⁻³) Al (10¹⁸ cm⁻³) n-type electrode Au, Au-Ti, Pt, Sn Au, Au-Ta (99:1) p-type electrode For InP Au, Ni, Cu Al-Si (89:11) For InGaAsP Au, Ag As will be understood from the foregoing description, the first to fifth reference examples of the present invention offer the following advantages. (1) Since the emitter and the base have different band gap widths (Npn structure), the rate of injection of carrier is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers injected into the base are accelerated to increase the level of kinetic energy. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Fabrication is facilitatedthanksto simple laminar structure. (4) Since the electron beam generator is produced from semiconductor materials, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. Furthermore, the electrons are changed into hot electrons by virtue of the spike caused by the hetero junction between the emitter region and the base region or a resonance tunnel in the base region, so that the efficiency of emission of electrons is further increased. Fig. 19 is a sectional view of a sixth embodiment of the solid-state electron beam generator of the present invention., In this embodiment, an AlP layer 202 and an AlGaP layer 203 are made to grow on an Si substrate 201 by MOCVD (Metalorganic Chemical Vapor Deposition) method and then a super-grid layer 204 of GaP and GaAsP and a super-grid layer 205 of GaAsP and GaAs are formed. Then, a GaAs layer 206 is made to grow on these super-grid layers. Subsequently, an n⁺-type GaAs layer 207 and an N-type Al x Ga (1-x) As layer 208 (0 < x ≦ 1) are made to grow. Oxygen ions are injected by an ion injector into the Al x Ga (1-x) As layer 208 so as to form an inert layer 209 in the regions of this layer 208 other than the electron beam generating region. A p-type GaAs layer 210 and an n-type GaAs layer 211 are formed on the N-type Al x Ga (1-x) As layer 208. A layer 212 of material for reducing work function, e.g., cesium oxide (Cs-O) is formed by deposition or diffusion on the surface of the n-type GaAs layer 211. The construction will be explained in more detail hereinunder. As mentioned above, this embodiment incorporates an N-type Al x Ga (1-x) As layer 208 serving as an emitter. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. A numeral 209 represents an inert layer formed by injecting oxygen ions into the N-type Al x Ga (1-x) As layer 208. The embodiment further has a p-type GaAs layer 210 which serves as a base. The small-letter symbol "p" is used to designate a p-type region with narrow band gap. In this embodiment, it is possible to add Al such that the p-type GaAs layer is substituted by a p-type Al z Ga( 1-z )As layer (0 < z < x), thereby allowing a control of the band gap of the layer 210. The embodiment further has an n-type GaAs layer 211 serving as a collector. The small-letter "n" is used here to designate an n-type region of a narrow band gap. The n-type GaAs layer may be substituted by an n-type Al t Ga (1-t) AS layer (0 < t ≦ 1). Thus, this embodiment of the solid-state electron beam generator in accordance with the present invention has a layered structure similar to that of a hetero-bipolar transistor. A reference numeral 212 designates a cesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the collector layer 211. This Cs-O layer serves as an electron-emission surface. The Cs-O layer 212 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The solid-state electron beam generator further has an SiO₂ insulating layer, an emitter electrode 213, a base electrode 214, and a collector electrode 215. Electrodes for n- or N-type semiconductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while the electrode for the p-type semi-conductor may be formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode of the p-type GaAs is formed directly on the surface of the p-type GaAs layer. This, however, is not exclusive and the electrode may be formed after doping the surface of this GaAs layer with Be ions so as to form a p⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer surface. Thus, in this sixth embodiment, an Npn-type epitaxial film of GaAs-Al x Ga (1-x) As system has grown on the Si substrate 201. The operation of this embodiment will be described with reference to Figs. 20 and 21 which are energy band diagrams showing the energy level of electrons as observed when the electron beam generator is in a thermally equilibrium state and when a bias voltage is applied, respectively. As explained before, the emitter layer 208 is formed of, for example, a Al x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the base layer 210. In the diagrams shown in Figs. 2 and 3, the crystal mixing ratio x of Al is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 208 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base region. It is, however, to be noted that the regions other than the electron beam generating region have been rendered inert by, for example, oxygen ion implantation. This high level of doping causes the state of the layer to be changed into a degenerating state and the Fermi level is set above the conductive band. Although the thickness of the emitter layer 208 is selected to be 150 nm (1500 Å) in Fig. 20, the thickness of this layer may be varied as desired insofar as it ensures a good ohmic contact between the emitter layer 208 and the electrode 213 in the region of the n⁺-type GaAs layer 207, as well as a large rate of injection of carriers into the base layer 210. Referring now to the base layer 210, this layer 210 is formed from a p-type GaAs layer having a narrow band gap, in order to ensure a high efficiency of injection of current into the base layer 210. The amount of dope in this p-type GaAs layer is selected to be on the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the base layer 210 is selected to be about 30 nm (300 Å) so as to reduce scattering in this layer. Since the emitter layer 208 and the base layer 210 have different band gap widths, a spike is formed at the boundary of these layers as shown in Fig. 20. When Al 0.3 Ga 0.7 As is used as the material of the emitter layer 208 while GaAs is used as the material of the base layer 210, the height ΔE C of the spike is about 0.318 eV. The work function at the collector surface is as small as 1.4 eV because the Cs-O layer is diffused in the surface of the collector layer 211. In order to realize an ohmic contact of a low resistance between the collector layer 211 and the collector electrode 215, a dope amount which is as large as 1 x 10¹⁸ cm⁻³ is applied to the collector layer 211. Although in this embodiment the collector layer 211 has a thickness of 100 nm (1000 Å), this thickness value is only illustrative. More specifically, the collector layer 211 has a smaller thickness provided that a good ohmic contact is attained between the collector electrode 215 and the collector layer 211. A high quality and uniformity of the collector layer 211 are obtainable by the use of a molecular beam epitaxy (MBE) device or a metalorganic chemical vapour deposition (MOCVD) device. Fig. 21 shows the state of the electron beam generator under application of a bias voltage. More specifically, Fig. 21 shows the energy band as obtained when a forward bias voltage is applied between the emitter and the base while a backward bias voltage V BC is applied between the base and the collector in the device shown in Fig. 19 which is in thermally equilibrium state. When a voltage of 1.45 V is applied as the voltage V EB between the emitter and the base, the quasi Fermi level E F in the emitter layer 208 approaches the conduction band of the base layer 210. Due to the presence of the spike as shown in Fig. 21, the carriers injected into the base layer 210 are changed into hot electrons due to thermal jumping or tunnel effect. The thus generated hot electrons are accelerated by the bias voltage V BC applied between the base and the collector, so as to have high level of kinetic energy. The level of the energy possessed by the electrons passing through the base layer 210 is about 0.7 eV higher than the vacuum level. Therefore, a large proportion of electrons is emitted into vacuum though a considerable part of energy is lost due to scattering in the collector layer 211. It is also to be noted that, in the described embodiment, the regions of the collector layer surface with diffusion of Cs-O other than the electron emitting region are provided with an SiO₂ insulating layer and an external acceleration electrode both of which are not shown. Therefore, the vacuum level is lowered by Δ B as shown by broken line in Fig. 21, as a result of application of an external electric field, whereby the electron emission efficiency is further increased. Fig. 22 shows a seventh embodiment of the solid-state electron beam generator in accordance with the present invention. Figs. 23 and 24 are energy band diagrams showing the energy levels of electrons as obtained when the electron beam generator is in the thermally equilibrium state and when a bias voltage is applied, respectively. The seventh embodiment shown in Fig. 22 is discriminated from the sixth embodiment shown in Fig. 19 in that the base region composed of the p-type GaAs layer 210 is provided with a resonance tunnel section 230 composed of a non-doped Al y Ga 1-y As (y=0,3) layer, serving as a barrier layer, a non-doped Al s Ga (1-s) As layer serving as a well layer, and a non-doped Al y Ga 1-y As (y=0,3) layer such as to meet the condition of 0 ≦ s < y ≦ 1, thereby forming a resonance tunnel level. Other portions are materially the same as those of the sixth embodiment. The principle and operation also are the same as those in the sixth embodiment shown in Figs. 19 to 21 so that description of principle and operation is omitted. When the thicknesses of the barrier layer and the well layer in the resonance tunnel section 230 are 3 nm (30 Å) and 2 nm (20 Å), respectively, the first resonance level appears at a point which is 0.11 eV above the conduction band in the base region. Therefore, as a forward voltage V EB is applied between the emitter and the base as shown in Fig. 7 so as to make the quasi Fermi level of the emitter region coincide with the resonance tunnel level, the hot electrons are made to pass through the base layer 210 past the resonance tunnel 230. When the amount of dope of the emitter layer 208 is on the order of 1 x 10¹⁸ cm⁻³, the difference between the quasi Fermi level of the emitter layer 208 and the energy level E C of the conduction band is given as follows. ΔE = E F - E C ≃ 0.01 (eV) This level difference coincides with the energy band width ΔE of the resonance tunnel level. In addition, since the p-type GaAs layer 210 constituting the base has a high rate of dope which is 1 x 10¹⁹ cm⁻³, the energy bands in the barrier layer and the well layer are flattened thus realizing a symmetrical double barrier structure. In consequence, the proportion of the electrons passing through the resonance tunnel 230 is increased. In the described embodiment of the present invention, the energy band width of the hot electrons is limited by the energy band width ΔE of the resonance tunnel level, so that carriers of low energy levels cannot flow into the base layer 210 and the collector layer 211. In consequence, the proportion of the carriers which fall to the level of the collector region surface, i.e., the proportion of electrons of low energy levels, is decreased, so that deterioration of the device can be suppressed advantageously. In the seventh embodiment explained in connection with Figs. 22 and 24, the hetero junction between the emitter region and the base region has a steep gradient so as to form a spike therebetween. This spike, however, is not essential because hot electrons can be formed also in the double-barrier structure which forms the resonance tunnel 230. When the spike is eliminated, the composition of the boundary between the emitter region and the base region is progressively changed so as to provide a graded layer. The described sixth and seventh embodiments make use of a buffer layer constituted by super-grid layer. This, however, is only illustrative and the buffer layer may be an extremely thin buffer layer grown on the Si substrate 201 at a low temperature (GaAs/ GaAs buffer layer ((20nm (200 Å))/Si system)/. It is also to be understood that, although the described embodiments utilize GaAs which is one of semiconductors of compounds of elements belonging to groups III to V, such a material is not exclusive and various other materials such as SiC/Si type materials can be used equally well. An example of construction of the solid-state electron beam generator of the invention which incorporates such material is shown in Table 3 below. SiC/Si type substrate Si or SiC Growth method Gaseous or liquid phase growth Emitter N⁺ type SiC N type SiC Base p-type Si Collector n-type Si n-type dopant N(≃ 10²⁰ cm⁻³) p-type dopant Al (10¹⁸ cm⁻³) n-type electrode Au, Au-Ta (99:1) p-type electrode Al-Si (89:11) As will be understood from the foregoing description, the sixth to seventh embodiments of the present invention offer the following advantages. (1) Since the emitter and the base have different band gap widths, the rate of injection of carrier is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers injected into the base are accelerated by the electric field so as to have greater kinetic energy. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Problems concerning heat generation are not so severe because the Si substrate exhibits only small heat resistance. (4) Since the electron beam generator is produced by using an Si substrate, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. Furthermore, the electrons are changed into hot electrons by virtue of the spike caused by the hetero junction between the emitter region and the base region or a resonance tunnel in the base region, so that the efficiency of emission of electrons is further increased. Fig. 25 is a sectional view of a eighth embodiment of the solid-state electron beam generator of the present invention. In this embodiment, an AℓP layer 302 and an AℓGap layer 303 are made to grow on an Si substrate 301 by MOCVD (Metalorganic Chemical Vapor Deposition) method and then a super-grid layer 304 of Gap and GaAsP and a super-grid layer 305 of GaAsP and GaAs are formed. Then, a GaAs layer 306 is made to grow on these super-grid layers. Subsequently, an n⁺-type GaAs layer 307 and an N-type Aℓ x Ga (1-x) As layer 308 (0 < x ≦ 1) are made to grow. Oxygen ions are injected into the Aℓ x Ga (1-x) As layer 308 so as to form an inert layer in the regions of this layer 308 other than the electron beam generating region. A p-type GaAs layer 310 is formed on the N-type Aℓ x Ga (1-x) As layer 308. A layer 312 of material for reducing work function is formed by deposition or diffusion on the surface of the n-type GaAs layer 310. The construction will be explained in more detail hereinunder. As mentioned above, this embodiment incorporates an N-type Aℓ x Ga (1-x) As layer 308. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. A numeral 309 represents an inert layer formed by injecting oxygen ions into the N-type Aℓ x Ga (1-x) As layer 308. The embodiment further has the p-type GaAs layer 310. The small-letter symbol "p" is used to designate a p-type region with narrow band gap. In this embodiment, it is possible to add Aℓ such that the p-type GaAs layer is substituted by a p-type Aℓ z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap. A reference numeral 312 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the collector layer. This Cs-O layer 312 serves as an electron-emission surface. The Cs-O layer 312 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. Reference numerals 311, 313, 314, and 315 denote an insulating region, an electrode for n-type Al X Ga 1-x As layer, an electrode for p-type GaAs layer and an external acceleration electrode, respectively. Electrodes for n- or N-type semiconductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while the electrode for the p-type semiconductor may be formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode of the p-type GaAs is formed directly on the surface of the p-type GaAs layer. This, however, is not exclusive and the electrode may be formed after doping the surface of this GaAs layer with Be ions so as to form a P⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer surface. Thus, in this eighth embodiment, an Npn-type epitaxial film of GaAs-Aℓ x Ga (1-x) As system is grown on the Si substrate. The operation of this embodiment will be described with reference to Figs. 26 and 27 which are energy band diagrams showing the energy level of electrons as observed when the electron beam generator is in a thermally equilibrium state and when a bias voltage is applied, respectively. As explained before, the layer 308 is formed of, for example, a Aℓ x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the base layer 310. In the diagrams shown in Figs. 2 and 3, the crystal mixing ratio of x of Aℓ is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the layer 308 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base region. It is, however, to be noted that the regions other than the electron beam generating region have been rendered inert by, for example, oxygen ion implantation. This high level of doping causes the state of the layer to be changed into degenerating state and the Fermi level is set above the conductive band. Although the layer 308 is formed by an MBE device or an MOCVD device such that its thickness is 150 nm (1500 Å) in Fig. 20, the thickness of this layer may be varied as desired insofar as it ensures a large rate of injection of carriers into the base layer 310. Referring now to the layer 310, this layer 310 is formed from a p-type GaAs layer having a narrow band gap, in order to ensure a high efficiency of injection of current into the layer 310. The amount of dope in this p-type GaAs layer 310 is selected to be in the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the layer 310 is selected to be about 30nm (300 Å) so as to reduce scattering in this layer. Since the layer 308 and the layer 310 have different band gap widths, a spike is formed at the boundary of these layers as shown in Fig. 26. When Aℓ 0.3 Ga 0.7 As is used as the material of the layer 308 while p-type GaAs is used as the material of the layer 310, the height ΔE C of the spike is about 0.318 eV. The work function at the surface of the layer 310 is as small as 1.4 eV because the Cs-O layer is diffused in the surface of the layer 310. Fig. 27 shows the state of the electron beam generator under application of a bias voltage. More specifically, Fig. 27 shows the energy band as obtained when a forward bias voltage V EB is applied between the layer 308 and the layer 310 while a voltage V a is applied between the layer 310 and an external acceleration electrode 315 (acceleration electrode constitutes plus side) when the device shown in Fig. 25 is in thermally equilibrium state. When a voltage of 1.45 V is applied as the voltage V EB between the layers 308 and 310, the quasi Fermi level E F in the layer 308 approaches the conduction band of the base layer 310. Due to the presence of the spike as shown in Fig. 27, the carriers injected into the base layer 310 are changed into hot electrons due to thermal jumping or tunnel effect. The work function of the p-type GaAs layer 310 with diffused CsO is 1.4 eV, while the electronic affinity of the p-type GaAs layer is 4.07 eV. Therefore, the band of the p-type GaAs is deflected downward at a region in the vicinity of the surface. However, the carriers injected into the layer 310 have been changed into hot electrons so that they are emitted into vacuum without dropping into the valley near the surface, as shown in Fig. 27. This is because the vacuum level is 1.4 eV which is lower than the band gap (1.42 eV) of the p-type GaAs. The vacuum level is deflected downward as shown in Fig. 27, because of application of the voltage Va between the external acceleration electrode 315 and the layer 310, so that an electric field is formed which acts to accelerate the emitted electrons. Fig. 28 shows a ninth embodiment of a solid-state electron beam generator in accordance with the present invention. Figs. 29 and 30 are energy band diagrams showing the energy levels of electrons as obtained when the electron beam generator is in the thermally equilibrium state and when a bias voltage is applied, respectively. The ninth embodiment shown in Fig. 28 is discriminated from the eighth embodiment shown in Fig. 25 in that the region composed of the p-type GaAs layer 310 is provided with a resonance tunnel section 330 composed of a non-doped Aℓ 0.3 Ga 0.7 As layer, serving as a barrier layer, a non-doped Aℓ s Ga (1-s) As layer serving as a well layer, and a non-doped Aℓ 0.3 Ga 0.7 As layer. Other portions are materially the same as those of the embodiment shown in Fig. 25. The principle and operation also are the same as those in the embodiment shown in Figs. 25 to 27 so that description of principle and operation is omitted. When the thicknesses of the barrier layer and the well layer in the resonance tunnel section 330 are 3nm (300 Å) and 2nm (20 Å), respectively, the first resonance level appears at a point which is 0.11 eV above the conduction band in the layer 310. Therefore, as a forward voltage V EB is applied between the layers 308 and 310 as shown in Fig. 30 so as to make the quasi Fermi level of the layer 308 coincide with the resonance tunnel level, the hot electrons are made to pass through the layer 310 past the resonance tunnel 330. When the amount of dope of the emitter layer 308 is on the order of 1 x 10¹⁸ cm⁻³, the difference between the quasi Fermi level of the layer 308 and the energy level E C of the conduction band is given as follows. ΔE = E F -E C ≃ 0.01 [EV] This level difference coincides with the energy band width ΔE of the resonance tunnel level. In addition, since the p-type GaAs 310 has a high rate of dope which is 1 x 10¹⁹ cm⁻³, the energy bands in the barrier layer and the well layer are flattened thus realizing a symmetrical double barrier structure. In consequence, the proportion of the electrons passing through the resonance tunnel 330 is increased. In the described embodiment of the present invention, the energy band width of the hot electrons is limited by the energy band width ΔE of the resonance tunnel level, so that carriers of low energy levels cannot flow into the layer 310 and the collector layer. In consequence, the proportion of the carriers which fall to the level of the surface of the layer 310, i.e., the proportion of electrons of low energy levels, is decreased, so that deterioration of the device can be suppressed advantageously. In the ninth embodiment, the hetero junction between the layers 308 and 310 has a steep gradient so as to form a spike therebetween. This spike, however, is not essential because hot electrons can be formed also in the double-barrier structure which forms the resonance tunnel 330. When the spike is eliminated, the composition of the boundary between the layers 308 and 310 is progressively changed so as to provide a graded layer. Although the described eighth and ninth embodiments make use of a buffer layer constituted by super-grid layer, this is only illustrative and the buffer layer may be an extremely thin buffer layer grown on the Si substrate 301 at a low temperature (GaAs/GaAs buffer layer (20nm (200 Å))/Si system). It is also to be understood that, although the described embodiments utilize GaAs which is one of semiconductors of III-V compounds of elements belonging to groups III and V, respectively, such a material is not exclusive and various other III-V-compound materials can be used equally well. Also SiC/Si type materials can be used equally well; such use of SiC/Si type materials is not an embodiment of the invention. A reference example of construction of the solid-state electron beam generator which incorporateS such material is shown in Table 4 below. SiC/Si type substrate Si or SiC Growth method Gaseous or liquid phase growth n-type region N⁺-type SiC N-type SiC p-type region p-type Si n-type dopant N(≃ 10²⁰ cm⁻³) p-type dopant Aℓ (10¹⁸ cm⁻³) n-type electrode Au, Au-Ta(99:1) p-type electrode Aℓ-Si (89:11) As will be understood from the foregoing description, the eighth to ninth embodiments of the present invention offer the following advantages. (1) Since two compound semiconductors have different band gap widths, the rate of injection of carrier from one semiconductor to another semiconductor is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers changed into hot electrons are directly emitted to the outside without propagating through the semiconductor. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The layers can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Problems concerning heat generation are not so severe because the Si substrate exhibits only small heat resistance. (4) The layered structure constitut'ing the operating portion has a simple structure so that the manufacture is facilitated. (5) Since the electron beam generator is produced by using an Si substrate, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. Furthermore, the electrons are changed into hot electrons by virtue of the spike caused by the hetero junction between the emitter region and the base region or a resonance tunnel in the base region, so that the efficiency of emission of electrons is further increased. Fig. 31 is a sectional view of a tenth embodiment of the solid-state electron beam generator of the present invention which employs an n-type or n⁺-type GaAs substrate 401. This embodiment has an N-type Aℓ x Ga (1-x) As layer 402 serving as an emitter. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. The embodiment further has an inert layer 403 which is formed by injecting oxygen into the N-type Aℓ x Ga (1-x) As layer 402. A reference numeral 404 designates a graded layer which is formed by progressively decreasing the crystal mixing ratio x of the Aℓ contained in the Aℓ X Ga (1-x) As layer which serves as the emitter layer 402. The embodiment further has a p-type GaAs layer 405 which serves as a base. The small-letter symbol "p" is used to designate a p-type region with a narrow band gap. In this embodiment, it is possible to add Aℓ such that the p-type GaAs layer 405 is substituted by a P-type Aℓ z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap of the layer 405. The embodiment further has an n-type GaAs layer 406 serving as a collector. The small-letter "n" is used here to designate an n-type region of a narrow band gap. The n-type GaAs layer 406 may be substituted by an n-type Aℓ t Ga (1-t) AS layer (0 < t ≦ 1). A numeral 407 denotes an n⁺-type GaAs layer for attaining an ohmic contact between the collector layer 406 and its electrode. A reference numeral 408 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the collector layer 406. This Cs-O layer 408 serves as an electron-emission surface. The Cs-O layer 408 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The solid-state electron beam generator further has an SiO₂ protection (insulation) layer 409, an emitter electrode 410, a base electrode 411, a collector electrode 412, and an external acceleration electrode 413 for accelerating electrons emitted from the surface of the collector layer 406. This embodiment is preferably produced by a process having the steps of: forming, on the n-type or n⁺-type GaAs substrate 401, the N-type AℓGaAs layer 402 by, for example, an MBE (Molecular Beam Epitaxy) device or an MOCVD (Metalorganic Chemical Vapor Deposition) device; injecting oxygen ions by an ion injector so as to form the inert region 403; successively conducting epitaxial growth of the graded layer 404, p-type GaAs layer 405, n-type GaAs layer 406 and n⁺-type GaAs layer 407; and forming a region where the base electrode 411 is to be deposited, by etching. Then, the SiO₂ protection layer and electrodes 410 to 412 are formed followed by formation of the Cs-O diffusion layer 408, thus completing the production. The electrodes 410, 412 for n-type GaAs may be formed of a composition,such as Au-Ge or Au-Ge-Ni, while the electrode 411 for the p-type semiconductor is preferably formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. The operation of this embodiment will be explained with reference to energy band diagram shown in Fig. 32. In this diagram, the full-line curve shows the energy level [eV] in the thermally equilibrium state of the electron beam generator, while broken-line curve shows the energy level [eV] in the state where a bias voltage is applied. As explained before, the emitter 402 is formed of, for example, a Aℓ x GA (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the base. The crystal mixing ratio x of Aℓ is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 402 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base layer 405. This high level of doping causes the state of the layer 402 to be changed into degenerating state and the Fermi level is set above the conductive band. Since the electrode 410 for the emitter layer 402 is formed on the reverse side of the n-type GaAs substrate 401, it is preferred that the rate of doping is increased so as to minimize the voltage drop across the substrate 401. Since the graded layer 404 is formed between the emitter layer 402 and the base layer 405, the crystal mixing ratio x of Aℓ is progressively decreased and reaches zero at the boundary on the base layer 405. As shown in Fig. 32, no spike is formed in the hetero junction between the emitter layer 402 and the base layer 405, by virtue of the provision of the graded layer 404. The elimination of the spike, which usually acts as a barrier, enables a large number of carriers to be injected into the base layer 405, thus assuring a high injection efficiency. Referring now to the base layer 405, this layer 405 is formed of a p-type GaAs layer having a narrow band gap, in order to ensure a high efficiency of injection of current into the base layer 405. The amount of dope in this p-type GaAs layer 405 selected to be in the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the base layer 405 is selected to be about 30nm (300 Å) so as to reduce scattering in this layer 405. The n-type GaAs collector layer 406 and the n⁺-type GaAs layer 407 are made to grow on the p-type GaAs base layer 405. Cs-O is diffused or deposited on the surface of the n⁺-type GaAs layer 407 so that the surface of the collector layer 406 exhibits a work function which is as small as 1.4 eV. As stated before, the Cs-O used as the material for reducing,the work function may be substituted by another material which contains an alkali metal other than Cs, one element selected from the group consisting of Sb, Bi, Se, As, P, Te, Cu, Ag, and Au, and oxygen. In order to attain an ohmic contact between the collector layer 406 and the collector electrode 412 and to reduce the level of the ohmic contact, the collector layer 406 is doped at a high rate of 1 x 10¹⁸/cm ³. The doping rate of the n⁺-type GaAs layer 407 is in the order of 1 x 10¹⁹/cm⁻³. In this embodiment, the n-type GaAs layer 406 and the n⁺-type GaAs layer 407 are formed to have a total thickness of 100nm (1000 Å). This thickness, however, is only illustrative. Namely, this total thickness is preferably reduced provided that a good ohmic contact is maintained between the electrode and these layers. It is possible to form these layers in high quality and uniformity by growing them using an MBE device or an MOCVD device. The operation under application of the bias voltage is as follows (See broken-line curve in Fig. 32). A forward bias voltage is applied between the emitter and the base, while a backward bias voltage is applied between the base and the collector. At the same time, a bias voltage which is positive with respect to the collector is applied to the external accelerating electrode 413. In consequence, the carriers (electrons) injected from the emitter into the base are accelerated by the electric field formed between the base and the collector and are emitted through the surface in which the material for reducing the work function, e.g., Cs-O, is diffused. The emitted electrons are further accelerated by the external electric field formed by the accelerating electrode 413 so as to have greater kinetic energy. In this embodiment, there is no spike nor other barrier between the emitter layer 402 and the base layer 405, because of the provision of the graded layer 404 between these layers. In consequence, the rate of injection of carriers from the emitter layer 402 into the base layer 405 is increased. In addition, a large number of carriers are accelerated by the backward bias between the base and the collector. In consequence, the efficiency of emission of electrons is remarkably increased. Fig. 33 is a sectional view of an eleventh embodiment which makes use of a semi-insulating substrate 421. This embodiment is formed by injecting elements similar to those used in the tenth embodiment shown in Fig. 31 by ion injection technique. Referring to Fig. 33, a numeral 421 denotes a semi-insulating GaAs substrate, 422 denotes an n⁺-GaAs layer for attaining an ohmic contact between the emitter electrode 410 and the emitter layer 402 formed of N-type Aℓ x Ga (1-x) As layer (0 < x ≦ 1), 404 denotes a graded layer in which the crystal mixing ratio of Aℓ is progressively decreased as the distance from the emitter layer 402 is increased, 405 denotes a p-type GaAs base layer, 406 denotes an n-type GaAs collector layer, 407 denotes an n⁺-type GaAs layer for attaining good ohmic contact between the collector layer 406 and a collector electrode 412, and 408 denotes a layer having diffused or deposited material such as Cs-O for reducing the work function. This embodiment can be produced, for example, by the following process. The n⁺-type GaAs layer 422, N-type Aℓ x Ga (1-x) As layer 402, graded layer 404, p-type GaAs layer 405, n-type GaAs layer 406 and the n⁺-type GaAs layer 407 are successively formed on the semi-insulating substrate 421. Then, Be ions are injected into the portion of the p-type GaAs base where the electrode is to be formed so as to form a p⁺-type region 423. At the same time B ions are injected to form a region 424 which serves to insulate the base and emitter from each other and to isolate the device. Then, an SiO₂ protection layer 409 is formed and the collector electrode 412 and the base electrode 411 are formed. The laminated structure is locally recessed to expose the n⁺-type GaAs layer 422 and the recess is filled with a material such as Au-Ge/Au thus forming the emitter electrode 410. Finally, the external acceleration electrode 413 is formed and Cs-O is diffused, thus completing the production process. This eleventh embodiment is advantageous over the tenth embodiment in that troublesome works such as etching down to the p-type GaAs base layer 405 (see Fig. 31) are eliminated and in that the device can have a flat surface. The principle of operation of this eleventh embodiment is not described because it is materially the same as that of the tenth embodiment. Although the described tenth and eleventh embodiments make use of GaAs which is one of semiconductors of compounds of elements belonging to groups III to V, such a material is not exclusive and various other materials such as InGaAsP/InP type materials can be used equally well. Examples of construction of the solid-state electron beam generator of the invention which incorporate such a material are shown in Table 5 below. InGaAsP/InP type Substrate InP Growth method Liquid phase growth Emitter N⁺-type InP N-type InP Graded layer In n-type In x Ga (1-x) AsyP (1-y) , the crystal mixing ratio y is progressively increased from 0 while the crystal mixing ratio x is progressively decreased from 1 so as to provide smooth gradient in base layer band gap Base p-type InGaAsP Collector n-type InGaAsP n-type dopant Te (≃ 2 x 10¹⁹ cm⁻³) p-type dopant Cd or Mg (≃ 5 x 10¹⁸ cm⁻³) n-type electrode Au, Au-Ti, Pt, Sn p-type electrode For InP Au, Ni, Cu For InGaAsP Au, Ag As will be understood from the foregoing description, the tenth to eleventh embodiments of the present invention offer the following advantages. (1) Since the emitter and the base have different band gap widths, and since a graded layer is disposed therebetween, the rate of injection of carrier from the emitter to the base is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers injected into the base are accelerated by the electric field so as to have greater kinetic energy. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Since the electron beam generator is produced from semiconductor materials, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. In particular, the eleventh embodiment offers advantages such as elimination of complicated process such as etching, flat surface of the produced device, and increase in the integration scale by forming this device together with other devices on the same substrate. Fig. 34 is a sectional view of a twelfth embodiment of the solid-state electron beam generator of the present invention which employs an n-type or n⁺-type GaAs substrate 501. This embodiment has an N-type Aℓ x Ga (1-x) As layer 502 serving as a source of carriers for supplying carriers. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. The embodiment further has an inert layer 503 which is formed by injecting oxygen into the N-type Aℓ x Ga (1-x) As layer 502. A reference numeral 504 designates a graded layer which is formed by progressively decreasing the crystal mixing ratio x of the Aℓ contained in the Aℓ x Ga (1-x) As layer 502. The embodiment further has a p-type GaAs layer 505. The small-letter symbol "p" is used to designate a p-type region with a narrow band gap. In this embodiment, it is possible to add Aℓ such that the p-type GaAs layer 505 is substituted by a p-type Aℓ z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap of the layer 505. A reference numeral 508 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the p-type GaAs layer 505. This Cs-O layer 508 serves as an electron-emission surface. The Cs-O layer 508 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The solid-state electron beam generator further has an SiO₂ protection (insulation) layer 509, electrodes 510, 511 for applying bias voltage, and an external acceleration electrode 513 for accelerating emitted electrons. A reference numeral 514 denotes a p⁺-type GaAs layer for attaining an ohmic contact between the electrode 511 and the associated layer 505. This embodiment is preferably produced by a process having the steps of: forming, on the n-type GaAs substrate 501, the N-type AℓGaAs layer 502 by, for example, an MBE (Molecular Beam Epitaxy) device or an MOCVD (Metalorganic Chemical Vapor Deposition) device; injecting oxygen ions by an ion injector so as to form the inert region 503; successively conducting epitaxial growth of the graded layer 504 and the p-type GaAs layer 505. Then, the SiO₂ protection layer 509 and electrodes 510, 511 are formed followed by formation of the Cs-O diffusion layer 508, thus completing the production. The electrode 510 for n-type GaAs may be formed of a composition such as, Au-Ge or Au-Ge-Ni, while the electrode 511 for the p-type GaAs is preferably formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. The operation of this embodiment will be explained with reference to energy band diagram shown in Fig. 35. In this diagram, the full-line curve shows the energy level [eV] in the thermally equilibrium state of the electron beam generator, while broken-line curve shows the energy level [eV] in the state where a bias voltage is applied. As explained before, the layer 502 is formed of, for example, a Aℓ x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of carriers into the layer 505. The crystal mixing ratio x of Aℓ is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 502 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base layer 505. This high level of doping causes the state of the layer 502 to be changed into degenerating state and the Fermi level is set above the conductive band. Since the electrode 510 for the emitter layer 502 is formed on the reverse side of the n-type GaAs substrate 501, it is preferred that the rate of doping is increased so as to minimize the voltage drop across the substrate 501. Since the graded layer 504 is formed between the layer 502 and the layer 505, the crystal mixing ratio x of Aℓ is progressively decreased and reaches zero at the boundary of the layer 505. As shown in Fig. 35, no spike is formed in the hetero junction between the n-type Al x Ga 1-x As layer 502 and the p-type GaAs layer 505, by virtue of the provision of the graded layer 504. The elimination of the spike, which usually acts as a barrier, enables a large number of carriers to be injected into the layer 505, thus assuring a high injection efficiency. Referring now to the layer 505, this layer 505 is formed from a p-type GaAs layer having a narrow band gap. The amount of dope in this p-type GaAs layer 505 is selected to be in the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the layer 505 is selected to be about 30nm (300 Å) so as to reduce scattering in this layer 505. Cs-O is diffused or deposited on the surface of the p-type GaAs layer 505 so that the surface of the layer 505 exhibits a work function which is as small as 1.4 eV. As stated before, the Cs-O used as the material for reducing the work function may be substituted by another material which contains an alkali metal other than Cs, one element selected from the group consisting of Sb, Bi, Se, As, P, Te, Cu, Ag, Au, Si and O. The operation under application of the bias voltage is as follows (See broken-line curve in Fig.35). A forward bias voltage is applied between the electrodes 510 and 511, while a voltage which is positive with respect to the electrode 511 is applied to the external accelerating electrode 513. As a result, the band of the p-type GaAs layer 505 is deflected downward as shown in Fig. 35, because the p-type GaAs layer 505 with Cs-O diffused thereon exhibits a work function of 1.4 eV while the electronic affinity of the p-type GaAs layer is 4.07 eV. Since the p-type GaAs layer 505 is in the highly doped state, the valence band and the Fermi level substantially coincide with each other. In addition, the band gap of GaAs is 1.428 eV which is greater than the work function (1.4 eV) of the surface having diffused Cs-O. Therefore, the carriers (electrons) of low energy injected from the N-type AℓGaAs layer 502 into the p-type GaAs layer 505 drop into the valley V which is formed in the vicinity of the surface as shown in Fig. 35. However, the absolute value of the number of the carriers injected into the layer 505 is increased by virtue of provision of the graded layer 504, so that the level of the current emitted also is increased correspondingly. The application of the external electric field by the external acceleration electrode 513 causes the vacuum level to be deflected downward as shown in Fig. 35, so that the emitted electrons are further accelerated by this electric field. In consequence, the carriers (electrons) injected from the n-type Al x Ga 1-x As layer into the p-type GaAs layer are accelerated by the electric field formed between the p-type GaAs layer and the external acceleration electrode 513 and are emitted through the surface in which the material for reducing the work function, e.g., Cs-O, is diffused. The emitted electrons are further accelerated by the external electric field formed by the accelerating electrode 513 so as to have greater kinetic energy. In this embodiment, there is no spike nor other barrier between the n-type Al x Ga 1-x As layer 502 and the p-type GaAs layer 505, because of the provision of the graded layer 504 between these layers. In consequence, the rate of injection of carrier from the n-type Al x Ga 1-x As layer 502 into the p-type GaAs layer 505 is increased. In addition, a large number of carriers are accelerated by the external electric field between the p-type GaAs layer and the external acceleration electrode 513. In consequence, the efficiency of emission of electrons is remarkably increased. Fig. 36 is a sectional view of an thirteenth embodiment which makes use of a semi-insulating substrate 521. This embodiment is formed by injecting elements similar to those used in the twelfth embodiment shown in Fig. 34 by ion injection technique. Referring to Fig. 36, a numeral 521 denotes a semi-insulating GaAs substrate, 522 denotes an n⁺-GaAs layer for attaining an ohmic contact with the electrode 510, a layer 502 formed of N-type Aℓ x Ga (1-x) As layer (0 < x ≦ 1), 504 denotes a graded layer in which the crystal mixing ratio of Aℓ is progressively decreased as the distance from the layer 502 is increased, 505 denotes a p-type GaAs base layer, and 508 denotes a layer having diffused or deposited material such as Cs-O for reducing the work function. This embodiment can be produced, for example, by the following process. The n⁺-type GaAs layer 522, N-type Aℓ x Ga (1-x) As layer 502, graded layer 504, and p-type GaAs layer 505 are successively formed on the semi-insulating substrate 521. Then, B ions are injected to form a region 524 which serves to insulate the base and emitter from each other and to isolate the device. Then, an SiO₂ protection layer 509 is formed and the electrode 511 is formed. The laminated structure is locally recessed to expose the n⁺-type GaAs layer 522 and the recess is filled with a material such as Au-Ge/Au thus forming the other electrode 510. Finally, the external acceleration electrode 513 is formed and Cs-O is diffused, thus completing the production process. This thirteenth embodiment is advantageous over the twelfth embodiment in that troublesome works such as etching down to the p-type GaAs base layer 505 (see Fig. 31) are eliminated and in that the device can have a flat surface. The principle of operation of this thirteenth embodiment is not described because it is materially the same as that of the twelfth embodiment. Thus, the thirteenth embodiment proposes a planar-type construction which makes it easy to produce a multiple-type device in which a plurality of devices are arranged on a common plane. Although the described twelfth and thirteenth embodiments make use of GaAs which is one of semiconductors compounds of elements belonging to groups III to V, such a material is not exclusive and various other materials such as InGaAsP/InP type materials can be used equally well. Examples of construction of the solid-state electron beam generator of the invention which incorporate such a material are shown in Table 6 below. InGaAsP/InP type Substrate InP Growth method liquid phase growth N-type region N⁺ type InP N type InP Graded layer In n-type In x Ga (1-x) AsyP (1-y) , the crystal mixing ratio y is progressively increased from 0 while the crystal mixing ratio x is progressively decreased from 1 so as to provide smooth gradient in base layer band gap Bp-type region p-type InGaAsP n-type dopant Te (≃ 2 x 10¹⁹ cm⁻³) p-type dopant Cd or Mg (≃ 5 x 10¹⁸ cm⁻³) n-type electrode Au, Au-Ti, Pt, Sn p-type electrode For InP Au, Ni, Cu For InGaAsP Au, Ag As will be understood from the foregoing description, the twelfth to thirteenth embodiments of the present invention offer the following advantages. (1) Since two compounds have different band gap widths, and since a graded layer is provided between these compounds, the rate of injection of carrier from the one to the other compound semiconductor is remarkably increased. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometers (several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Since the electron beam generator is produced from semiconductor materials, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. In particular, the thirteenth embodiment offers advantages such as elimination of complicated process such as etching, flat surface of the produced device, and increase in the integration scale by forming this device together with other devices on the same substrate. Fig. 37 is a sectional view of a fourteenth embodiment of the solid-state electron beam generator of the present invention. In this embodiment, an AℓP layer 602 and an AℓGaP layer 603 are made to grow on an Si substrate 601 by MOCVD (Metalorganic Chemical Vapor Deposition) method and then a super-grid layer 604 of GaP and GaAsP and a super-grid layer 605 of GaAsP and GaAs are formed. Then, a GaAs layer 606 is made to grow on these super-grid layers. Subsequently, an n⁺-type GaAs layer 607 and an N-type Aℓ x Ga (1-x) AS layer 608 (0 < x ≦ 1) are made to grow. Oxygen ions are injected by an ion injector into the Aℓ x Ga (1-x) As layer 608 so as to form inert layer 609 in the regions of this layer 608 other than the electron beam generating region. On the N-type Aℓ x Ga (1-x) As layer 608 is formed a graded layer 620 in which the crystal mixing ratio x of Aℓ is progressively decreased towards the GaAs. A p-type GaAs layer 610 and an n-type GaAs layer 611 are formed on the graded layer 620. A layer 612 of material for reducing work function, e.g., caesium oxide (Cs-O) is formed by deposition or diffusion on the surface of the n-type GaAs layer 611. The construction will be explained in more detail hereinunder. As mentioned above, this embodiment incorporates an N-type Aℓ x Ga (1-x) As layer 608 serving as a emitter. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. A numeral 609 represents an inert layer formed by injecting oxygen ions into the N-type Aℓ x Ga (1-x) As layer 608. The embodiment further has a p-type GaAs layer 610 which serves as a base. The small-letter symbol "p" is used to designate a p-type region with a narrow band gap. In this embodiment, it is possible to add Aℓ such that the p-type GaAs layer 610 is substituted by a p-type Aℓ z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap. The embodiment further has an n-type GaAs layer 611 serving as a collector. The small-letter "n" is used here to designate an n-type region of a narrow band gap. The n-type GaAs layer 611 may be substituted by an n-type Aℓ t Ga (1-t) As layer (0 < t ≦ 1). A reference numeral 612 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the collector layer 611. This Cs-O layer 612 serves as an electron-emission surface. The Cs-O layer 612 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. Numerals 613, 614 and 615 denote, respectively, the electrodes for the emitter, base and the collector. Electrodes for n- or N-type semiconductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while the electrode for the p-type semiconductor may be formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode 614 of the p-type GaAs is formed directly on the surface of the p-type GaAs 610 layer. This, however, is not exclusive and the electrode 614 may be formed after doping the surface of this GaAs layer 610 with Be ions so as to form a p⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer 610 surface, Thus, in this fourteenth embodiment, an Npn-type epitaxial film of GaAs-Aℓ x Ga (1-x) As system has grown on the Si substrate 601. The operation of this embodiment will be described with reference to Fig. 38 which is an energy band diagram. In this diagram, the full-line curve shows the energy level [eV] in the thermally equilibrium state of the electron beam generator, while broken-line curve shows the energy level [eV] in the state where a bias voltage is applied. As explained before, the emitter layer 608 is formed of, for example, a Aℓ x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of current into the base. The crystal mixing ratio x of Aℓ is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 608 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base layer 610. This high level of doping causes the state of the layer 608 to be changed into degenerating state and the Fermi level is set above the conductive band. Since the graded layer 604 is formed between the emitter layer 608 and the base layer 610, the crystal mixing ratio x of Aℓ is progressively decreased and reaches zero at the boundary to the base layer 610. As shown in Fig. 38, no spike is formed in the hetero junction between the emitter layer 608 and the base layer 610, by virtue of the provision of the graded layer 604. The elimination of the spike, which usually acts as a barrier, enables a large number of carriers to be injected into the base layer 610, thus assuring a high injection efficiency. Referring now to the base layer 610, this layer 610 is formed of a p-type GaAs layer having a narrow band gap. The amount of dope in this p-type GaAs layer 610 is selected to be on the order of 5 x 10¹⁸ cm⁻³ so as to reduce the resistance, and the thickness of the base layer 610 is selected to be about 30nm (300 Å) so as to reduce scattering in this layer 610. The n-type GaAs collector layer 611 is made to grow on the p-type GaAs base layer 610. Cs-O is diffused or deposited on the surface of the n-type GaAs layer 611 so that the surface of the collector layer 611 exhibits a work function which is as small as 1.4 eV. As stated before, the Cs-O used as the material for reducing the work function may be substituted by another material which contains an alkali metal other than Cs, one element selected from the group consisting of Sb, Bi, Se, As, P, Te, Cu, Ag, Au, Si and O. In order to attain an ohmic contact between the collector layer 611 and the collector electrode 615 and to reduce the level of the ohmic contact, the collector layer 611 is doped at a high rate of 1 x 10¹⁸/cm⁻³. In this embodiment, the collector layer 611 has a thickness of 100nm (1000 Å). This thickness, however, is only illustrative. Namely, this thickness is preferably reduced provided that a good ohmic contact is maintained between the collector layer 611 and the collector electrode 615. It is possible to form these layers in high quality and uniformity by growing them using an MBE device or an MOCVD device. The operation under application of the bias voltage is as follows (See broken-line curve in Fig. 38). A forward bias voltage is applied between the emitter and the base, while a backward bias voltage is applied between the base and the collector. At the same time, a bias voltage which is positive with respect to the collector is applied to the external accelerating electrode (not shown). In consequence, the carriers (electrons) injected from the emitter into the base are accelerated by the electric field formed between the base and the collector and are emitted through the surface in which the material for reducing the work function, e.g., Cs-O, is diffused. The emitted electrons are further accelerated by the external electric field formed by the accelerating electrode so as to have greater kinetic energy. In this embodiment, there is no spike nor other barrier between the emitter layer 608 and the base layer 610, because of the provision of the graded layer 604 between these layers. In consequence, the rate of injection of carriers from the emitter layer 608 into the base layer 610 is increased. In addition, a large number of carriers are accelerated by the backward bias between the base and the collector. In consequence, the efficiency of emission of electrons is remarkably increased. Fig. 39 is a sectional view of a fifteenth embodiment which makes use of a semi-insulating substrate 630. This embodiment is formed by injecting elements similar to those used in the fourteenth embodiment shown in Fig. 37 by ion injection technique. Referring to Fig. 39, a numeral 630 denotes an Si substrate, 632 denotes an AℓP layer, 634 denotes an AℓGaP layer, 636 denotes a super-grid layer of GaP and GaAsP, 638 denotes a super-grid layer of GaAsP and GaAs, and 640 denotes a GaAs layer. This structure is substantially the same as that in the fourteenth embodiment shown in Fig. 37. A numeral 642 denotes an n⁺-GaAs layer for attaining an ohmic contact between the emitter electrode 644 and the emitter layer 646 formed of N-type Aℓ x Ga (1-x) As layer (0 < x ≦ 1), 648 denotes a graded layer in which the crystal mixing ratio x of Aℓ is progressively decreased as the distance from the emitter layer 646 is increased, 650 denotes a p-type GaAs base layer, 652 denotes an n-type GaAs collector layer, 654 denotes an n⁺-type GaAs layer for attaining good ohmic contact between the collector layer 652 and a collector electrode 656, and 658 denotes a layer having diffused or deposited material such as Cs-O for reducing the work function. Numerals 666 and 662 denotes, respectively, a base electrode and an external acceleration electrode. This embodiment can be produced, for example, by the following process. After forming the n⁺-type layer 654, a p⁺-type region 664 is formed by injecting Be ions into the electrode-forming portion of the p-type GaAs (base). At the same time, a region 668 is formed by injecting B ions for the purpose of insulation between the base and the emitter and the isolation of the device. Then, an SiO₂ protection layer 660 is formed and the collector electrode 656 and the base electrode 666 are formed. The laminated structure is locally recessed to expose the n⁺-type GaAs layer 642 and the recess is filled with a material such as Au-Ge/Au thus forming the emitter electrode 644. Finally, Cs-O is diffused or deposited, thus completing the production process. This fifteenth embodiment is advantageous over the fourteenth embodiment in that troublesome works such as etching down to the p-type GaAs base layer 650 (see Fig. 37) are eliminated and in that the device can have a flat surface. The principle of operation of this fifteenth embodiment is not described because it is materially the same as that of the fourteenth embodiment. Thus, the fourteenth embodiment offers a planar type structure which makes easy to produce a multiple device having a multiplicity of devices formed on a common plane. Although the described fourteenth and fifteenth embodiments make use of a buffer layer incorporating a super-grid layer, this is not exclusive and these embodiments may instead incorporate an extremely thin buffer layer (GaAs/GaAs buffer layer ((20nm (200 Å))/Si system) which is made to grow on the Si substrate at a low temperature. As will be understood from the foregoing description, the fourteenth to fifteenth embodiments of the present invention offer the following advantages. (1) Since the emitter and the base have different band gap widths, and since a graded layer is disposed therebetween, the rate of injection of carrier from the emitter to the base is remarkably increased as compared with the case where the band gap width is equal. In addition, the carriers injected into the base are accelerated by the electric field so as to have greater kinetic energy. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The emitter region and the base region can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom (Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Problems concerning heat generation are not so serious because of the use of the Si substrate which exhibits a small heat resistance. (4) Since the electron beam generator is produced from semiconductor materials, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. In particular, the fifteenth embodiment offers advantages such as elimination of complicated process such as etching, flat surface of the produced device, and increase in the integration scale by forming this device together with other devices on the same substrate. Fig. 40 is a sectional view of a sixteenth embodiment of the solid-state electron beam generator of the present invention. In this embodiment, an AℓP layer 702 and an AℓGaP layer 703 are made to grow on an Si substrate 701 by MOCVD (Metalorganic Chemical Vapor Deposition) method and then a super-grid layer 704 of GaP and GaAsP and a super-grid layer 705 of GaAsP and GaAs are formed. Then, a GaAs layer 706 is made to grow on these super-grid layers. Subsequently, an n⁺-type GaAs layer 707 and an N-type Aℓ x Ga (1-x) As layer 708 (0 < x ≦ 1) are made to grow. Oxygen ions are injected into the Aℓ x Ga (1-x) As layer 708 so as to form an inert layer 709 in the regions of this layer 708 other than the electron beam generating region. On the N-type Aℓ x Ga( 1-x )As layer 708 is formed a graded layer 720 in which the crystal mixing ratio x of Aℓ is progressively decreased towards the GaAs. A p-type GaAs layer 710 is formed on the graded layer 720. A layer 712 of material for reducing work function is formed by deposition or diffusion on the surface of the p-type GaAs layer 710. An external acceleration electrode 715 is formed on the p-type GaAs layer 710 through the intermediary of an SiO₂ insulating layer 711. Then, electrodes 713 and 714 are formed on the n⁺-type GaAs layer 707 and on the p-type GaAs layer 710, respectively. The construction will be explained in more detail hereinunder. As mentioned above, this embodiment incorporates an N-type Aℓ x Ga (1-x) As layer 708 serving as a source for supplying carriers. The symbol x represents the crystal mixing ratio which is selected to meet the condition of 0 < x ≦ 1. The capital-letter symbol N represents an n-type region having a wide band gap. A numeral 709 represents an inert layer formed by injecting oxygen ions into the N-type Aℓ x Ga (1-x) AS layer 708. The embodiment further has the p-type GaAs layer 710. The small-letter symbol "p" is used to designate a p-type region with a narrow band gap. In this embodiment, it is possible to add Aℓ such that the p-type GaAs layer 710 is substituted by a p-type Aℓ z Ga (1-z) As layer (0 < z < x), thereby allowing a control of the band gap. A reference numeral 712 designates a caesium oxide (Cs-O) layer formed by deposition or diffusion on the surface of the layer 710. This Cs-O layer 712 serves as an electron-emission surface. The Cs-O layer 712 may be substituted by another type of layer formed by deposition or diffusion from a material containing an alkali metal such as Cs and at least one element selected from the group consisting of Cu, Ag, Au, Sb, Bi, Se, As, P, Te, Si and O. The electrode 713 for N-type semiconductor may be formed of a composition such as Au-Ge or Au-Ge-Ni, while the electrode 714 for the p-type semiconductor may be formed of Au-Sn, Ag-Zn, Au-Be or Au-Zn. In the illustrated embodiment, the electrode 714 of the p-type GaAs layer 710 is formed directly on the surface of the p-type GaAs layer 710. This, however, is not exclusive and the electrode 714 may be formed after doping the surface of this GaAs layer 710 with Be ions so as to form a p⁺-type region or may be formed on a p⁺-type GaAs layer grown on the surface of the p-type GaAs layer 710 surface. Thus, in this sixteenth embodiment, an epitaxial film of GaAs-Aℓ x Ga (1-x) As system is grown on the Si substrate 701. The operation of this embodiment will be explained with reference to energy band diagram shown in Fig. 41. In this diagram, the full-line curve shows the energy level [eV] in the thermally equilibrium state of the electron beam generator, while broken-line curve shows the energy level [eV] in the state where a bias voltage is applied. As explained before, the layer 708 is formed of, for example, a Aℓ x Ga (1-x) As layer which has a wide band gap so as to ensure high efficiency of injection of carriers into the layer 710. The crystal mixing ratio x of Aℓ is selected to be x = 0.3 for attaining a good hetero junction and considering also influences of the L-band and X-band. This value of the crystal mixing ratio, however, is only illustrative. The doping rate of the emitter layer 708 is as high as 5 x 10¹⁷ to 1 x 10¹⁹ cm⁻³ so as to allow a large number of carriers to be injected into the base layer 710. This high level of doping causes the state of the layer to be changed into degenerating state and the Fermi level is set above the conductive band. Since the graded layer 720 is formed between the layer 708 and the layer 710, the crystal mixing ratio x of Aℓ is progressively decreased and reaches zero at the boundary to the layer 710. As shown in Fig. 41, no spike is formed in the hetero junction between the layer 708 and the layer 710, by virtue of the provision of the graded layer 720. The elimination of the spike, which usually acts as a barrier, enables a large number of carriers to be injected into the layer 710, thus assuring a high injection efficiency. Referring now to the layer 710, this layer 710 is formed from a p-type GaAs layer having a narrow band gap. The amount of dope in this p-type GaAs layer 710 is selected to be on the order of 5 x 10¹⁸ cm⁻³ so as to reduce resistance, and the thickness of the layer 710 is selected to be about 30nm (300 Å) so as to reduce scattering in this layer 710. Cs-O is diffused or deposited on the surface of the p-type GaAs layer 710 so that the surface of the layer 710 exhibits a work function which is as small as 1.4 eV. As stated before, the Cs-O used as the material for reducing the work function may be substituted by another material which contains an alkali metal other than Cs, one element selected from the group consisting of Sb, Bi, Se, As, P, Te, Cu, Ag, Au, Si and O. These layers can be formed in high quality and uniformity by an MBE device or an MOCVD device. The operation under application of the bias voltage is as follows (See broken-line curve in Fig. 41). A forward bias voltage is applied between the electrodes 713 and 714, while a bias voltage which is positive with respect to the electrode 714 is applied to the external accelerating electrode 715. As a result, the band of the p-type GaAs layer 710 is deflected downward as shown in Fig. 41, because the p-type GaAs layer with Cs-O diffused thereon exhibits a work function of 1.4 eV while the electronic affinity of the p-type GaAs layer is 4.07 eV. Since the p-type GaAs layer 710 is in the highly doped state, the valence band and the Fermi level substantially coincide with each other. In addition, the band gap of GaAs is 1.428 eV which is greater than the work function (1.4 eV) of the surface having diffused Cs-O. Therefore, the carriers (electrons) of low energy injected from the N-type AℓGaAs layer 708 into the p-type GaAs layer 710 drop into the valley V which is formed in the vicinity of the surface as shown in Fig. 41. However, the absolute value of the number of the carriers injected into the layer 710 is increased by virtue of provision of the graded layer 720, so that the level of the current emitted also is increased correspondingly. The application of the external electric field by the external acceleration electrode 715 causes the vacuum level to be deflected downward as shown in Fig. 41, so that the emitted electrons are further accelerated by this electric field. Fig. 42 is a sectional view of a seventeenth embodiment which makes use of a Si substrate 730. This embodiment is formed by injecting elements similar to those used in the sixteenth embodiment shown in Fig. 41 by ion injection technique. Referring to Fig. 42, a numeral 730 denotes an Si substrate, 732 denotes an AℓP layer, 734 denotes an AℓGaP layer, 736 denotes a super-grid layer of GaP and GaAsP, 738 denotes a super-grid layer of GaAsP and GaAs, and 740 denotes a GaAs layer. These layers are substantially the same as those in the sixteenth embodiment shown in Fig. 40. A numeral 742 denotes an n⁺-GaAs layer for attaining an ohmic contact with the electrode 744, a numeral 746 denotes a layer formed of N-type Aℓ x Ga (1-x) As (0 < x ≦ 1), 748 denotes a graded layer in which the crystal mixing ratio of Aℓ is progressively decreased as the distance from the layer 746 is increased, 750 denotes a p-type GaAs base layer, and 758 denotes a layer having diffused or deposited material such as Cs-O for reducing the work function. Numerals 766 and 762 denote, respectively, a bias applying electrode and an external acceleration electrode. This embodiment can be produced, for example, by the following process. A p⁺-type region 764 is formed in the electrode-forming portion of the p-type GaAs by injecting Be ions. At the same time, a region 768 is formed by injecting B ions for the purpose of insulation between the layers 746 and 750 and isolation of the device. Then, the SiO₂ protection layer 760 is formed, followed by formation of the external acceleration electrode 762 and the electrode 766. Then, a hole is formed to reach the n⁺-type GaAs layer 742 and is filled with, for example, Au-Ge/Au, thus forming the electrode 744. Finally, the external acceleration electrode 762 is formed and Cs-O is diffused, thus completing the production process. This seventeenth embodiment is advantageous over the sixteenth embodiment in that troublesome works such as etching down to the p-type GaAs base layer 505 (see Fig. 31) are eliminated and in that the device can have a flat surface. The principle of operation of this seventeenth embodiment is not described because it is materially the same as that of the sixteenth embodiment. Thus, the seventeenth embodiment proposes a planar-type construction which makes it easy to produce a multiple-type device in which a plurality of devices are arranged on a common plane. Although the described sixteenth and seventeenth embodiments make use of GaAs which is one of a buffer layer incorporating a super-grid layer, this is not exclusive and the embodiments may employ an extremely thin buffer layer (GaAs/GaAs buffer layer ((20nm (200 Å))/Si system) grown on the Si substrate at a low temperature. As will be understood from the foregoing description, the sixteenth to seventeenth embodiments of the present invention offer the following advantages. (1) Since two compound semiconductors have different band gap widths, and since a graded layer is provided between these semiconductors, the rate of injection of carrier from one semiconductor to another semiconductor is remarkably increased. In consequence, the efficiency of emission of electrons is remarkably increased. (2) The layers can be formed as epitaxial films having thicknesses on the order of several nanometer (several tens of angstrom Å)), by making an efficient use of an MBE device or an MOCVD device. Thus, the layered structure of the device in accordance with the invention has high quality and uniformity. Since the thicknesses of layers can be reduced, it is possible to decrease the driving voltage. (3) Problems concerning heat generation is not so severe because the Si substrate exhibits only small heat resistance. (4) Since the electron beam generator is produced by using an Si substrate, it becomes easy to obtain a device having a plurality of electron beam generators on a common substrate or to couple the electron beam generator to other device or devices. This obviously contributes to an enlargement in the scale of integration of semiconductor devices. (5) The layered structure constituting the operating portion has a simple structure so that the manufacture is facilitated. In particular, the embodiment which makes use of ion injection offers advantages such as elimination of complicated works such as etching, flat surface of the product device and possibility of formation together with other devices on a common substrate so as to assure a larger scale of integration.

Claims: A solid-state electron beam generator having: a hetero bipolar transistor comprising an emitter region (2; 32; 208; 402; 608; 646) having a first band gap on a substrate, a base region (6; 35; 210; 405; 610; 650) having a second band gap narrower than said first band gap on said emitter region (2; 32; 208; 402; 608; 646), a base region electrode (16; 41; 214; 411; 614; 666) electrically connected to said base region (6; 35; 210; 405; 610; 650), a collector region (8; 64; 211; 406; 611; 652) on said base region (6; 35; 210; 405; 610; 650) with an electron emission surface (10; 38; 212; 408; 612; 658) on said collector region (8; 64; 211; 406; 611; 652), and a collector region electrode (18; 42; 215; 412; 615; 656) electrically connected to said collector region (8; 64; 211; 406; 611; 652), wherein electrons are to be injected from said emitter region (2; 32; 208; 402; 608; 646) into said base region (6; 35; 210; 405; 610; 650), a reverse bias source is connected between said base region electrode (16; 41; 214; 411; 614; 666) and said collector region electrode (18; 42; 215; 412; 615; 656), and said electrons are to be injected from said base region (6; 35; 210; 405; 610; 650) to said collector region (8; 64; 211; 406; 611; 652) by use of said reverse bias, thereby being injected into said electron emission surface (10; 38; 212; 408; 612; 658) to emit said electrons from said electron emission surface (10; 38; 212; 408; 612; 658) as the electron beam; and wherein a material is included in said electron emission surface (10; 38; 212; 408; 612; 658) on said collector region (8; 64; 211; 406; 611; 652) for reducing the workfunction of said electron emission surface (10; 38; 212; 408; 612; 658) with respect to the work function of said collector region (8; 64; 211; 406; 611; 652). A solid-state electron beam generator according to claim 1, wherein said solid-state beam generator is provided on a GaAs epitaxial film (206; 606; 640) on a Si substrate (201; 601; 630). A solid-state electron beam generator according to claim 1 or 2, further comprising a gradient region (404; 620; 648) wherein a mixed crystal ratio of predetermined materials is gradually changed between said emitter and base regions (402, 405; 608, 610; 646, 650). A solid-state electron beam generator according to claim 1, 2 or 3, wherein said solid-state electron beam generator is provided on a n- or n⁺-type GaAs substrate (22; 401), a semi-insulating GaAs substrate (26; 421), or a Si substrate (201; 601; 630), said emitter region (2; 32; 208; 402; 608; 646) is a n-type Al x Ga 1-x As layer (0<x<1) with a first band gap; said base region (6; 35; 210; 405; 610; 650) is a p-type Al z Ga 1-z As layer (0<z<x) with a second band gap; and said collector region (8; 64; 211; 406; 611; 652) is a n-type Al t Ga 1-t As layer (0<t<1). A solid-state electron beam generator according to claim 4, wherein said p-type Al z Ga 1-z As layer (0<z<x) constituting said base region (6; 210) is provided with a resonant tunnel section (30; 230) substantially composed of a barrier layer of non-doped Al y Ga 1-y As, a well layer of non-doped Al s Ga 1-s As, and a barrier layer of non-doped Al y Ga 1-y As (0<s<y<1). A solid-state electron beam generator according to claim 4, wherein said n-type Al x Ga 1-x As layer (0<x<1) (2; 32; 208; 402; 608) has a predetermined inactive area (4; 62; 209; 403; 609) including oxygen. A solid-state electron beam generator according to claim 4 when dependent on claim 3, wherein said gradient region (404; 620; 648) is substantially formed of Al x Ga 1-x As. A solid-state beam generator according to any of the preceding claims 1 to 7, wherein said material for reducing the workfunction is an alkali metal material. A method of emitting and using an electron beam from a hetero bipolar transistor, using as said hetero bipolar transistor a hetero bipolar transistor comprising an emitter region (2; 32; 208; 402; 608; 646) having a first band gap on a substrate, a base region (6; 35; 210; 405; 610; 650) having a second band gap narrower than said first band gap on said emitter region (2; 32; 208; 402; 608; 646), a base region electrode (16; 41; 214; 411; 614; 666) electrically connected to said base region (6; 35; 210; 405; 610; 650), a collector region (8; 64; 211; 406; 611; 652) on said base region (6; 35; 210; 405; 610; 650) with an electron emission surface (10; 38; 212; 408; 612; 658) on said collector region (8; 64; 211; 406; 611; 652), and a collector region electrode (18; 42; 215; 412; 615; 656) electrically connected to said collector region (8; 64; 211; 406; 611; 652), said method comprising the steps of: injecting electrons from said emitter region (2; 32; 208; 402; 608; 646) into said base region (6; 35; 210; 405; 610; 650); applying a reverse bias voltage between said base region electrode (16; 41; 214; 411; 614; 666) and said collector region electrode (18; 42; 215; 412; 615; 656); and injecting electrons from said base region (6; 35; 210; 405; 610; 650) to said collector region (8; 64; 211; 406; 611; 652) by use of said reverse bias, thereby injecting electrons into said electron emission surface (10; 38; 212; 408; 612; 658) and emitting said electrons from said electron emission surface (10; 38; 212; 408; 612; 658) as the electron beam. A solid-state electron beam generator comprising: a first region (502; 708; 746) with a first band gap; a second region (505; 710; 750) with a second band gap narrower than said first band gap forming a heterojunction with said first region (502; 708; 746); and a gradient region (504; 720; 748) wherein a mixed crystal ratio of predetermined material is gradually changed between said first and second regions (502, 505; 708, 710; 746, 750), and electrons are to be injected from said first region (502; 708; 746) into said second region (505; 710; 750), thereby emitting said electrons from an electron emission surface of said second region (505; 710; 750) as the electron beam directly to the outside. A solid-state electron beam generator according to claim 10, wherein said first region (502; 708; 746) is a n-type Al x Ga 1-x As layer (0<x<1) with a first band gap; and said second region (505; 710; 750) is a p-type Al z Ga 1-z As layer (0<z<x) with a second band gap. A solid-state electron beam generator according to claim 10, wherein an alkali metal material is included in said electron emission surface (508; 712; 758) of said second region (505; 710; 750). A solid-state electron beam generator according to claim 10, wherein said first region (502; 708) is a n-type Al x Ga 1-x As layer (0<x<1) and has a predetermined area (503; 709) including oxygen. A solid-state electron beam generator according to claim 10, wherein said gradient region (504; 720; 748) is an Al x Ga 1-x As layer. A solid-state electron beam generator comprising: a first region (308; 502; 708; 746) with a first band gap; and a second region (310; 505; 710; 750) with a second band gap narrower than said first band gap forming a heterojunction with said first region (308; 502; 708; 746) on an epitaxial film of III-V-compound semiconductor material on a Si substrate; wherein electrons are to be injected from said first region (308; 502; 708; 746) into said second region (310; 505; 710; 750), thereby emitting said electrons from an electron emission surface (312; 508; 712; 758) of said second region (310; 505; 710; 750) as the electron beam directly to the outside. A solid-state electron beam generator according to claim 15, wherein a gradient region (504; 720; 748) wherein a mixed crystal ratio of predetermined materials between said first and second regions (502, 505; 708, 710; 746, 750) is changed gradually is provided. A solid-state electron beam generator according to claim 16, wherein said gradient region (504; 720; 748) is a Al x Ga 1-x As layer. A solid-state electron beam generator according to claim 15, wherein a p-type Al z Ga 1-z As layer (0<z<x) constituting said second region (310) is provided with a resonant tunnel area (330) comprising a barrier layer of non-doped Al y Ga 1-y As, a well layer non-doped Al s Ga 1-s As, and a non-doped barrier layer of Al y Ga 1-y (0<s<y<1). A solid-state electron beam generator according to claim 15 or 16, wherein said first region (308; 502; 708; 746) is a n-type Al x Ga 1-x As layer (0<x<1) with a first band gap; and said second region (310; 505; 710; 750) is a p-type Al z Ga 1-z As layer (0<z<x) with a second band gap. A solid-state electron beam generator according to claim 15 or 16, wherein an alkali metal material is included in said electron emission surface (312; 508; 712; 758) of said second region (310; 505; 710; 750). A solid-state electron beam generator according to claim 15 or 16, wherein said first region (308; 502; 708) is a n-type Al x Ga 1-x As layer (0<x<1) and has a predetermined inactive region (309; 503; 709) including oxygen.

Inventors: MIYAWAKI MAMORU, MIYAWAKI, MAMORU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H01J1
IPC codes: H01J1