Electron emission element

Abstract: An electron emission element comprises a P-type semiconductor substrate (1) and electrodes (2, 3) formed on both ends of the semiconductor substrate. A voltage is applied between said electrodes. The P-type semiconductor substrate is irradiated with light to emit the electrons, generated in the P-type semiconductor substrate by photoexcitation, from an electron emitting face (4) at an end of the P-type semiconductor substrate.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electron emission device. Related Background Art There are already known various electron emission elements, such as an element utilizing an avalanche phenomenon of a PN junction, one utilizing electron injection into a P-layer of a PN junction under forward bias, one having a thin insulator sandwiched between metal layers (MIM structure), and ones utilizing electric field emission or surface conduction. From document H.-D., Junge "Lexikon Elektronik", 1st Edition , 1978, Physik-Verlag, Weinheim (DE), pages 217-218, headword "Fotodiode" photodiodes, especially PIN- and avalanche photodiodes, are known. Document EP-A-0 041 119 discloses an electron emission element with a semiconductor body and an electron confinement barrier wherein a small opening with low workfunction is provided. By means of light irradiation or electrical injection (application of an electric field between two electrodes being positioned at opposite sides of the semiconductor body) hole-electron pairs are generated. Recombination is only possible at the small opening. Therefore, due to a concentration gradient being generated due to emission of electrons through the small opening, electrons are swept in the direction of the opening. For effecting electron emission in a certain direction a grid electrode can be positioned opposite to the small opening. An electric field then is applied between the grid electrode and the upper electrode. Additionally, document US-A-3 872 489 discloses an electron emission element with a P-type semiconductor which is positioned on a positively biased contact. In this case electron emission is effected by means of an applied electric field. The electrons are emitted through an opening in the electrode. The emitted electrons can be directed to a positive potential outside the electron emission element. Furthermore, document FR-A-1 460 237 discloses an electron emission element, wherein a thin transparent metal layer is provided on a transparent substrate. On this thin transparent metal layer an insulating layer and a metal layer are provided. The two metal layers and the insulating layer provide a MIM structure. A voltage is applied between the two metal layers. Due to light irradiation on the transparent substrate electrons are generated which can tunnel through the MIM structure by means of the applied electric field. This electron emission element can be used within an electrostatic recording system. Document US-A-3 624 273 discloses an array of electron emission elements which are used in an flat display panel. The electron emission elements can consist of thin-film devices, solid state structures, thermally emitting cathode structures or field emission cathode structures. Due to a potential difference between two electrodes electron emission is effected. Due to the array structure it is possible to select a certain electron emission element by means of selecting the corresponding electrodes. Electron emission is only effected by means of application of an electric field to the electrodes. The electron emission elements activate phosphor elements of the display screen. Moreover, from document US-A-3 119 947 an electron emission element is known which comprises three contiguous layers of semiconductive material, wherein adjacent layers of opposite conductivity type form two rectifying junctions. A voltage is applied to the two layers of the same conductivity type. Electron emission is effected by means of the applied electric field. Furthermore, document FR-A-2 235 496 discloses an electron emission element including a hetero junction. Electrons are emitted due to application of a voltage. It is also possible to effect emission of electrons by means of irradiation with light. The electron emission becomes easier due to a layer with reduced workfunction which is provided on the electron emission surface. Finally, document GB-A-1 023 257 discloses an electron emission element which emits electrons in response to incident irradiation with light. In this element a MIM structure is used. A voltage is applied to the MIM structure and, therefore, the electrons can tunnel through the MIM structure. Due to the use of a photoconductor positioned between the metal layer of the MIM structure and the insulating layer, electron emission is only possible when the electron emission element is irradiated with light and, therefore, the resistance of the photoconductive layer decreases. However, in these known elements, in which electrons of high energy are electrically generated and emitted, the electron emission can only be controlled electrically, and it has not been possible to correlate the electron emission with light. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide an electron emission element allowing electron emission control with light. According to the invention this object is accomplished by an electron emission device comprising: a plurality of electron emission elements and a plurality-of shutter elements corresponding thereto; each of said plurality of electron emission elements comprising a P-type semiconductor area and electrodes formed on both ends of said electron emission element in the direction of incident light radiation; wherein a voltage is to be applied between said electrodes for accelerating electrons which are excited to the conduction band by the irradiation of said P-type semiconductor area with light, to emit electrons from an electron emitting surface at one end of said ends of said electron emission element; and each of said plurality of shutter elements, being positioned in the light irradiating direction in front of a light incident surface of said corresponding semiconductor area, being suitable for on-off controlling said light irradiation of said corresponding P-type semiconductor area. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of an electron emission element not constituting an embodiment of the present invention; Fig. 2 is a schematic cross-sectional view of an electron emission element not constituting an embodiment of the present invention; Fig. 3 is a schematic view of an example not constituting an embodiment of the present invention; Fig. 4A is a partial plan view of an electron emission device not constituting an embodiment; Fig. 4B is a cross-sectional view thereof along a line B-B; Fig. 5 is a partial cross-sectional view of an electron emission device constituting an embodiment; Fig. 6 is a schematic view showing a general structure of an MIM electron emission element not constituting an embodiment; Fig. 7 is a schematic cross-sectional view of a conventional MIM electron emission element not constituting an embodiment; Fig. 8A shows a schematic cross-sectional view of the basic structure of a photo-exciting electron emission element not constituting an embodiment; Fig. 8B shows an expanded cross-sectional view of a recess in the metal layer in Fig. 8A; Fig. 9 is a schematic view showing the structure of a photoinduced electron emission element of Fig. 8A; Fig. 10A is a schematic view of an electron emission element, not constituting an embodiment, in which a forward bias is applied to a PN junction and electrons are injected into the P-layer; Fig. 10B is a chart schematically showing current-voltage characteristic thereof; Fig. 11 is a schematic view of an MIM electron emission element not constituting an embodiment; Fig. 12 is a schematic view of a surface conduction type electron emission element not constituting an embodiment; Fig. 13 is a schematic cross-sectional view showing the structure of an electron emission device not constituting an embodiment; Fig. 14 is a schematic view showing another electron emission device not constituting an embodiment; Fig. 15 is a schematic view of a photoswitchable electron emission element not constituting an embodiment; and Fig. 16 is a schematic view of a photoswitchable electron emission element not constituting an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be explained by an embodiment and examples not being embodiments thereof shown in the attached drawings. Fig. 1 is a schematic cross-sectional view of an electron emission element not constituting an embodiment of the present invention. On both ends of a P-type semiconductor substrate 1 are attached metal electrodes 2, 3, and an electron emission aperture is formed in the metal electrode 2. On the P-type semiconductor substrate 1 in said electron emission aperture there is formed a layer 4 of a material for reducing the work function, such as Cs or CsO. In such structure, a driving voltage is applied across the metal electrodes 2, 3 with a positive side at the metal electrode 2, and the P-type semiconductor substrate 1 is irradiated with light hν. If hν ≧ Eg which is the band gap of the semiconductor, the electrons are excited to the conduction band, accelerated by the electric field and emitted from the surface of the work function reducing material 4. A high electron emission efficiency can be achieved if the practical work function is reduced, by the work function reducing material 4, to a level lower than the conduction band of the semiconductor, namely a state of negative electron affinity. Also, since electron emission does not occur in the absence of light irradiation, there can be achieved a high-speed photoswitching operation in which the electron emission is turned on or off by the on-off control of light. Fig. 2 is a schematic cross-sectional view of an example not being an embodiment of the present invention. One of the electrodes formed on the P-type semiconductor substrate 1 is composed of a transparent electrode 5, for example of ITO, through which light is introduced. Thus the electrons are excited in the P-type semiconductor substrate 1 as explained above, accelerated to the metal electrode 2 and emitted from the surface of the layer 4 of the work function reducing material. It is possible to easily produce an electron emission device having an array of plural electron emission elements, by forming a transparent electrode 5 on a glass substrate, then forming plural P-type semiconductor substrates 1 in the form of islands or in mutually insulated forms, and forming electrodes 2 with electron emission apertures and layers 4 of work function reducing material respectively corresponding to said P-type semiconductor substrates 1. Fig. 3 is a schematic view of an example not being an embodiment of the present invention. In this example, the electron emission elements shown in Fig. 2 are formed on a transparent substrate, and are mutually separated by insulating opaque layers 6. Thus said electron emission elements can be independently driven by the independent on-off control of the lights entering said electron emission elements. Also the electron emission efficiency can be controlled by regulating the energy of the incident light. As explained in the foregoing, the electron emission element of the present example easily enables electron emission control with light, such as photoswitching operation. Also an extremely simple structure consisting solely of the P-type semiconductor substrate 1 and electrodes 2,5 enables easy manufacture even of an array of plural elements. In the following there will be explained, as an example not being an embodiment of the present invention, an electron emission device having plural electron emission elements and usable as a pattern generator for a display device. As an electron source there has been utilized thermal electron emission from a heated cathode. However the electron emission element utilizing such heated cathode has been associated with the drawbacks of a large energy loss in heating, necessity of heating means, and inability for immediate function after switching on, due to considerable time required for pre-heating. For this reason various electron emission elements not relying on heating have been developed and proposed. For example there are already proposed an element in which an inverse bias voltage is applied to a PN junction to emit electrons from the electron emission element by electron avalanche yield phenomenon; an MIM element having a metal layer-insulator-metal layer structure in which a voltage is applied between said two metal layers to emit the electrons, which have passed the insulator by tunnel effect, from the metal layer to outside; a surface conduction element (SCE) in which a voltage is applied perpendicularly to a thin high resistance layer to emit the electrons from the surface of said thin layer to outside; and a field effect (FE) element in which a voltage is applied to a metal of a form facilitating the concentration of electric field to create a local high-density electric field thereby emitting the electrons from said metal. As an application of such electron emission elements, plural electron emission elements are arranged two-dimensionally and are suitably on-off controlled to emit the electrons in a desired pattern, and the emitted electrons are accelerated and deflected toward a fluorescent surface to obtain a display. In such electron emission elements, the electron emission is generated by applying a voltage exceeding a threshold value. Thus, if a device for patterned electron emission is formed by an array of a plurality of the electron emission elements, it becomes necessary to attach a wire for voltage control for each electron emission element. Consequently the density of arrangement of the electron emission elements becomes lower, so that a pattern of high-density pixels is difficult to achieve. The present example not being an embodiment is to resolve such drawbacks. Fig. 4A is a partial plan view of an electron emission device constituting an example not being an embodiment of the present invention, and Fig. 4B is a cross-sectional view along a line B-B. In these figures there are shown a substrate 102, an MIM structure 104, and a light shutter 106. The substrate 102 is transparent, and is composed, for example, of glass, ceramics, or an insulating crystal such as GaAs, GaSb, InAs, GaP or spinell (MgAl 2 O 4 ). The MIM structure 104 has a first metal layer 108, an insulator layer 110 and a second metal layer 112. The first metal layer 108 is composed for example of Al, Be, Mo, Pt, Ta, Au, Ag, W, Cr, Mg or nichrome. Said metal layer 108 should preferably be as thin as possible, for example in a range of 0.001 to 1 µm, in order that the electrons excited by the light irradiation from below can be efficiently supplied to the insulator layer 110. The insulator layer 110 is composed for example of SiO 2 , Ta 2 O 5 , Al 2 O 3 , BeO, SiC, SiO x N y H z , SiN x H y or AlN. The thickness of said insulator layer 110 is preferably small as long as the destruction of insulation does not occur, but is suitably selected in consideration of the insulating material used in said insulator layer 110 and the metal employed in the second metal layer 112 to achieve a desired electron emission characteristic, for example in a range of 1-200 nm (10 - 2000 Å). The second metal layer 112 is composed for example of Au, Pt, Al or Ag. Said second metal layer 112 is preferably as thin as possible, for example in a range of 10 - 300nm (100 - 3000 Å), in consideration of the electron emission efficiency. As shown in Fig. 4B, a power source 114 is provided for applying a voltage between the first metal layer 108 and the second metal layer 112, with positive side at the second metal layer 112. The shutter 106 is provided with a first transparent electrode 116 formed on the lower face of the substrate 102; a transparent plate 118 positioned opposite to said substrate 102; a second transparent electrode layer 120 formed on said transparent plate 118 so as to face said first transparent electrode layer 116; and a liquid crystal layer 122 provided between said substrate 102 and transparent plate 118. The first transparent electrodes 116 extend in a direction perpendicular to the line B-B and are arranged in parallel manner with a suitable pitch. Similarly the second transparent electrodes 120 extend along the line B-B and are arranged in parallel manner with a suitable pitch. Each first transparent electrode 116 is connected to a terminal 124 for applying the driving voltage, and each second transparent electrode 120 is connected to a similar terminal 125. Thus there is formed a unit area 126 for independent voltage application by so-called matrix drive, at the crossing position of each first transparent electrode 116 and each second transparent electrode 120 of the shutter 106. Each unit area 126 in the above-explained MIM structure 104 corresponds to an electron emission element of the present example not being an embodiment of the present invention, and each unit area 126 of the shutter 106 corresponds to a shutter element in the present example not being an embodiment of the present invention. As shown in Fig. 4B, a light source is provided for illuminating the shutter 106 through the transparent plate 118. The electron emission device of the present example not being an embodiment is driven in the following manner. In the MIM structure 104, a voltage is applied by the power source 114 between the first metal layer 108 and the second metal layer 112. The voltage of said power source 114 is suitable determined according to the specific parameters of the MIM structure and the desired electron emission characteristic, but is selected at such level that said second metal layer 112 does not emit electrons solely by the voltage from said power source 114, for example in a range from 3 to 20 V. In the shutter 106, in a unit area 126 not subjected to voltage application between the first electrode 116 and the second electrode 120, the liquid crystal layer 122 remains transparent whereby the light from the light source is transmitted, through the substrate 102, to the MIM structure 104. Consequently, in said unit area 126, the electrons in the first metal layer 108 of the MIM structure 104 are subjected to the energy of irradiating light, pass through the insulator layer 110 easily by the voltage applied by said power source 114, and are emitted through the second metal layer 112 to the outside. On the other hand, in each unit area 126 of the shutter 106 not subjected to the voltage application between the first transparent electrode 116 and second transparent electrode 120, the liquid crystal layer 122 becomes light-scattering, so that the light from the light source is mostly scattered and scarcely reaches the MIM structure 104 through the substrate 102. Therefore no electron emission takes place from the MIM structure 104 in such unit area 126. As explained above, the electron emission device of the example not being an embodiment only requires application of a common voltage to all the electron emission elements, and the electron emission can be obtained from desired unit areas 126 by matrix voltage application to the shutter elements 106. The voltage applied to the electron emission element may be selected smaller or larger as the intensity of light source becomes lower or higher, respectively. Fig. 5 is a partial cross-sectional view of an embodiment of the present invention, showing a similar part as shown in Fig. 4B. In Fig. 5 there are shown a PN junction 130, and a light shutter 132. The PN junction 130 has an N-type semiconductor layer 134, a P-type semiconductor layer 136, a first electrode layer 138, and a second electrode layer 140. The N-type semiconductor layer 134 has a thickness for example in a range of 0.001 to 1 pm, while the P-type semiconductor layer 136 has a thickness for example in a range of 0.001 to 1 µm. The first electrode 138 is substantially transparent. Such transparent electrode can be realized by suitable selection of material and thickness. Said first electrode 138 is for example composed of ITO, SnO 2 or ZnO and has a thickness for example of 10 to 300 nm (100 to 3000 Å). The second electrode 140 covers a part of the P-type semiconductor layer 136, the remaining part of which is coated with an alkali metal such as Cs or another compound in order to facilitate electron emission. As shown in Fig. 5, a power source 144 is provided to apply a voltage between the first electrode 138 and the second electrode 140, with the positive side at the second electrode 140. The shutter 132 is basically same as the shutter in the foregoing example of Fig. 4A, and is composed of two transparent plates 146, 148 positioned in mutually opposed relationship; a first electrode layer 150 and a second electrode layer 152 respectively formed on mutually opposed faces of said transparent plates 146,148; a liquid crystal layer 154 provided between said transparent plates 146, 148; and voltage applying terminals connected to said first and second electrodes 150, 152. In the present embodiment, the area coated with a material 142 such as Cs corresponds to a unit area 156 formed at the crossing point of each first electrode 150 and each second electrode 152 in the shutter 132. The electron emission device of the present embodiment is driven in the same manner as in the example of Fig. 4A. The voltage applied across the PN junction 130 is selected at such level that the PN junction 130 does not emit the electrons toward above in the absence of light irradiation to said junction, but emits the electrons only in the presence of light irradiation. In the present embodiment the electrons are emitted from the coated area of the material 142 such as Cs. The foregoing embodiment and example, respectively, employ a liquid crystal shutter, but the device of the present invention may utilize other shutters utilizing electrooptical crystals such as PLZT, lithium niobate (LiNbO 3 ) or lithium tantalate (LiTaO 3 ). Also in the foregoing embodiment and examples respectively, there have been disclosed an electron emission element utilizing negative electron affinity of a PN junction, but there can also be utilized, in the electron emission device of the present invention, an element in which a reverse bias voltage is applied to a PN junction and an electron avalanche yield is induced by light irradiation, or an element in which a P-layer is photoexcited in an NPP + structure. As explained in the foregoing, in the present embodiment, the plural electron emission elements only require a relatively low common voltage, so that said electron emission elements can be constructed in simple manner and arranged with a high density, thus enabling the formation of a pattern with high-density pixels. In the following there will be explained an example not being an embodiment of the present invention. Fig. 6 schematically shows a general structure of an MIM electron emission element. The MIM electron emission element is composed of a metal M1, a thin insulating layer I formed thereon, and a thin metal M2. Under the application of a voltage V, larger than the work function  m of the metal M2, between the metals M1, M2, the electrons which have passed the insulating layer I by tunnel effect and having an energy larger than the vacuum level are emitted from the surface of the metal M2. In order to obtain a high electron emission efficiency in such electron emission element, the insulating layer I should be formed as thin as possible within an extent not causing destruction of insulation, and the metal M2 should also be formed as thin as possible within an extent allowing sufficient current flow. Fig. 7 is a schematic cross-sectional view of a conventional MIM electron emission element not being an embodiment, wherein the metal M2 is formed thinner in an electron emitting area W to improve the electron emission efficiency. The present example not being an embodiment is to enable control by light of the quantity of electron emission from such MIM electron emission element and to realize a simple structure enabling high integration. Fig. 8A is a schematic partial cross-sectional view showing the basic structure of a photoexciting electron emission element not being an embodiment, and Fig. 8B is a cross-sectional view of a recess in the metal layer. Refering to Fig. 8A, a translucent substrate 201 such as glass is provided thereon with a transparent electrode 202 composed for example of ITO, on a part of which there are formed, in succession, an N-type semiconductor area 203, a P-type semiconductor area 204, and a conductive area 205 composed of metal such as Al or semiconductor such as Si. The transparent electrode 202, N-type semiconductor area 203, P-type semiconductor area 204 and conductive area 205 constitute a photoelectric converting junction area. On said conductive area 205 and at the side of the photoelectric converting junction area, there is formed an insulating area 206, which is advantageously composed of SiO 2 in case the conductive area 205, P-type semiconductor area 204 and N-type semiconductor area 203 are composed of Si. On said insulating area 206 there is formed a metal layer 207 for example of Al, Au or Pt, which has a recess 209 in an area above said photoelectric converting junction area. In said recess 209 there is formed an area 208 of a work function reducing material composed for example of an alkali metal or alkali earth metal such as Cs. The conductive area 205, insulating area 206 and metal layer 208 constitute an MIM electron emission element. In the present example not being an embodiment, the conductive area 205 serves as an electrode of the photoelectric converting junction area and also as the conductive layer of the MIM electron emission element, thus simplifying the entire structure. The insulating area 206 serves as the insulating layer of the MIM electron emission element, and also insulates the lateral faces of the-photoelectric converting junction area, thus preventing the deterioration of characteristic by current leak. Also in the present example not being an embodiment, the photoelectric converting junction area for the photoelectric conversion and the MIM electron emission element for electron emission are mutually superposed and unified to enable high integration, and said junction area is formed only under the recess for electron emission, thereby improving the efficiency thereof. Besides said efficiency is further improved by the presence of the area of the work function reducing material 208 in the recess 209. It is, therefore, rendered possible to produce, in integrated manner, photoexcitable electron emission elements having plural electron emission sources such as an image sensor. In the present example not being an embodiment of the above-explained structure, a voltage V is applied between the conductive area 205 and the metal layer 207, and the photoelectric converting junction area is irradiated with light through the translucent substrate 201, thus a photoelectromotive force is generated between the P-type semiconductor area 204 and the N-type semiconductor area 203, to inject the electrons into the conductive area 205 constituting the MIM electron emission element, and the electrons which have passed the insulating area 206 are emitted from the recess 209 in which the work function of the metal layer 207 is lowered by the work function reducing material 208. In this manner efficient electron emission is made possible with a lower energy. Fig. 9 is a schematic view showing detailed structure of the photoexcitable electron emission device not being an embodiment. As shown in Fig. 9, the metal layer 207 is provided with plural recesses 209, for each of which there are provided an MIM electron emission area and a converting junction area. The pitch of the recesses 209 can be as small as the order of micrometer. As explained in the foregoing, the photoexcitable electron emission element of the example not being an embodiment is featured by a unified structure of a photoelectric converting junction area and an MIM electron emission element wherein a conductive area 205 is commonly used as the electrode of said photoelectric converting junction area and that of the MIM electron emission element and an insulating area 206 is formed for electric insulation, so that a simple structure enabling high integration can be obtained. Also the photoelectric converting junction area is formed only below the recess 209 of the metal layer 207 constituting the electron emitting portion of said MIM electron emission element to achieve efficient electron emission. Thus it is rendered possible to produce integrated a photoexcitable electron emission device provided with plural photoelectron emission elements such as an image sensor. Said recess 209 may be provided with an area of a work function reducing material 208 for emitting the electrons from the metal layer 207 with a lower energy, thereby improving the efficiency of electron emission. In the following there will be explained an example not being an embodiment of the present invention. Fig. 10A is a schematic view of an electron emission element in which a forward bias voltage is applied to a PN junction to inject electrons into the P-layer, and Fig. 10B is a schematic chart showing the current-voltage characteristic thereof. In Fig. 10A, a forward bias voltage V applied to the PN junction induces a forward current I as shown in Fig. 10B, whereby a part of electrons injected from the N-layer into the P-layer is emitted into the vacuum from the surface of the P-layer. The surface of said P-layer is coated for example with Cs in order to reduce the work function, thereby increasing the quantity of the electrons emitted. Fig. 11 is a schematic view of an MIM electron emission element not being an embodiment, and Fig. 12 is a schematic view of a surface conduction electron emission element not being an embodiment. The MIM electron emission element has a laminate structure composed of a metal electrode 306, an insulating layer 307 and a thin metal electrode 308, and the electrons are emitted from the thin metal electrode 308, by a voltage application between the electrodes 306 and 308. The surface conduction electron emission element is composed of electrodes 310, 311 formed on an insulating substrate 309, and a high-resistance thin film 312 formed therebetween. A voltage application between the electrodes 310, 311 induces electron emission from the surface of said high-resistance thin film 312. However, in such known elements, in which electrons of high energy are electrically generated and emitted, the electron emission can only be controlled electrically, and it has not been possible to correlate the electron emission with light. The present example not being an embodiment aims at an electron emission element characterized by a simple structure allowing integration, and allowing electron emission control with light. In the following description reference is to be made to Fig. 2, as the basic structure of the present example not being an embodiment is similar to that shown in Fig. 2. On a transparent electrode 5 composed for example of ITO there are provided a P-type semiconductor area 1 and a metal electrode 2 having an electron emitting aperture. A layer 4 of a work function reducing material such as Cs or CsO is formed on the surface of the P-type semiconductor area 1 in said aperture. In an electron emission element of such structure, the P-type semiconductor area 1 is irradiated by light hν through the transparent electrode 5, and a voltage is applied between the transparent electrode 5 and the metal electrode 2, with the positive side at the metal electrode 2. If hν ≧ Eg which is the band gap of the semiconductor, the electrons are excited to the conduction band by the light, accelerated by the electric field and emitted from the surface of the layer 4 of work function reducing material. The presence of said layer 4 of the work function reducing material lowers the practical work function to a level lower than the conduction band of the P-type semiconductor area 1, or a state of negative electron affinity (NEA), thereby achieving a high electron emission efficiency. The electron emission is not induced if a voltage is not applied between the transparent electrode 5 and the metal electrode 2 even in the presence of irradiating light, or if the irradiating light is not present even in the presence of said voltage. Thus the amount of electron emission can be controlled either by the voltage applied to the P-type semiconductor area 1 or by the irradiating light hν. Also said control may be achieved by the combination of the applied voltage and the irradiating light. Fig. 13 is a schematic cross-sectional view of an example not being an embodiment of the electron emission device of the present invention, wherein same components as those in Fig. 2 are represented by same numbers. As shown in Fig. 13, on a transparent electrode 5, there are provided plural P-type semiconductor areas 1 mutually separated by opaque insulating areas 305. Then a metal electrode 2 is provided thereon, and layers 4 of a work function reducing material are formed respectively corresponding to said P-type semiconductor areas 1. In such electron emission device, a voltage V is applied between the transparent electrode 5 and the metal electrode 2 with the positive side at the metal electrode 2, and desired P-type semiconductor areas 1 are irradiated with light, whereby the electrons are emitted only from the layers 4 of work function reducing material of thus irradiated P-type semiconductor areas 1. Consequently each electron emission element can be independently controlled by the on-off control of the light entering each P-type semiconductor area 1, and dot-shaped or linear electron emission element can therefore be obtained. Also the independent control of the electron emission element can be achieved by irradiating the P-type semiconductor areas 1 with constant light, dividing the metal electrode 2 corresponding to the P-type semiconductor areas 1 and on-off controlling the voltage supplied thereto. Also the on-off control may be achieved by the combination of the applied voltage and the irradiating light. Fig. 14 is a schematic view showing the function of another example not being an embodiment of the electron emission device of the present invention. The structure of the electron emission device is same as that shown in Fig. 13 and will not therefore be explained in detail. In the present example not being an embodiment transparent electrodes 301 1 - 301 4 and electrodes 303 1 - 303 4 are made to cross each other to form a matrix, and a P-type semiconductor area (not shown) is formed at each crossing point. A voltage V can be applied to desired P-type semiconductor areas by applying said voltage to desired transparent electrodes 301 1 - 301 4 and desired electrodes 303 1 - 303 4 through control transistors T21 - T24 and T11 - T14. As in the foregoing examples not being embodiments and in the embodiment of Fig. 5, it is possible to obtain an electron emission element of linear or planar form by the on-off control of light irradiating the P-type semiconductor areas and possibly the on-off control of the applied voltage V. As detailedly explained in the foregoing, the electron emission device of the present example not being an embodiment can achieve electron emission control with light such as a photoswitching. Because of an extremely simple structure composed solely of a P-type semiconductor area and electrodes and irradiated through the transparent electrode, it enables easy manufacture and high integration, and it is advantageously employed in a multiple electron emission element having plural P-type semiconductor areas. More specifically, a linear or dot-shaped electron emission device can be obtained by forming plural P-type semiconductor areas explained above on a same transparent electrode, mutually insulating said semiconductor areas with insulating areas and forming an electrode on said semiconductor areas wherein the electron emission from each semiconductor area can be controlled by on-off control of said electrode or irradiating light. Also a dot-shaped, linear or planar electron emission device can be obtained by positioning said transparent. electrodes and said electrodes in a matrix form, forming a P-type semiconductor area at each crossing point and mutually insulating said semiconductor areas with insulating areas, wherein the electron emission from each P-type semiconductor area can be controlled by the on-off control of said transparent electrodes and said electrodes, or by the on-off control of irradiating light. Also it is possible to form a recess in said electrode and to form an area of a work function reducing material therein, thereby enabling electron emission at a lower energy and thus improving the efficiency of electron emission. In the following there will be explained an example not being an embodiment of the present invention, providing a photoswitchable electron emission element in which the amount of electrons emitted by an MIM electron emission element can be controlled with light and which enables a laminate structure and compact designing. Fig. 15 is a schematic view of a photoswitchable electron emission element not constituting an embodiment of the present invention. As shown in Fig. 15, the photoelectric converting junction element is a PIN photodiode composed of a P + -layer 401, an I-layer 402, and an N + -layer 403. Because of the presence of the I-layer 402, the PIN photodiode can reduce the junction capacity and the running time of carriers in the depletion layer. On said PIN photodiode there is formed an electron emission element, composed of a conductive layer 404, an insulating layer 405 of a thickness of 3 -10 nm (30 - 100 Å) formed thereon, and a metal layer 406 of a thickness of 1 - 10 nm (10 - 100 Å) formed on said insulating layer 405. The conductive layer 404 is composed of a metal such as Al or a semiconductor such as Si. The insulating layer 405 is preferably composed, also from the standpoint of manufacture, of Al 2 O 3 if the conductive layer 404 is composed of Al, or of SiO 2 if the conductive layer 404 is composed of Si. The metal layer 406 is composed of Al, Au or Pt. In a photoswitchable electron emission element composed of the PIN photodiode and the electron emission element as explained above, an inverse bias voltage V R is applied between the P + -layer 401 and the N + -layer 403 to maintain the carrier generating area of the I-layer 402 at a low impurity level, to expand the depletion area and reduce the junction capacity. In such condition, the carriers are mostly generated in the depletion area and move by an electric field in said depletion area. By another voltage V applied between the conductive area 404 and the metal layer 406, the carrier electrons generated by light absorption are injected into the conductive layer 404, accelerated and emitted from the metal layer 406, corresponding to the incident light. Fig. 16 is a schematic view of a photoswitchable electron emission element not constituting an embodiment of the present invention, wherein the electron emission element is same as that in the example not being an and will not therefore be explained further. As shown in Fig. 16, the photoelectric converting junction element in the present example of Fig. 15 embodiment is an avalanche photodiode composed of an N + -layer 407, a P - -layer 408, a P-layer 409, a P -layer -410 and a P + -layer 411. In addition to the high-speed performance of a PIN diode, the avalanche diode is featured by an internal amplification effect, wherein the electrons injected from the N + -layer 407 are accelerated by a strong electric field in the P - -layer 408 to cause an avalanche phenomenon, inducing electron multiplication. On said avalanche photodiode there is formed an electron emission element mentioned above. In a photoswitchable electron emission element composed of such avalanche photodiode and electron emission element, an inverse bias voltage V R is applied between the P + -layer 411 and the N + -layer 407 to form a depletion layer in the carrier generating area of the P -layer 410. Then a voltage V applied between the conductive layer 404 and the metal layer 406 injects the carrier electrons, generated by light absorption and multiplied by an avalanche phenomenon in the P -layer 408, into the conductive layer 404, and thus injected electrons are accelerated and emitted from the metal layer 406, corresponding to the incident light. As detailedly explained in the foregoing, the photoswitchable electron emission element of the present example not being an embodiment can emit electrons corresponding to the incident light, and is compact since the photoelectric converting junction element and the electron emission element can be unified in a laminate structure. Also as said junction element, there may be employed a PIN photodiode for reducing the junction capacity due to the presence of an i-layer, and for reducing the running time of carriers in the depletion layer under a high voltage, and a high-speed photoswitchable electron emissibn element can thus be obtained. Also as said junction element, there may be employed an avalanche photodiode to achieve internal amplification by electron multiplication through avalanche phenomenon, in addition to the above-mentioned high-speed performance.

Claims: An electron emission device comprising: a plurality of electron emission elements and a plurality of shutter elements (106; 132) corresponding thereto; each of said plurality of electron emission elements comprising a P-type semiconductor area (1; 136; 204) and electrodes (2, 5; 138, 140; 202, 205) formed on both ends of said electron emission element in the direction of incident light radiation; wherein a voltage is to be applied between said electrodes (2, 5; 138, 140; 202, 205) for accelerating electrons (e) which are excited to the conduction band by the irradiation of said P-type semiconductor area (1; 136; 204) with light (hν), to emit electrons (e) from an electron emitting surface (4; 142; 208) at one end of said ends of said electron emission element; and each of said plurality of shutter elements (106; 132), being positioned in the light irradiating direction in front of a ligh incident surface of said corresponding semiconductor area (1; 136; 204), being suitable for on-off controlling said light irradiation of said corresponding P-type semiconductor area (1; 136; 204). A device according to claim 1, characterized in that each of said electron emission elements comprises in successive superposition: a translucent substrate (201); a transparent electrode (202) formed on said translucent substrate (201); a photoelectric converting junction structure formed by laminating at least a N-type semiconductor area (203) and said P-type semiconductor area (204); an electrode layer (205) formed on said P-type semiconductor area (204); an insulating layer (206) formed on said electrode layer (205); and a metal layer (207) arranged on said insulating layer positioned on said electrode layer (205); said electrode layer (205), said insulating layer (206) and said metal layer (207) forming a MIM structure; and said metal layer (207) having a recess (209) in a portion thereof at a major side thereof opposite said insulating layer (206), said portion being positioned in registry with said electrode layer (205). A device according to claim 2, characterized in that an area of a work function reducing material is formed in said recess (209). A device according to claim 1, characterized in that , in said electron emission element, said electrode (5; 138; 202) formed on the light incident side is transparent and light irradiation is conducted through said transparent electrode (5; 138; 202). A device according to claim 1 or 4, characterized in that said electrode (2; 140) at the one end of said electron emission element is provided with a recess (209) in which an area (4; 142) of a work function reducing material is formed. A device according to any of the preceding claims 1 to 5, characterized in that said plurality of electron emission elements have a transparent electrode (5) formed at the light incident side in common; and insulation areas (6; 206; 305) are formed between neighboring P-type semiconductor areas (1; 204). A device according to any of the preceding claims 2 to 4, characterized in that said transparent electrodes (5; 138; 202) and said electrode layers (2; 140; 205) formed on said P-type semiconductor area are disposed to intersect each other so as to form an electrode matrix, and said P-type semiconductor areas (1; 136; 204) are formed at the crossing points of said electrodes (2, 5; 138, 140; 202, 205) while insulating areas (6; 206; 305) are formed between neighboring P-type semiconductor areas (1; 136; 204). A device according to any of the preceding claims 2, 3, 6 or 7, characterized in that said photoelectric converting junction structure is a PIN photodiode. A device according to any of the preceding claims 2, 3, 6 or 7, characterized in that said photoelectric converting junction structure is an avalanche photodiode.

Inventors: OKUNUKI MASAHIKO, SHIMIZU AKIRA, SHIMODA ISAMU, SUGATA MASAO, SUZUKI AKIRA, TSUKAMOTO TAKEO, OKUNUKI, MASAHIKO, SHIMIZU, AKIRA, SHIMODA, ISAMU, SUGATA, MASAO, SUZUKI, AKIRA, TSUKAMOTO, TAKEO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H01J1
IPC codes: H01J1