Abstract:
This invention relates to an electron tube having a structure for enabling a stable operation for a long time. In the electron tube, at least a confining mechanism is arranged between a photocathode and the electron incident surface of a semiconductor device, which are arranged to oppose each other through a container. Particularly, the area of the opening of the confining mechanism is smaller than that of the electron incident surface, thereby confining the orbits of photoelectrons from the photocathode. This structure avoids bombardment of electrons arriving at portions other than the electron incident surface of the semiconductor device and prevents the semiconductor device from being unnecessarily charged.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electron tube used as a photodetector for quantitatively measuring weak light and particularly having a sensing device such as a semiconductor device for multiplying photoelectrons emitted from a photocathode and outputting the electric signals. 
     2. Related Background Art 
     Conventionally, an electron tube which causes an electron lens to accelerate and focus electrons emitted from a photocathode upon incidence of light and makes the photoelectrons incident on, e.g., a semiconductor device to obtain a high gain is known. This electron tube is disclosed in, e.g., U.S. Pat. No. 5,120,949, Japanese Patent Laid-Open No. 6-318447, U.S. Pat. Nos. 5,374,826 or 5,475,227. Particularly, U.S. Pat. No. 5,475,227 discloses a structure for preventing a phenomenon that ions generated from gas molecules adsorbed on the electron incident surface of the semiconductor device due to electrons incident on the semiconductor device are accelerated and fed back to the photocathode to result in a large degradation in photocathode. More specifically, a semicylindrical ion deflecting electrode is arranged immediately before the semiconductor device to bend the orbits of ions generated on the electron incident surface of the semiconductor device, thereby preventing the ions from returning to the photocathode. 
     SUMMARY OF THE INVENTION 
     The present inventors examined the prior arts and found the following problems. In the prior art disclosed in U.S. Pat. No. 5,475,227, ions generated from the semiconductor device are bent in orbit and prevented from being fed back to the photocathode. With this structure, although the photocathode can be prevented from degrading, the ions bent in orbit collide with the insulating side wall, so no stable operation can be obtained. This is because secondary electrons are emitted from the insulating side wall of the container upon collision of ions to charge the side wall to a positive potential, thus affecting the orbits of electrons propagating from the photocathode to the semiconductor device. Particularly, with the arrangement of each prior art, only a specific portion of the side wall of the container is charged upon collision of ions to make the electron lens asymmetric. Therefore, the orbits of electrons are largely bent. In addition, the secondary electrons generated upon collision of ions are incident on the semiconductor device to generate a pseudo signal or stray to produce a new unstable state. 
     An object of the present invention is to provide an electron tube having a structure for enabling a stable operation for a long time. 
     According to the present invention, there is provided an electron tube comprising, at least, a container having a first opening and a second opening opposing the first opening, a photocathode arranged on the first opening side of the container to emit photoelectrons in correspondence with incident light, a semiconductor device having an electron incident surface for receiving the photoelectrons from the photocathode, the electron incident surface being arranged to face the photocathode, a conductive stem arranged on the second opening side of the container to define a distance between the photocathode and the electron incident surface of the semiconductor device, and a confining mechanism arranged between the photocathode and the electron incident surface to confine orbits of the photoelectrons from the photocathode. Particularly, the confining mechanism has an opening which contributes to confine the spread of the photoelectrons (the photoelectrons from the photocathode pass through this opening and arrive at the electron incident surface of the semiconductor device). The area of the opening is set to be smaller than that of the electron incident surface of the semiconductor device. Therefore, the opening of the confining mechanism is arranged at a position close to the electron incident surface. 
     The electron tube further comprises an electron lens constituted by a cathode electrode arranged on the first opening side of the container and having a through hole for passing the photoelectrons from the photocathode toward the electron incident surface, and an anode electrode arranged between the cathode electrode and the conductive stem. The anode electrode has a first surface facing the photocathode, a second surface opposing the first surface, and a through hole extending from the first surface to the second surface. 
     In this arrangement, the confining mechanism includes the anode electrode, and the opening of the confining mechanism corresponds to a second-surface-side opening of the through hole of the anode electrode. In other words, the opening having smallest area within the openings of the electron lens corresponds to the opening of the confining mechanism. 
     In this electron tube, external light is converted into electrons by the photocathode. The electrons (photoelectrons) emitted from the photocathode pass through the opening portion of the anode electrode and then arrive at the electron incident surface of the semiconductor device. At this time, positive ions are generated on the electron incident surface. The anode electrode is set at a positive potential with respect to the electron incident surface of the semiconductor device. Since the anode electrode is reverse-biased with respect to the positive ions generated on the electron incident surface, the generated positive ions cannot return to the photocathode or case through the through hole of the anode electrode. 
     In this case, preferably, a cylindrical collimator portion extending toward the photocathode is arranged on the first surface of the anode electrode concentrically with the first-surface-side opening of the through hole of the anode electrode. When the collimator portion is arranged on the anode electrode in use of the semiconductor device (e.g., an avalanche photodiode: APD), extension of the electric field from the photocathode toward the semiconductor device through the through hole of the anode electrode can be minimized. Therefore, ion feedback can be effectively suppressed. 
     More preferably, a conductive mesh electrode is arranged in the through hole of the anode electrode. When the mesh electrode is arranged in the anode electrode in use of the semiconductor device (e.g., a photodiode: PD), extension of the electric field from the photocathode toward the semiconductor device through the through hole of the anode electrode can be minimized. Therefore, ion feedback can be effectively suppressed. 
     The electron tube according to the present invention may further comprise a collimator electrode supported by the anode electrode. The collimator electrode has a third surface facing the photocathode, a fourth surface opposing the third surface, and a through hole extending from the third surface to the fourth surface. The confining mechanism includes the collimator electrode, and the opening of the confining mechanism corresponds to a fourth-surface-side opening of the through hole of the collimator electrode. The orbits of the photoelectrons incident from the photocathode on the third-surface-side opening of the collimator lens at a predetermined angle are collimated by the collimator electrode, and its spread is confined by the collimator lens. The photoelectrons which have passed through the collimator electrode are incident on the electron incident surface along the normal of the electron incident surface. When the collimator electrode is arranged, arrival of the photoelectrons at portions other than the electron incident surface is effectively suppressed. 
     The semiconductor device has an n-type substrate and a p-type semiconductor layer formed on the n-type semiconductor substrate and having the electron incident surface. In the semiconductor device, the n-type semiconductor substrate and the anode electrode are electrically connected to the conductive stem. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view (partially cutaway view) showing the structure of an electron tube according to the first embodiment of the present invention, in which the structures of main parts of the electron tube are common to the first to third embodiments; 
     FIG. 2 is a sectional view of the electron tube (first embodiment) shown in FIG. 1 taken along a line I--I in FIG. 1; 
     FIG. 3 is a sectional view showing a detailed structure near a semiconductor device in the electron tube shown in FIG. 2; 
     FIG. 4 is a view for explaining the structural relationship between the electron incident surface of the semiconductor device and the opening of a confining mechanism; 
     FIG. 5 is a sectional view showing the structure of an electron tube according to the second embodiment of the present invention, which corresponds to the sectional view (FIG. 2) taken along the line I--I in FIG. 1; 
     FIG. 6 is a plan view showing the structure of a mesh electrode arranged in the through hole of an anode electrode; 
     FIG. 7 is a sectional view of the anode electrode shown in FIG. 6 taken along a line II--II in FIG. 6; 
     FIG. 8 is a sectional view showing the detailed structure near a semiconductor device in the electron tube shown in FIG. 6; 
     FIG. 9 is a view showing the process of assembling a collimator electrode supported by the anode electrode (third embodiment); and 
     FIG. 10 a sectional view of the anode electrode and the collimator electrode shown in FIG. 9 taken along a line III--III in FIG. 9. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of an electron tube according to the present invention will be described below with reference to FIGS. 1 to 10. 
     FIGS. 1 and 2 are a perspective view and a sectional view, respectively, showing an electron tube according to the first embodiment of the present invention. Particularly, the sectional view in FIG. 2 shows the section of the electron tube (FIG. 1) taken along a line I--I in FIG. 1. Referring to FIGS. 1 and 2, an electron tube 1 has a cylindrical case 10. The case 10 is constituted by a hollow cylindrical cathode electrode 11 of a Kovar metal and a welded flange portion 13, which are respectively fixed at two ends of a ceramic portion 12 extending along an axis AX to sandwich the ceramic portion 12. The cathode electrode 11, the ceramic portion 12, and the welded flange portion 13 are integrated by brazing. In consideration of an electron lens (to be described later), when the case 10 has an outer diameter of 15 mm, an inner diameter of 12 mm, and a total length of 13 mm, the length of the cathode electrode 11 is preferably 5 mm. 
     An input surface plate 21 made of glass to transmit light is fixed to the cathode electrode 11 of the case 10. The input surface plate 21 has a photocathode 22 (photoelectric surface) inside and is arranged on the side of a first opening 14 of the case 10. After the photocathode 22 is formed, the input surface plate 21 is fixed to the cathode electrode 11 while the photocathode 22 and the cathode electrode 11 are electrically connected via a photocathode electrode 25 consisting of a chromium thin film. The photocathode electrode 25 has an inner diameter of 8 mm, with which the effective diameter of the photocathode electrode 25 is defined. 
     A disk-shaped stem 31 of a conductive material (e.g., a Kovar metal) is fixed to the welded flange portion 13 of the case 10. The stem 31 is arranged on the side of a second opening 15 of the case 10. A lead pin 32 insulated by glass 34 is fixed to the stem 31. The peripheral portion of the stem 31 is resistance-welded to the welded flange portion 13 and integrated with the case 10. Therefore, the electron tube 1 is constituted by integrating the case 10, the input surface plate 21, and the stem 31, and a predetermined vacuum state is held in the electron tube 1. 
     As shown in FIG. 3, a semiconductor device 40 operating as an APD (Avalanche PhotoDiode) is fixed on the surface of the stem 31 on the photocathode side with a conductive adhesive 50. The semiconductor device 40 uses a substrate 41 formed of heavily doped n-type silicon as a substrate material. A disk-shaped p-type carrier multiplication layer 42 is formed at the central portion of the substrate 41. A guard ring layer 43 consisting of a heavily doped n-type semiconductor and having the same thickness as that of the carrier multiplication layer 42 is formed outside the carrier multiplication layer 42. A breakdown voltage control layer 44 of a heavily doped p-type semiconductor is formed on the surface of the carrier multiplication layer 42. The surface of the breakdown voltage control layer 44 serves as an electron incident surface 44a. An oxide film 45 and a nitride film 46 are formed to connect the peripheral portion of the breakdown voltage control layer 44 to the guard ring layer 43. To apply an anode potential to the breakdown voltage control layer 44, an incident surface electrode 47 is formed on the outermost surface of the semiconductor device 40 by depositing aluminum into an annular shape. A peripheral electrode 48 rendered conductive with the guard ring layer 43 is also formed on the outermost surface of the semiconductor device 40. The peripheral electrode 48 is separated from the incident surface electrode 47 by a predetermined interval. The diameter of the electron incident surface 44a is preferably 3 mm inside the incident surface electrode 47. 
     The silicon substrate 41 of the semiconductor device 40 is fixed to the stem 31 with the conductive adhesive 50. The stem 31 and the silicon substrate 41 are electrically connected to each other by using the conductive adhesive 50. The incident surface electrode LO 47 of the semiconductor device 40 is connected to the lead pin 32 insulated from the stem 31 through a wire 33. 
     As shown in FIGS. 1 to 3, a plate-like anode electrode 60 is arranged between the semiconductor device 40 and the photocathode 22. The anode electrode 60 is fixed to the welded flange portion 13 and positioned near the semiconductor device 40. The distance between the anode electrode 60 and the semiconductor device 40 is preferably 1 mm. A through hole 61 (a confining mechanism for confining the photoelectrons in orbit) for passing photoelectrons from the photocathode 22 toward the electron incident surface 44a of the semiconductor device 40 is formed at the central portion of the anode electrode 60. A cylindrical collimator portion (collimator electrode) 62 projecting to the photocathode side is integrated with the anode electrode 60 to surround the through hole 61. The collimator portion 62 projects toward the photocathode 22 and is arranged to surround the photoelectric-surface-side opening of the through hole 61. The through hole 61 has a diameter of 2 mm. The collimator portion 62 has an inner diameter of 2 mm and a height of 1 mm. 
     As shown in FIG. 4, the effective area of the electron incident surface 44a is limited by the collimator electrode 62 to an area S1 (the area S1 matches the area of the stem-side opening of the through hole 61 of the anode electrode 60) smaller than that (S2) of the electron incident surface 44a. More specifically, the diameter of the electron incident surface 44a capable of receiving incident electrons is 3 mm, as described above. However, the diameter of a region on which electrons can actually be incident is limited to about 2 mm. 
     The diameter (2 mm) of the through hole 61 of the anode electrode 60 is made smaller than that (3 mm) of the electron incident surface 44a such that incidence of electrons on the unnecessary portion, i.e., the peripheral portion of the electron incident surface 44a of the semiconductor device 40 does not charge the oxide film 45 or nitride film 46, or does not damage the p-n junction interface or the contact face between the semiconductor layer 44 and the metal electrode 47 to degrade the device characteristics. The collimator portion 62 is added to the anode electrode 60 such that extension of the electric field from the photocathode 22 toward the semiconductor device 40 through the through hole 61 is minimized, and the effect of suppressing ion feedback (to be described later) is increased. The collimator portion 62 functions to return the direction of electrons which are emitted from the peripheral portion of the photocathode 22 to be obliquely incident on the semiconductor device 40 to the vertical direction. Electrons obliquely incident on the semiconductor device 40 cross the larger dead layer (the upper layer portion of the breakdown voltage control layer 44) of the semiconductor device 40, so the ratio of incident electrons reaching the depletion layer lowers to decrease the multiplication gain. By adding the collimator portion 62 to correct the orbits of electrons, variations in multiplication gain depending on the electron emission position are suppressed. The anode electrode 60 is formed by pressing a 0.3-mm thick stainless steel plate. The anode electrode 60 may be integrated with the welded flange portion 13. 
     The assembly of the electron tube 1 having the above structure will be described next. The semiconductor device 40 is die-bonded to the stem 31. The incident surface electrode 47 is connected to the lead pin 32 by the wire 33. The anode electrode 60 is fixed to the welded flange portion 13 of the case 10 by resistance welding. The welded flange portion 13 is fixed to the stem 31 by resistance welding. The input surface plate 21 and the stem 31 are set in a vacuum unit called a transfer unit together with the case 10 (these members 21, 31, and 10 are separated) and baked at 300° C. for about 10 hours. Thereafter, the photocathode 22 is formed on one side of the input surface plate 21. The input surface plate 21, the stem 31, and the case 10 are integrated in the vacuum atmosphere in this unit. Finally, the vacuum state in the transfer unit is canceled to hold a predetermined vacuum state in the electron tube 1. 
     As shown in FIGS. 1 and 2, a voltage of -12 kV is applied to the photocathode 22 and the cathode electrode 11 of the electron tube 1, and the anode electrode 60 is grounded (applied with a voltage of 0 V). At this time, the cathode electrode 11 and the anode electrode 60 form an electron lens. Electrons emitted from the photocathode 22 having the effective diameter of 8 mm are focused to a diameter of 1.5 mm smaller than the inner diameter of the collimator portion 62 and the through hole 61 and received by the electron incident surface 44a of the semiconductor device 40. In the semiconductor device 40, a voltage of -150 V is applied to the breakdown voltage control layer (anode) 44 of the semiconductor device 40, and the silicon substrate 41 (cathode) is grounded (applied with a voltage of 0 V) such that the p-n junction is reverse-biased. With this structure, the APD 40 obtains an avalanche multiplication gain of about 50. 
     When light is incident on the electron tube 1, electrons are emitted from the photocathode 22 into the vacuum (inside the electron tube 1). The electrons (photoelectrons) are accelerated and focused by the electron lens and incident on the electron incident surface 44a of the APD 40 with an energy of about 12 keV. The incident electrons generate one electron-hole pair every time the electrons lose an energy of 3.6 eV in the APD 40. In this first multiplication process, the electrons are multiplied to about 3,000 times and further 50 times in the subsequent avalanche multiplication process (the avalanche multiplication gain is about 50). The secondary electron gain reaches a total of about 2×10 5 . 
     In the electron tube 1, the multiplication factor at the first stage is 3,000, i.e., higher than that of the conventional photomultiplier (to be referred to as a &#34;PMT&#34; hereinafter) by about three orders of magnitude. Therefore, detection with a high S/N ratio can be performed. In fact, when about four electrons are emitted from the photocathode 22 on the average upon incidence of very weak pulse light, the electron tube can discriminate the number of input photoelectrons (the number of incident photons), which is beyond the discrimination ability of the conventional PMT. Such characteristics obtained by the electron tube 1 according to the present invention are very effective in quantitative observation of fluorescence emitted from a trace of biosubstance. In addition, it is very important that the electron tube 1 itself stably operates for a long time. 
     In the electron tube 1 of the first embodiment, a voltage of -150 V is applied from the power supply to the electron incident surface 44a of the semiconductor device 40 through the lead pin 32, the wire 33, and the incident surface electrode 47. On the other hand, the anode electrode 60 is grounded (applied with a voltage of 0 V) through the welded flange portion 13. That is, the anode electrode 60 is set at a positive potential with respect to the breakdown voltage control layer 44 of the semiconductor device 40. This means that, since the anode electrode 60 is reverse-biased with respect to the positive ions generated on the electron incident surface 44a, the generated positive ions cannot return to the photocathode 22 or the case 10 through the opening portion 61 of the anode electrode 60. 
     More specifically, since the anode electrode 60 is kept at the positive potential (reverse potential with respect to the positive ions generated on the electron incident surface 44a) with respect to the electron incident surface 44a in the electron tube 1 according to the present invention, the positive ions generated on the electron incident surface 44a cannot return to the insulating portion of the photocathode 22 or the case 10 beyond the anode electrode 60. Since the photocathode 22 of the electron tube 1 is not affected by ion feedback, the photocathode 22 does not degrade even during a long-time operation. In addition, since the positive ions do not return to the insulating portion of the case 10, the case 10 is not charged. The orbits of electrons emitted from the photocathode 22 toward the semiconductor device 40 are not affected by charge, and no pseudo signal is generated by secondary electrons emitted from the case 10. Therefore, the electron tube 1 realizes a very stable operation for a long time. 
     Assume that ions generated on the electron incident surface 44a of the semiconductor device 40 return to the photocathode 22. The positive ions returning to the photocathode 22 have a high energy of about 12 keV because of the potential difference between the photocathode 22 and the electron incident surface 44a, so the material of the photocathode 22 is sputtered by the positive ions. Therefore, if ions generated on the electron incident surface 44a return to the photocathode 22, the photocathode sensitivity largely degrades during a short-time operation. 
     An electron tube 100 according to the second embodiment of the present invention will be described below with reference to FIGS. 5 to 8. Only differences from the first embodiment will be described below. The same reference numerals denote the same parts throughout the drawings, and a detailed description thereof will be omitted. 
     As shown in FIG. 5, a cathode electrode 18 is as short as about 2 mm. At the central portion of a case 90, intermediate flanges 15a and 15b are inserted between insulating rings 16a, 16b, and 16c. A PD having a large electron incident surface area is used as a semiconductor device 80. A large through hole 71 is formed in an anode electrode 70. A mesh electrode 72 shown in FIG. 6 is arranged in the through hole 71. By shortening the cathode electrode 18, an electron lens for guiding electrons which are emitted from a photocathode 22 and rarely focused to the semiconductor device 80 can be constituted. More specifically, the electron tube 100 is assumed to be used in a strong magnetic field of about 2 T (tesla) along a tube axis AX passing through the center of the case 90. 
     Since, in such a strong magnetic field, the propagation direction of electrons is determined by the direction of the magnetic field, the electric field can be used to just accelerate the electrons. More specifically, no electron lens can be formed by the electric field, and the substantial effective diameter of the photocathode 22 is limited by the opening portion 71 of the anode electrode 70 or an electron incident surface 84a (to be described later; FIG. 8) of the semiconductor device 80. To ensure the maximum effective diameter of the photocathode 22, both the anode electrode 70 having the large through hole 71 and the semiconductor device 80 having the large electron incident surface 84a are required. This use condition is required for a high-energy experiment or the like using an accelerator. However, in the second embodiment as well, an area S3 of the stem-side opening of the through hole 71 is smaller than an area S2 of the electron incident surface 84a (FIGS. 4 and 6). 
     The intermediate flanges 15a and 15b arranged in the case 90 function to suppress the unstable state due to charge of the case 90. Voltages obtained by uniformly distributing a voltage of -12 kV applied to the photocathode 22, i.e., voltages of -8 kV and -4 kV are applied to the intermediate flanges 15a and 15b, respectively. 
     As shown in FIGS. 6 and 7, the mesh electrode 72 is arranged in the through hole 71 of the anode electrode 70. The mesh electrode 72 is formed by partially etching the anode electrode 70 made of stainless steel. In this case, the line width of the mesh electrode 72 is 50 μm, and the pitch is 1.5 mm. Electrons are transmitted through the mesh electrode 72 in correspondence with the opening ratio (93%) of the mesh electrode 72. 
     The mesh electrode 72 is arranged in the through hole 71 of the anode electrode 70 because the through hole 71 of the anode electrode 70 is made large in correspondence with the electron incident surface 84a of the semiconductor device 80. More specifically, when the through hole 71 of the anode electrode 70 is made large, the valley of the negative potential on the side of the photocathode 22 extends to the side of the stem 31 through the through hole 71. This degrades the effect of suppressing feedback of positive ions generated on the electron incident surface 84a of the semiconductor device 80. When the mesh electrode 72 is added, the negative potential from the photocathode 22 can be prevented from extending to the side of the electron incident surface 84a, so that the ion feedback suppressing effect can be maintained. The maximum diameter of the through hole 71 of the anode electrode 70 is smaller than the electron incident surface 84a of the PD 80 (S3&lt;S2). 
     As shown in FIG. 8, the semiconductor device 80, i.e., the PD uses, as the substrate material, a diffusion wafer obtained by heavily and deeply diffusing phosphorus as an n-type impurity from the lower surface of a high-resistance n-type wafer. 
     Therefore, the diffusion wafer is constituted by a heavily doped n-type contact layer 81 formed on the lower surface and a high-resistance n-type layer 82. An n-type channel stop layer 83 is formed by heavily ion-implanting phosphorus in the peripheral portion of the surface of the high-resistance n-type layer 82. A disk-shaped p-type incident surface layer (breakdown voltage control layer) 84 is formed by heavily diffusing boron at the central portion of surface of the layer 82. An oxide film 85 and a nitride film 86 are formed so as to cover the surface of the channel stop layer 83 and the peripheral portion of the incident surface layer 84. An incident surface electrode 87 consisting of an aluminum film is formed to contact the incident surface layer 84 and apply a voltage to the incident surface layer 84. A charge prevention electrode 88 consisting of an aluminum film contacting the channel stop layer 83 is formed at a position separated from the incident surface electrode 87. The electron incident surface 84a of the PD 80 is substantially defined by the inner diameter of the incident surface electrode 87. 
     A voltage of -12 kV is applied to the photocathode 22 of the electron tube 100, and a voltage of 0 V is applied to the anode electrode 70. Since the contact layer 81 of the semiconductor device 80 is at the same potential as that of the anode electrode 70, the contact layer 81 is applied with the voltage of 0 V. The electron incident surface 84a is applied with a voltage of -50 V through the lead pin 32, the wire 33, and the incident surface electrode 87. The operation of the electron tube 100 upon incidence of light is the same as in the first embodiment. By arranging the mesh electrode 72 in the through hole 71, ion feedback can be appropriately suppressed even when the through hole 71 of the anode electrode 70 is made large. More specifically, even when the through hole 71 of the anode electrode 70 is made large, extension of the electric field can be suppressed, i.e., the valley of the low potential from the photocathode 22 which is biased to the negative potential can be prevented from entering the side of the electron incident surface 84a through the through hole 71 of the grounded anode electrode 70 in the presence of the mesh electrode 72. For this reason, gas molecules ionized on the electron incident surface 84a upon incidence of electrons can be effectively prevented from returning to the photocathode 22 or the case 90 through the through hole 71. 
     Since the light-receiving surface of the input surface plate 21 is large, the electron tube 100 of the second embodiment stably operates in a high magnetic field for a long time and is used for a high-energy experiment using an accelerator. 
     An electron tube according to the third embodiment of the present invention has a collimator electrode 65 supported by an anode electrode 60 (70), as shown in FIGS. 9 and 10. The collimator portion 62 in the first embodiment differs from the collimator electrode 65 in the third embodiment in the following point. The collimator portion 62 is integrated with the anode electrode 60 (70) to constitute part of the anode electrode 60 (70) while the collimator electrode 65 is a conductive ring member directly attached to the anode electrode 60 (70). Therefore, the collimator portion 62 and the collimator electrode 65 have no functional difference therebetween. The collimator electrode 65 forms an electric field for returning photoelectrons e -  which are emitted from the peripheral portion of a photocathode 22 to be obliquely incident on a semiconductor device 40 toward a tube axis AX (the tube axis AX corresponds to the direction of light incidence). With this structure, the photoelectrons e -  emitted from the entire region in the photocathode 22 uniformly lose the energy in the dead layer of the semiconductor device 40. For this reason, the electron tube can maintain a high ability of discriminating the number of electrons. Note that the structure of the third embodiment can be applied to both electron tubes of the first and second embodiments shown in FIGS. 1, 2, and 5. 
     To further increase the above effect, the sectional area (the area of a through hole 650 defined by a plane perpendicular to the tube axis AX) of the through hole 650 of the collimator electrode 65 reduces from the photocathode 22 toward a stem 31, as shown in FIG. 10. In other words, the area of the photoelectric-surface-side opening of the through hole 650 of the collimator electrode 65 is larger than that of the stem-side opening of the through hole 650 of the collimator electrode 65. 
     The structural relationship between the collimator electrode 65 and an electron incident surface 44a (84a) of the semiconductor device 40 (80) will be described. An area S4 of the stem-side opening of the through hole 650 of the collimator electrode 65 is smaller than an area S2 of the electron incident surface 44a (84a) of the semiconductor device 40 (80) (FIGS. 4 and 9). That is, the region for receiving the electrons emitted from the photocathode 22 has an area smaller than the effective area of the electron incident surface 44a (84a) of the semiconductor device 40 (80). With this structure, electrons accidentally emitted from portions other than the photocathode 22 are never incident on portions other than the electron incident surface of the semiconductor device 40 (80) to degrade the semiconductor device 40 (80) itself (degradation due to electron bombardment) or result in unnecessary charge. 
     When the collimator electrode 65 has a total length of 3.5 mm, the diameter of the photoelectric-surface-side opening of the through hole 650 is preferably 3 mm, and the diameter of the stem-side opening of the through hole 650 is preferably 2 mm. (At this time, the area of the stem-side opening is set to be smaller than that of the electron incident surface of the semiconductor device 40 (80)). 
     According to the present invention, for the opening of the confining mechanism arranged between the photocathode and the semiconductor device, e.g., the area of the stem-side opening of the through hole of the anode electrode is set to be smaller than the incident area of the electron incident surface of the semiconductor device. In addition, in the semiconductor device having the p-type electron incident surface and the n-type substrate, the n-type substrate is electrically connected to the stem to set the anode electrode at the same potential as that of the stem, and the semiconductor device is reverse-biased. With this structure, an electron tube which enables a stable operation for a long time can be realized. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. 
     Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 
     The basic Japanese Application No. 111656/1996 filed on May 2, 1996, and 128723/1996 filed on May 23, 1996 are hereby incorporated by reference.