Camera tube having a target formed by an array of phototransistors
United States Patent [191 Bell [ CAMERA TUBE HAVING A TARGET FORMED BY AN ARRAY OF PHOTOTRANSISTORS [75] Inventor: Ronald L. Bell, Woodside, Calif.  
 [73] Assignee: Varian Associates, Palo Alto. Calif.  
 [22] Filed: Oct. 23, 1968 [21] Appl. No.: 769,895  
 [52] US. Cl. 313/368 [51] Int. Cl H0lj 29/45, HOlj 31/38 [58] Field of Search 178/71. E, 7.2 A, 7.2; 313/65 AB, 66  
 [56] References Cited UNITED STATES PATENTS 3,403,284 9/1968 Buck et a1. 313/65 AB X 3,433,994 3/1969 Gibson 313/65 AB X 3.458.782 7/1969 Buck et a1. 313/65 AB X 3,467,880 9/1969 Crowell 313/66 X 3,569,758 3/1971 Horiuchi et a1 313/66 Primary Examiner-Robert Segal Attorney, Agent, or Firm-Stanley Z. Cole; D. R. Pressman; Robert K. Stoddard BEA 11 Apr. 1, 1975 [57] ABSTRACT A vidicon camera tube is disclosed. The camera tube includes an electron gun at one end thereof for projecting a beam of electrons over a beam path to a target electrode formed by a photosensitive array of phototransistors. Deflection plates are provided intermediate the electron gun and target for scanning the electron beam over the array of phototransistors to derive an electrical signal in accordance with the photon image which is illuminating the array of phototransistors. The phototransistors each include one exposed terminal facing the scanning electron beam. A capacitor structure is formed on the beam scanned side of the array of phototransistors for providing substantially increased capacitance between the scanned terminals of the phototransistors and a source of stable potential such as ground potential. By providing the capacitance between the scanned terminals and ground the photon gain of the phototransistors is greatly increased above 1, thereby substantially increasing the sensitivity of the camera tube and therefore its resolution.  
 2 Claims, 7 Drawing Figures CAMERA TUBE HAVING A TARGET FORMED BY AN ARRAY OF PI-IOTOTRANSISTORS DESCRIPTION OF THE PRIOR ART Heretofore, vidicon camera tubes have been proposed utilizing a target comprised of an array of phototransistors having floating terminals scanned by the electron beam to produce the video output signal. Such a vidicon tube is described in the Journal of Applied Physics, Vol. 34, No. 10, page 2923, and following of the Oct. I963 issue. In this article the vidicon tube is analyzed and it is concluded that the floating electrodes limit the gain during exposure of light to one charge per quantum absorbed.  
  The vidicon electronic camera tube has two significant advantages. namely, the tube is relatively inexpensive to produce and has the ability, in principle, to extend the wavelength range of imaging devices well into the infrared range. Therefore, there exists a need to increase the operating sensitivity, i.e., to increase the quantum efficiency to a value in excess of unity in a device wherein there are essentially only two terminals of the phototransistor elements available for circuit connection.  
 SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved camera tube having a target formed by an array of phototransistors.  
  One feature of the present invention is the provision, in a camera tube of the type having a target formed by an array of phototransistors, of providing a capacitor on the electron beam scanned side of the array of floating transistor terminals, such capacitor providing extra capacitance to ground or to a source of stable potential, whereby the quantum efficiency of the phototransistors is improved to provide quantum gain in excess of unity.  
  Another feature of the present invention is the same as the preceding feature wherein the capacitor structure includes a metallic film disposed overlaying an insulative dielectric layer, which in turn overlays the array of phototransistors.  
  Another feature of the present invention is the same as the first feature wherein the capacitor structure includes an array of p-n junctions formed on the beam scanned side of the phototransistors such that the capacitor structure is formed by the extra p-n junctions formed on the phototransistors.  
  Another feature of the present invention is the same as any one or more of the preceding features wherein the phototransistors are p-n-p transistors having emitter, base and collector terminals and wherein the collector terminals are scanned by the electron beam.  
  Another feature of the present invention is the same as any one or more of the preceding features except the next preceding feature wherein the phototransistors are n-p-n transistors having emitter, base and collector terminals and wherein the emitter terminals are scanned by the electron beam.  
  other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram, partly in section,  
 with the target greatly expanded out of scale, depicting the prior art vidicon camera tube,  
  FIG. 2 is a schematic circuit diagram for the target of the vidicon tube of FIG. 1,  
  FIG. 3 is an enlarged cross sectional view ofa portion of the structure of FIG. I delineated by line 3-3 and modified to show a feature of the present invention,  
  FIG. 4 is an alternative embodiment of a portion of the structure of FIG. 3 delineated by line 44,  
  FIG. 5 is a simplified schematic circuit diagram derived from the circuit diagram of FIG. 2,  
  FIG. 6 is a schematic circuit diagram similar to that of FIG. 2 for a p-n-p phototransistor target, and  
  FIG. 7 is a schematic simplified circuit diagram derived from the circuit diagram of FIG. 6.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown the prior art vidicon camera tube 10. The vidicon tube 10 comprises three sections, an electron gun section 20, a scanning section 30 and a target section 4 all contained within an evacuated envelope structure 5. The electron gun section 20 contains a conventional thermoionic cathode emitter which emits electrons and several electrodes for focusing and modulating the electron beam. The scanning section 30 contains coils or plates for deflecting the beam of electrons to any desired position or sequence of positions on the target section 4 by magnetic or electrostatic action. The target section 4 contains a photoconducting layer 6 and a transparent conductive coating 7 attached to the inside of a glass face plate 8. An electron collecting mesh 9 is disposed adjacent the beam scanned face of the photoconductive layer 6.  
  In operation, the photoconductive layer 6, in the dark, behaves as a capacitor. The transparent conductive coating 7 serves as one electrode of the capacitor and the scanning electron beam serves as the other electrode of the capacitor. The electron gun 20 projects the beam, at a beam voltage of a few kilovolts provided by beam voltage supply II, at the mesh 9 which is located just in front of the photoconductive target. This high velocity scanning beam passes through mesh openings and is decelerated toward the potential at the target surface. The exposed target surface will be rapidly charged to cathode potential by a collection of electrons from the scanning beam, since it is insulated from the front target electrode by the resistance of the body of target material. If a potential is applied between the front target electrode 7 and the cathode of the electron gun 20, this potential will appear across the elements of the target which are struck by the electron beam. If the beam scans every element of the target sequentially, the entire target will experience the applied potential across its thickness, if the time taken to scan the surface is less than the dielectric relaxation time of the photoconductive material 6. Otherwise, the part of the target which was scanned first will lose its charge before the last part of the target is scanned. Since a typical scan time is one-thirtieth of a second, successful television operation requires that the vidicon target material have a charge relaxation time of at least one-thirtieth of a second for no incident illumination.  
  When a photon image is focused on the vidicon target 4 through the front transparent electrode 7 and face plate 8, the target elements which are struck by photons will lose their charge if the target material is photoconductive. This occurs because the light-induced electron-hole pairs reduce the resistance of the target body, thereby decreasing the RC relaxation time. When the scanning electron beam returns to an element which has been discharged by light during the previous scanning cycle, it quickly recharges this element to cathode potential. ln this case, a current flows in the external circuit through the load resistor 12 and it is this current which provides the television signal indicating the presence of light at that particular point on the screen.  
  The photoconductive target is formed by an array, such as 540 X 540 phototransistor elements 13, shown in greatly magnified scale. The electron beam charges an element 13 of the target in microseconds. as it continuously moves over all the screen elements in succession, but each element has one-thirtieth of a second to be discharged by incoming light. This gives the vidicon a one-thirtieth of a second integration feature which is very important in building up detectable signals at low light levels. The aforedescribed prior art vidicon camera tube is essentially the same as that disclosed in the aforecited Journal of Applied Physics article of Oct l), [963. The equivalent circuit for a phototransistor element 13 of the photoconductive target 4 and external load resistor 12 is as shown in FIG. 2 where C, is the emittento-base junction capacitance, C is the base-to-collector junction capacitance, and C is the stray capacitance from the floating emitter electrode to ground. In the prior art, C is negligible and the quan tum gain during exposure to light is limited to 1.  
  It has been discovered that the quantum efficiency can be substantially increased above unity, for a phototransistor target employing floating target electrode. if the capacitance from the target electrode to ground is substantially increased. The mathematical proof for this discovery is adduced below in the section titled Mathematical Analysis. Suffice it to say, that the quantum gain and thus the sensitivity and resolution of the vidicon camera tube 1 can be substantially improved by incorporating a capacitor structure on the scanned face of the target 6 to greatly increase the value of C,, the capacitance from the scanned terminal of the phototransistor to a stable source of potential, such as ground potential. The capacitor structure may take any one of a number of various forms, two of which are described in greater detail in FIGS. 3 and 4.  
  Referring now to FIG. 3, there is shown in enlarged scale a portion of the target electrode structure 6 delineated by line 3-3 of FIG. 1. The photosensitive target 6 includes the array of phototransistors 13, such transistors each including, for example, an n region 16 forming the emitter of an np-n phototransistor, a base portion 17 ofp type material. and a collector region 18 of an n type material. Alternatively, the phototransistor target may comprise an array of p-n-p phototransistors 13, in which case region 16 forms a collector, region 17 forms the base, and region 18 forms the emitter: such regions being constituted of p-type, ntype, and p-type materials, respectively.  
  The emitter or collector region 16 includes an exposed portion 19 which is scanned by the electron beam for charging the capacitance of the phototransistor, as previously described. A capacitor structure 21 is formed on the electron beam scanned face of the phototransistor target 6 by first depositing a layer of insulative material 22, such as silicon dioxide or by merely oxiding the exposed surface area of a silicon target electrode 6. Holes 23 are photoetched in layer 22 for passage of the beam therethrough to the scanned terminal of the n-p-n or p-n-p phototransistor 13. A thin metallic film 24, as of gold, is deposited overlaying the dielectric layer 22 to form the capacitor structure by the mutually opposed areas of the film 24 and the annular coextensive regions of each of the emitter or collector regions 16. The conductive film 24 is connected to ground by means of a suitable conventional connection.  
  Referring now to FIG. 4, there is shown an alternative embodiment of the present invention. In this embodiment the phototransistor 13 is substantially the same as that shown in FIG. 3 with the exception that the n or p region 16 includes an annular region at the inner surface of the opposite type semiconductive material, namely, p-type or n-type, respectively, to form a junction capacitance at the junction between the annular p or n-type region 25 and the n or p region 16. An insulative passivating coating 26 such as silicon dioxide is either deposited on or formed on the electron beam scanned side of the phototransistor. As in the embodiment of FIG. 3, holes 23 are etched in the passivating layer 26 to permit the scanning electron beam to be incident upon the scanned terminal of the phototransistor in the region 19. In addition, the passivating layer includes an annular etched portion 27 to permit a metallic layer 28 to be deposited over the insulative layer 26 to make contact with the annular p or n-region 25. The metallic layer 28 is connected to ground potential or some other source of stable potential via conventional connection techniques. The p-n or n-p junction formed between layer 25 and layer 16 forms an additional capacitance to ground or to a stable potential for increasing the gain and, thus, the sensitivity and resolution of the vidicon tube 10.  
 MATHEMATICAL ANALYSIS N-PN ARRANGEMENT Referring now to FIGS. 2 and 5, FIG. 2 depicts a schematic circuit diagram for an n-p-n phototransistor element of the target 6. To simplify the analysis, it is assumed that the impedance 12 in the output (collector) lead can be neglected, which is a very close approximation. The equivalent circuit is then as shown in FIG. 5 where l, 2 and 3 represent emitter base and collector terminals respectively. The injected node currents are i, GV  
 i GV (l-a) i ozGV respectively, where G is the transconductance of the emitter/base junction and a is the transistor a factor. The useful current into the load 12 is i An incoming photon generates an electron-hole pair, the hole is collected by the base region, and if the event occurred in the collector region, the electron remains there and contributes to the collector current i;,. Suppose now that we represent these events by placing a charge q on the base node 2 at time t 0. We have 2 l/ 2 Eq. (4) where 2 23 rz ia/( i-z 13) Eq- (5) and i 2 l2/( l2 m) Eq (6) i.e., at time zero,  
 To calculate the time-behaviour of the system, note that for the nodal charges q and ([2.  
  ig; V (0) exp(-t/T)dc Eq. (17) -fGqC T/C (C C Eq. (18) Using the expression above for T and inserting an additional single charge for the electron originally generated in the collector region, we have Os 1 m/( m 1a)( 2/ 1) 4&#39; C and c are the junction capacitances of an individual phototransistor, but C is an added shunt capacitance to ground from the emitter, which can in principle be very large. For this case, we have CE/CI 23 r-zll ra qand we have a quantum gain /(l- Eq. (22 i.e., the current gain of the device used as a transistor.  
 On the other hand, ifC is only moderately large and not very large, the quantum gain can be largely independent of the transistor gain, but still respectably greater than unity. The value is Qn q l lfl l 2( l2 +1)- Eq- (2 The value of the transistor a is of course very close to unity.  
 P-N-P ARRANGEMENT Here we have the beam scanning the collectors of the array, and a common emitter output to the video amplifier, as shown in FIG. 6.  
  The output impedance l2 seen is very low, and the equivalent circuit is therefore approximately as shown in FIG. 7. If we place a charge Q; on 2, Q, on 3, and Q 0n 1, then 3 a1+ 2: i-z m 23)- Eq. (29] Thus,  
  2 2 2 s a Eq. (30) In operation, an incoming photon generates an electron-hole pair somewhere in the device. and the electron finds its way to the base 2. The hole will be collected either on the collector (0 or on the emitter (Q The effect of Q, is small (zero in the case of the second equivalent circuit). The probability of the hole being collected on 3 is small and its effect is diminishable by increasing C We might neglect the effects of Q; for these reasons, although in practice the presence of an opposite polarity charge on 3 reduces the effective gain (see below) and fluctuations in the fraction of charge collected on 3 will produce an effective gain fluctuation from event to event.  
  For the p-n-p structure, the diffusion and conduction nodal currents are:  
 Here V the voltage appearing across the emitter junction (conductance G), is equal to the quantity V calculated above.  
 Differentiating Eq. (30),  
 dv dQ RdQ; */c, HC, E (34] and inserting values from Eqs. (32) and (33) dv, GV|- -(la))C,RaG\ n/Cg. Eq. (35) The solution of this is i: 12 c Eq. (36) where l =G( ai/C,+R 1G/C,. Eq. 37  
 The charge transferred to the terminal 1 (output) is Q 0o G v (0) e Eq. 38  
  2(Q2) 2 (Q by superposition. Eq, 24  
  2( 2) Q2 2 Eq. (25) where 2 12 23 3l 23 ar) Eq. (26) and 2 3) 03 3 Eq. (27) where R 23 I 12 23) Eq. (28) and and from Eq. (37),  
 GT={(l-or)/C Ra/C }-1 E 40 If ll/C 0 (zero influence of collector charge on operatlon) V (0) (12( 2 and Q1 1( 02( q i.e., the full gain of the phototransistor is realized. If on the other hand R/C is finite, the time constant Eq. (40) and the gain are both reduced. In the limit when Roz/C l-an/C GT C /Ra Eq. (42) and  Q1 w Q3 3O2( 2 01( Eq: 3) This can still represent a substantial gain if C is made large and RC is small.  
  Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description of shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.  
 What is claimed is:  
  I. In a camera tube for converting a photon image into a sequence of electrical signals. the combination comprising:  
 a cathode means for providing an electron beam current;  
 an array of phototransistor devices conductively responsive to the photon image, one element of each phototransistor device having at least a portion thereof directly exposed to the electron beam current;  
 means for scanning the electron beam current periodically over the phototransistor array to cause the electron beam current to strike the exposed portions;  
 a shunt capacitor proximate the exposed portions and formed by an insulative layer of dielectric material positioned over the phototransistor array with a film of conductive material positioned over the insulative layer, both the insulative layer and the conductive film having an array of apertures in registration with the exposed regions to permit passage of the electron beam current therethrough to the exposed portions, the shunt capacitor connected in parallel circuit relationship with respect to the cathode means and initially charged and periodically recharged by the electron beam current while discharged through the phototransistor array by the photon image to establish variations in the electron beam current; and  
 means for developing a sequence of electrical signals in response to the electron beam current variation.  
 2. The camera tube specified in claim 1, wherein:  
 the insulative layer is provided with passages therethrough proximate each aperture in the array of apertures;  
 the conductive film contacts each photosensitive device through the insulative layer passage; and  
 a p-n junction is provided in each photosensitive device at the points of contact between the conductive film and the photosensitive devices to provide shunt junction capacitances in parallel circuit relationship with the cathode means to assist the shunt dielectric capacitor.