Photosensitive Junction Controlled Electron Emitter

Abstract

A sandwich structure of photosensitive junctions in series with a mosaic of hotoemitters. An external grid is positioned adjacent the mosaic of photoemitters and has the high voltage side of a step up voltage divider thereto with the low voltage side connected to the input side of the sandwich structure. The sandwich structure and external grid are enclosed in a vacuum envelope for converting an input optical radiant image into an electron image for display on an electroluminescent screen. A bias light is uniformly flooded over the mosaic of photoemitters to provide saturation electron current therefrom. The flow of electrons emitted from the photoemitters are in proportion to the intensity of infrared light incident on the input side of the sandwich structure. The input side of the structure has an antireflection coating thereof for aiding the incident infrared light in producing electron-hole pairs across the photosensitive junctions.

Description

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention is in the field of electronic imaging devices which rely on the internal photoelectric effect in a mosaic of photodiode-photocathode junctions wherein the useful spectral response extends into the intermediate infrared spectrum. These photodiode-photocathode junctions comprise a P-type substrate having an antireflection coating on an input side and a plurality of N-type material islands on the output side. Each of the plurality of N-type material islands has a discrete electrically isolated photoemitter associated therewith on the output side. A voltage divider is connected in step up fashion from the antireflection coating, to a focusing grid laid on the output side of the sandwich structure, and to an external grid that is adjacent the plurality of discrete electrically isolated photoemitters. Also, a bias light floods the output side of the plurality of photoemitters to provide electron current saturation therefrom into the vacuum envelope and toward an electroluminescent screen.

The electron current from each of the mosaic of discrete electrically isolated photoemitters is in direct relation to the intensity of the infrared image incident on the P-type substrate at a position directly opposite each photoemitter because the infrared light creates electronhole pairs in the photodiodes in accordance with the intensity of the infrared light. The higher the concentration of electron-hole pairs the larger the reverse bias on the P-N photosensitive junctions. As the reverse bias increases, a higher relative voltage difference exists between the external grid and the discrete electrically isolated photoemitter islands, thus causing acceleration of the electrons toward the electroluminescent screen. A visible image is formed on the screen according to the incident infrared image present on the antireflection coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a circuit depicting a single image detector element of the present invention;

FIG. 2 shows curves of the photocathode current versus the grid voltage;

FIG. 3 shows the photodiode current versus photodiode voltage curves;

FIG. 4 shows a curve of the Log of the photocathode current versus the log of the photodiode irradiance;

FIG. 5 illustrates a sectional view of the sandwich structure of the present invention;

FIG. 6 shows a frontal view of sandwich structure;

FIG. 7 is a schematic diagram of a second embodiment comprising a circuit depicting a single image detector element where the P-N photosensitive junction is the reverse biased collector-base junction of a phototransistor;

FIG. 8 is a schematic diagram of a third embodiment comprising a pulsing voltage source; and

FIG. 9 illustrates a schematic diagram of a fourth embodiment of the present invention comprising a pulsed bias light source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is a photosensitive junction controlled electron emitter consisting of a mosaic made up of a plurality of electrically isolated photosensitive junctions with each single junction being in electrical contact with a single electron emitter of a mosaic of discrete electrically isolated photoemitters. The photosensitive junctions and emitters form a composite sandwich structure. The photosensitive junction is either of the semiconductor P-N junction type or of the metal-semiconductor Schottky barrier type. The photosensitive junction is simply a photodiode, or one of the opposite junctions of a transistor. The junction can be operated in either the reverse biased or the photovoltaic mode. The electron emitter is made of a photoelectron material, such as cesium antimonide or cesiated silicon, which is irradiated with radiation from a uniform biasing light to provide current saturation therefrom. A cold cathode emitter, such as a tunnel emitter, may also be used as the photoemitter with the use of the biasing light.

Earlier efforts to increase the infrared spectrum response of electronic imaging devices centered on the use of photoconductors to control electron emission from light biased photocathodes. However, these efforts failed to yield a useful infrared sensitive electron imaging device because the bulk resistivity of infrared sensitive photoconductors is insufficient relative to the reciprocal of the low transconductance of phototubes. The present invention resulted from a concentrated effort to discover a better means than bulk photoconductivity for controlling the electron emission from a light biased photocathode.

FIG. 1 illustrates a schematic diagram of the equivalent circuit for a single image detector element in the simplest arrangement of the photosensitive junction controlled electron emitter of the mosaic of image detector elements of the present invention. The circuit consists of a semiconductor photodiode 10 in series with the photocathode 12 of an electronic imaging tube enclosed in a vacuum envelope (not shown). Electron current emitted from photocathode 12 and through external grid 16 may enter an electron multiplying structure (not shown) before being collected by a charge storage film or a cathodoluminescent phosphor at the other end of the vacuum envelope. A steady bias light, whose radiation is represented by .phi.B, floods the photocathode produces a current density of 1 to 10 microamperes per square centimeter from the photocathode making the total current flow out the photocathode wholly determined by current flow through the photodiode. The photodiode 10 is reverse biased by battery 36 and while in this saturated condition acts as a current limiter to the flow of current from photocathode 12. Variations in voltage of battery 36 when the photodiode is operating in the saturation region have little effect on the flow of current from photocathode 12. In the dark, the photodiode 10 current is the leakage current and is therefore, the current flowing through the photocathode 12. Looking now at FIGS. 2 and 3, when the image tube is operated in the dark the photocathode will be operating at some point such as A on the photodiode current Ic VS grid voltage V.sub.G curve as shown in FIG. 2. When infrared signal radiation falls on photodiode 10, the photodiode saturation current increases from the dark value I.sub.D to a new value I.sub.F in proportion to the intensity of the infrared radiation on the photodiode. The photocathode will then be operating at a new point B that correspond to the photodiode current I.sub.F as illustrated in the curve of FIG. 3. The change in V.sub.D, or diode voltage, due to the change in V.sub.G when the operating point has moved from A to B in FIG. 2 has negligable feedback effect on the photodiode current, thus the photocathode current is determined by the high quantum efficiency of the photodiode. A photocathode of very high quantum efficiency in the infrared is realized. The dynamic range of operation will depend on whether the photocathode is used in a direct view image intensifier or is used in a camera tube. In both cases saturation current is determined by the intensity of the bias light that is projected on the photocathodes. However, the threshold current in the direct view image intensifier is determined by the dark current of the photodiode, while in the camera tube only the fluctuations, or shot noise, in the dark current determine the threshold. The dark current density of a silicon photodiode array as low as 10.sup.-.sup.9 amp/cm.sup.2 has been obtained at room temperature. The shot noise current for an average of 10.sup.-.sup.9 amp/cm.sup.2 is equal to 10.sup.-.sup.13 amp/cm.sup.2 assuming one thirtieth of a second integration time. Cooling the photodiode array will further decrease both the average and shot current densities.

The feasibility of the photodiode controlled photoemitter and the quantum efficiency which is obtained therefrom have been demonstrated using a silicon photodiode and an S-20 photocathode. The dynamic range of this combination is shown in FIG. 4 where the Log of the photocathode current Ic VS the Log of the photodiode irradiance .phi. is plotted. The quantum efficiency was measured and found to be 44 percent at the signal radiation wavelength of 8,000 A. The room temperature dark current of the photodiode, with voltage of battery 36 at 10 volts, is 2 .times. 10.sup.-.sup.10 amperes, and determines the threshold signal for a direct view image intensifier at room temperature. In FIG. 4, the curve representing the straight line extension below the dark current limit was obtained by balancing the dark current in a bridge circuit to determine the threshold signal for a camera tube.

A partial section of the composite photodiode-photocathode mosaic sandwich is indicated in FIG. 5. The image signal radiation, represented as .phi.IR in the case of infrared radiation, passes through the antireflection coating 8 an into the P-type substrate 10 where the photons from the image are absorbed, thus generating electron-hole pairs therein. The electrons diffuse to the vicinity of the P-N junctions formed by P-type substrate 10 and the N-type islands 11. These electrons are carried across the various discrete P-N junctions, through the gold islands 15, and into the photoemitter film 17 to replace the electrons that are constantly being emitted from 17 into the vacuum envelope by the bias light irradiating the photoemitter islands 17. The emitted electrons will pass through the external grid 16 to enter some secondary electron multiplication structure (not shown). A mesh of focussing grids 14, made of a good conductor such as gold, are laid alongside the photoemitter islands 17 and are electrically isolated therefrom by a layer of insulation 13. Focussing grids 14 are connected to the negative side of battery 36 to accelerate electrons emitted from photoemitter islands 17 through external grid 16 since, except near photocurrent saturation, a retarding electric field exists in the space between the photoemitters 17 and grid 16.

FIG. 6 illustrates an output side of the sandwich structure. An ohmic contact 20 surrounds the entire output side of the sandwich structure and is used to replenish electrons in the P-type substrate 10. Only one photoemitter island 17 is shown, but there is actually one island 17 located inside each square formed by the mesh of focussing grids 14.

FIG. 7 is schematic diagram of a single image detector element for controlling photocathode emission wherein the P-N junction in this embodiment is the reverse biased collector-base junction of a phototransistor 30. The operation of transistor 30 is the same as the photodiode of FIG. 1 with the exception that the dark currents and the photocurrents are amplified by phototransistor 30. Electrically, either the N-P-N or P-N-P transistor will operate equally well. However, the P-N-P transistor would be most favorable for high quantum efficiency since the reverse biased collector-base junction is nearest the incident surface of the signal radiation .phi.IR and therefore, transistor 30 efficiently collects the photogenerated electrons in its collector P-type substrate, represented by 30c. The base of transistor 30 is represented by 30b, and the emitter is represented by 30a.

FIG. 8 illustrates another embodiment showing a schematic diagram of a single image element photodiode 40 and photocathode 44 in a circuit designed to improve on the operation and simplify construction of the basic device as described with reference to FIG. 1. The embodiment shown in FIG. 8 differs from the embodiment shown in FIG. 1 in that the focusing wire grids 14 are omitted and the direct current battery 36, which normally biases grid 16, is replaced by a pulse generator 48 that applies a chain of positive pulses to grid 16. The positive pulses help avoid the focusing problem caused by the inherent retarding electric field between grid 16 and the photocathode 44 by operating the phototube in pulse saturation with the duration of the pulse saturation current being determined by charge integration of the photodiode current between pulses and the amount of saturation current of the phototube being commensurate with a given bias light .phi..sub.B intensity. The charge integration depends on the capacitance of the photodiode junction. Thus, initially when the voltage pulse of polarity indicated in FIG. 8 is applied to grid 16, the grid-to-cathode capacitance of the phototube, represented by C.sub. GC, charges quickly to the amplitude of the pulse voltage from pulse generator 48. After capacitance C.sub.GC charges to the amplitude of the voltage pulse from 48, the much larger capacitances of the photodiode array, represented by C.sub.J, is charged. The capacitance per unit area of the photodiode array C.sub.J is from 100 to 1,000 times the capacitance per unit area of the grid-to-cathode capacitance C.sub.GC. While the junction capacitance C.sub.J charges, the grid-to-cathode voltage will decrease from the full pulse voltage until the phototube current drops to a value equal to the reverse biased photodiode current. During most of the charging time of the junction capacitance C.sub.J an accelerating field exists between grid and cathode and focussing, rather than defocussing, of the photocurrent occurs. When the voltage pulse terminates, the junction capacitance C.sub.J begins to discharge through the junction at a rate determined by the intensity of the signal irradiance .phi.IR on the junction. The discharge of C.sub.J continues until the next voltage pulse from 48 is applied. An amount of charge equal to that lost by the junction capacitance then flows through the tube in the form of a short saturation current pulse. The photodiode junction 40 as shown in FIG. 8 may be replaced with a phototransistor with its collector-base receiving the infrared signal radiation. The photodiode current is amplified by the phototransistor.

In the circuits of both FIGS. 2 and 8, there is still the possibility of the undesirable effect in the finished mosaic of leakage of the more intense bias light .phi.B into the region of the photosensitive junction. This leakage effect is avoided in the embodiment illustrated with the improved circuit shown in FIG. 9. In this embodiment, 57 is a direct current voltage source and the bias light is pulsed by a pulse generator 56 intermittently forward biasing a photodiode 58 thus causing intermittent radiation .phi.B to be emitted from 58. Before the bias light is turned on, the grid-to-cathode capacitance C.sub.GC charges to the voltage of source 57 with the junction voltage of photodiode 50 being zero. When the pulsed bias light is turned on, saturation current flows through the phototube and the junction capacitance C.sub.J charges until the grid-to-cathode voltage across capacitance C.sub.GC decreases to a value such that the photocathode 52 current equals the reverse biased junction capacitance C.sub.J discharges by an amount determined by the photodiode current and the interval of time between the bias light pulses. When the bias light is turned on again, an amount of charge equal to that discharged from the junction capacitance flows through the tube as saturation current when an accelerating field is present between the grid and the cathode to focus the photoelectrons. During the integration of the infrared signal current in the photodiode 50 junction, the bias light .phi.B is off and the bias light leakage into the photodiode 50 junction is thus avoided. Leakage of bias light during the pulse time has no effect on operation as long as the phototube current is much larger than the photodiode current due to bias light leakage, a condition which is easily fulfilled.

The photosensitive junction controlled electron emitter of the present invention has the advantage that the resistance of a reverse biased diode junction can be very much larger than the intrinsic resistance of a bulk photoconductor such that the photocathode current is entirely determined by the electro-optical properties of the photosensitive junction, except when the photocathode is operating in the current saturation mode.

1. The defocussing difficulty in any photoconductor controlled photoemitter that arises from the retarding electric field between grid and cathode over most of the dynamic range of operation is readily overcome by making use of the junction capacitance along with low leakage current to operate the junction in the charge storage-pulse readout mode.

2. The speed of response of photosensitive junctions at all signal radiation levels is fast whereas photoconductors are notoriously sluggish at low radiation levels.

3. The use of infrared sensitive junctions to obtain controlled emission of electrons into the vacuum is a very great advantage over the use of infrared sensitive junctions in a camera tube target because electron multiplication can be readily accomplished before storage and electron beam readout and neither beam, load resistor, nor video preamplifier noise will limit sensitivity.

4. In the charge storage pulse readout mode of operation it may be feasible to operate a channel plate multiplier in the high gain-low noise pulse saturation mode since the channel plate voltage may be pulsed in synchronization with the photocathode current pulses to avoid overheating of the channel plate when the channel plate is operating in a high direct current voltage condition.

5. When compared to other solid state image intensifier devices, the construction of the photosensitive junction controlled electron emitter is greatly simplified since no interconnecting wires or scan generators are required for the solid state vidicon and no complex amplifiers at each detector element are required as for the direct view solid state image intensifiers.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alternations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

Claims

1. An electronic imaging device enclosed in a vacuum envelope comprising:

a plurality of reverse biased rectifying photosensitive junctions;

a plurality of discrete electrically isolated photoemitters serially connected with said plurality of reverse biased rectifying photosensitive junctions to form a mosaic of discrete electrically isolated photocathodes to form a composite sandwich structure;

a cathode luminescent phosphor screen;

a bias light source for uniform illumination of the front surface of said plurality of discrete electrically isolated photoemitters for producing electron current saturation therefrom;

an external grid positioned adjacent to and separated from said plurality of discrete electrically isolated photoemitters;

voltage means connected to said fine mesh grid for accelerating electrons emitted from said plurality of discrete electrically isolated photoemitters to said cathode luminescent phosphor screen; and

an electronic focusing means for proximity focusing the photocathode current in the field between said fine mesh grid and said plurality of

2. An electronic imaging device as set forth in claim 1 wherein said plurality of reverse biased rectifying photosensitive junctions are

3. An electronic imaging device as set forth in claim 1 wherein said plurality of reverse biased rectifying photosensitive junctions are

4. An electronic imaging device as set forth in claim 1 wherein said plurality of reverse biased rectifying photosensitive junctions are

5. An electronic imaging device as set forth in claim 2 wherein said bias

6. An electronic imaging device as set forth in claim 2 wherein said bias

7. An electronic imaging device as set forth in claim 5 wherein said

8. An electronic image device as set forth in claim 5 wherein said voltage

9. An electronic imaging device as set forth in claim 6 wherein said

10. An electronic imaging device as set forth in claim 6 wherein said

11. An electronic imaging device as set forth in claim 10 wherein said electronic focussing means is a wire grid electrically isolated photoemitters wherein said wire grid is biased negative relative to said

12. An electronic imaging device as set forth in claim 10 wherein said electronic focussing means comprises operating said photosensitive rectifying junctions in a charge storage pulse readout mode wherein a photogenerated charge is periodically stored in the capacitance between said rectifying junctions during an integration time and is discharged by application of said pulsed voltage means to cause a pulse of photocathode saturation current to flow to said cathodoluminescent phosphor screen until a charge equal to the stored charge has passed through said

13. An electronic imaging device as set forth in claim 12 wherein said pulsed light output from said bias light source is on during the discharge period of said photogenerated charge and is off during the photogenerated charge period.