Junction target monoscope

Abstract

A signal display system having a visual display and a display signal generator in which a semiconductor junction target has a high conductivity P layer, a low conductivity N layer, and a surface layer of insulating material having holes in the shape of letters or other characters. The target semiconductor junction is reverse biased so that when an electron beam striking the target is scanned over the character apertures, it will produce carrier multiplication in the target and an output signal several orders of magnitude greater than a conventional monoscope. The same principle may be used for a camera pickup tube when beam electrons returning from a light sensitive target are multiplied on striking a reverse biased junction target.

Description

BACKGROUND OF THE INVENTION

Conventional monoscopes display systems are well known and comprise a cathode ray tube driven by a monoscope wherein a target electrode of aluminum has a pattern of characters deposited thereon in the form of carbon, which has a different secondary emission characteristic from the aluminum base material.

Under constant use such a monoscope will deteriorate in quality within a few thousand hours, and, in addition, the signal level output from the monoscope is relatively low and changes as the monoscope ages.

SUMMARY OF THE INVENTION

This invention provides for an improved monoscope or light sensitive tube display system using a target comprising semiconductor material. In the monoscope, the target comprises a wafer having a first layer of, for example, silicon, germanium or any other suitable semiconductor material, coated on one side with a conducting layer of metal, such as gold and attached to a base support plate. The other side of the wafer has a layer of low resistivity semiconductor material of the opposite conductivity type from said first layer and is covered by a mask which, at the velocity of the electron beam, is substantially impervious to electron bombardment, and may be of any desired material such as a metal, semiconductor or insulator or combinations thereof. Apertures through the mask expose the semiconductor material, and the shape of the apertures is such that a beam scanning across the apertures will produce an output signal for presentation on a cathode ray tube of the desired character. A back bias is applied to the junction by an external voltage source in series with a load impedance across which the signal output is generated.

Preferably, the target is made of a layer of N type silicon on the order of 10 mils thick having a realtively high resistance such as 10.sup.13 charge carriers per cubic centimeter. One side is coated with a metal conductor and the other side has diffused into it a thin layer of P type impurity, in sufficient concentration to produce a high conductivity P layer, such as for example, 10.sup.19 carriers per cubic centimeter. The P layer is, for example, less than one micron thick. The high impurity concentration in the P layer causes the P layer to act substantially as a conductor so that an additional conductive coating over the P layer is not required but may be used if desired. Rather, a contact made with the P layer and the external circuitry at one point will produce good contact with the entire useful surface of the target. The mask when made, for example, of silicon dioxide is of sufficient thickness, for example, several microns, that the electron beam will not penetrate into the semiconductor body to the region of the junction.

A substantial back bias voltage is applied across the junction in series with an external load so that when the electron beam impinges on the semiconductor, it will penetrate into the junction region where a substantial voltage gradient is produced by the back bias. Electron-hole pairs are generated in proportion to the electron velocity as determined by its accelerating voltage divided by the theoretical voltage required to generate one electron-hole pair. In the case of a silicon target, this is approximately 3.6 volts, and therefore when an electron beam accelerated, for example, by 3600 volts strikes the target an output signal is produced which is approximately 1000 times the beam current. Because of the high output signal possible by relatively high beam voltages, beam current may be reduced, thereby improving definition and permitting reduction of the size of the target. This in turn reduces the output capacitance produced primarily by the target junction region thereby increasing the possible frequency response. A video frequency response extending from substantially 0 to over 100 megahertz is practicable in a display system using such a monoscope.

In a further embodiment of the invention, the semiconductor junction target is positioned on or adjacent to the gun structure and the main target structure is made separate therefrom. The main target structure produces a return stream of electrons modulated with information on the main target structure. The main target structure is, for example, a photoconductive target comprising for example, a layer of antimony trisulfide which is charged by the electron beam and areas of which are discharged by the impingement of a light pattern thereon. The discharged portions of the target accept recharging electrons when scanned by the beam whereas those portions which are not discharged reflect electrons from the beam which strike the junction target. The velocity at which the electrons strike the junction is approximately that produced by cathode to anode potential, and may be, for example, 500 to 5000 volts. The resulting current multiplication produces an output signal across an external load impedance connected in series with an external back bias voltage source across the junction.

The invention provides for an improved display system in which any tube, such as a monoscope, light sensitive tube, informational storage tube, or other tube which can be made to produce an informational signal generated by deflection of an electron beam and by charge carrier multiplication in the tube, supplies an information signal output to a display device such as a cathode ray tube intensity modulated by said informational signal.

Further details and advantages of the invention will be apparent as the description thereof progresses reference being had to the accompanying drawings wherein:

FIG. 1 illustrates a transverse cross-sectional view of a monoscope embodying the invention,

FIG. 2 illustrates a portion of the target electrode structure of the monoscope illustrated in FIG. 1,

FIG. 3 is a transverse cross-sectional of the portion of the target structure illustrated in FIG. 2,

FIG. 4 is a transverse cross-sectional view of a camera tube embodying a further feature of the invention,

FIG. 5 illustrates an elevation view from the end of a gun structure, such as that shown in FIG. 4, and

FIG. 6 is a transverse cross-sectional view of the gun structure shown in FIG. 5.

FIG. 7 illustrates a data information display system embodying the invention.

Referring now to FIG. 1 there is shown a monoscope tube 10 comprising an envelope 11. A cathode 12 heated by a heater 13, a control grid 14, accellerating anode structure 15, focusing electrode 16, vertical deflection plates 17, and horizontal deflection plates 18 are all mounted inside the envelope 11 in a gun assembly of well known construction used in conventional monoscopes. The raster scan 19 feeds deflection voltages to the plates 17 and 18 to position the electron beam on particular positions on a target structure 20.

An electrode structure 20, shown in greater detail in FIGS. 2 and 3, is supported by a metal backing plate 21 on a rod 22 extending through the envelope 11. Attached to plate 21 is a layer of silicon semiconductor material 23 preferably of N impurity type and having a relatively high resistance. For example, layer 23 may be approximately 10 mils thick and have an impurity of approximately 10.sup.13 atoms per cubic centimeter. A junction is formed at 24 in a well known manner by thermal diffusion of P impurity material into the layer 23 to form a relatively low resistance P type impurity layer 25. Layer 25 may be of any desired thickness, but is sufficiently thin that electrons from the beam will readily penetrate to the junction region and for silicon is preferably between 0.1 and 1 microns in thickness. The conductivity of the P layer 25 region is made very substantially greater than the conductivity of the N region 23 so that it will serve as a relatively low resistance conductor to all points of the layer 25. A lead in 26 is connected to the P type semiconductor material and lead 26 extends out to the envelope 11. A back bias is supplied across the junction formed by the portions 23 and 25 by means of a battery 27 applied between leads 26 and 22 through an output load impedance 28.

Characters 29 illustrated in FIG. 2 are formed on the face of the target 20 in the following manner. A silicon dioxide layer 30 is formed over the complete surface of the layers 25 and 23 by heating the target structure in an oxidizing atmosphere. A photoresist is applied over the silicon dioxide, the letters and other characters 29 are put onto the photoresist and the photoresist is developed in a well known manner to remove the portions of the photoresist defining the characters. The portions of layer 30 defining characters 29 are then etched away to expose the P layer portions. The layer 30 is made sufficiently thick, for example, 1 micron thick or less, that substantially no electrons from the beam will penetrate through layer 30. Since the layer 25 is formed by oxidizing the P layer, the P layer is first formed to a thickness somewhat greater than 1 micron and this thickness is reduced by oxidation to the final thickness.

When a suitable voltage of, for example, 500 to 5,000 volts is applied between the cathode 12 and the target electrode 20, electrons will strike the target with sufficient velocity to penetrate the P layer 25 of the target 20 when the electron beam passes over an exposed region of P material, but when the electron beam strikes the layer 30, electrons will not penetrate the P layer 25. The back bias supplied across the junction 24 by the battery 27 produces a voltage gradient in layer 25 extending substantially from junction 24 to the plate 21. As a result when beam electrons penetrate the semiconductor material to the region of junction 24, they produce electron-hole pairs substantially in direct proportion to their electron velocity. For example, a voltage of 3600 volts applied between the cathode 12 and the target 20 will produce approximately 1000 electron-hole pairs in the silicon semiconductor target 20. Because of the voltage gradient in layer 23 produced by battery 27, 1000 electrons would be moved to the positive side of the battery 27 and 1000 electrons would be pulled through the load resistor 28 to neutralize the positive charge carriers moving through layer 23 to the plate 21. Accordingly, a current gain of approximately 1000 times the beam current is produced by this target structure.

Because of this current gain the electron beam current can be much lower than present devices while still producing a sufficient output signal across the resistor 28 to drive an amplifier or even a cathode ray tube display device directly. By reducing the beam current, for example, to the order of 1 microampere or so, the spot size of the electron beam on the target 20 may be made much smaller than with previous monoscopes for the same signal output. As a result, smaller characters can be used while still maintaining the same definition. The net result of using smaller characters is that the total target size on which a given number of characters are positioned may be reduced, and this in turn results in a low interelectrode capacitance between plate 21 and the P layer 25 so that the upper frequency of the output signal in a display system using this monoscope can be much higher than in previous practical systems. For example, the target structure 20 in a practical device can have the portion thereof on which the characters are displayed approximately 3/8 inch across whereas normal monoscope targets structures are an inch or two in diameter.

Alternatively, a larger number of characters may be positioned on a given size of target.

A target area reduction of 4 or 5 to 1 produces a corresponding reduction in interelectrode capacitance, and a high signal current output permits use of a lower load resistance 28 than present systems for the same signal output power. Since the product of the output load resistance and the target interelectrode capacitance varies as an inverse function of the practical upper frequency response limit of the system, reducing this product increases the maximum frequency output. The upper frequency response of the target 20 is also limited by the transit time of a charge carrier through the semiconductor layer 23. The resistance 28 may be reduced if the cathode to anode voltage is increased such that a higher signal current is produced through the resistor 28 due to a higher current multiplication in the target 20. The capacitance may be reduced by making the thickness of the semiconductor wafer 23 greater. However, this increases transit time and therefore a point is reached at which, for practical values of the back-bias voltage produced by battery 27, an upper frequency limit is defined. For example, with a bias voltage of from 100 to 300 volts and a thickness of layer 23 of 10 mils, a transit time permitting operation of frequencies in excess of 100 megacycles is possible.

Referring now to FIG. 4, there is shown a further embodiment of the invention wherein a light sensitive tube generally of the vidicon type is illustrated at 40. Tube 40 comprises a glass envelope 41 and an electrode gun assembly 42 comprising a cathode 43, a heater 44, a control grid 45, and a focusing and accellerating anode structure comprising tubular portions 46 and 47. Envelope 41 has a flat glass faceplate 48 attached thereto by means of a metal ring 49. A transparent conduction layer 50 is deposited on the inside of faceplate 48 in contact metal ring 49. On transparent layer 50 is a layer of photoconductive material 51 such as antimony sulfide. A focus coil 52 is provided to focus the electron beam on layer 51 and a deflection coil 53 is provided to deflect the beam across layer 51, in any desired deflection pattern. Conductive layer 50 is maintained positive with respect to cathode 43 by a small voltage of, for example 10 volts, by means of a battery 54 such that when the electron beam scans across the face of the target it will charge the exposed surface of the layer 51 to approximately 10 volts negative with respect to the transparent layer 50. Light striking the target in the form of an image, for example, through a lens 55 will cause those portions of the conductive layer 51 illuminated to become photoconductive thereby discharging those portions of layer 51. When the electron beam scans across the face of the tube it will recharge those portions of the layer 51 which have been discharged due to the local conductivity produced in layer 51 by the light pattern and this current will flow through the battery 54 to the cathode. The portion of the beam which is reflected from the target 51 by those regions where no light has impinged on the target such that the target is completely charged, is accelerated back toward the gun 42. Because the tube has a magnetic deflection system the electrons returning from the target will return to generally the area of the end of the electron gun but will have a spot size several times the size of the aperature indicated at 56 in the end of the gun 42. The diameter of the gun is made sufficiently large, for example, 1/2 inch in diameter so that substantially all the returning electrons will impinge on a current multiplying target at the end of the gun.

Referring now to FIGS. 5 and 6, there is shown the current multiplying target comprising a semiconductor layer 57 having a hole 56 therein matching the hole in the end plate 58 of the gun 42 through which the electron beam passes. Semiconductor layer 57 is made, for example, of N type silicon approximately 10 mils thick and of relatively high resistance such as 10.sup.13 carriers per cubic centimeter. Diffused in exposed surface of layer 57 is a low resistance layer of material 59 to form a junction 60 in the wafer 57. Conductive ring 61 around the outer rim of the layer in contact therewith, is connected to a signal output lead 62 which extends out through the envelope 41. A lead 63 supplies high voltage of, for example, 500 to 5,000 volts to the anode 47 by means of a battery 64 and is connected through a battery 65 and an output load resistor 66 to lead 62 to provide a back bias across the junction 60. Electrons returning from the target 51 and impinging on the layer 59 at high velocity will pass through junction 60 and produce a current multiplication in accordance with the principles discussed previously in connection with FIGS. 1 through 3. For example, if battery 64 has a voltage of 1500 volts a signal output current of approximately 400 times the beam current reflected from the target layer 51 will be produced through load resistor 66.

From the foregoing, it may be seen that this invention discloses a structure in which information signals produced inside an evacuated envelope may be multiplied by a semiconductor target to produce a wide frequency response low noise output signal. Previous current multipliers inside tubes such as cascaded electron multipliers using a secondary emission produce substantial amounts of noise. In addition, such multipliers have phase distortion since electron transit time varies, and hence the frequency response of such devices is limited. Furthermore, such multipliers are bulky and require a large number of individual voltages to be applied to the cascade stages.

Signal generating tubes used in systems of this invention lower noise and higher output signals with higher frequency response than previous devices. Also, the power supply required for the signal output amplifiers, if indeed such amplifiers are required, need not be as closely regulated with respect to ripple, hum or other interference as is necessary when low level signals are processed resulting in a substantial reduction in cost of the overall system.

Referring now to FIG. 7 there is shown a digital display system embodying the invention wherein the device of FIGS. 1, 2, and 3 or the device of FIGS. 4, 5 and 6 may be used. A cathode ray tube display device 70 has a cathode 71 driven by a video amplifier 72 whose input is driven by the output of a monoscope 10 having a target electrode 20 and a cathode 12. Horizontal deflection plates 18 are driven by an X deflection amplifier 73 and the Y deflection plates 17 are driven by a Y deflection amplifier 74. The Y deflection amplifier is driven by a Y expansion amplifier 75 driven by a 1.18 megahertz squarewave to vertically scan across each individual character and by a Y-D to A converter 76 which vertically positions the monoscope beam in accordance with digital input signals. The X deflection amplifier is driven by a character ramp generator 77 which generates a deflection across the individual character in response to an input synchronizing signal and is also driven by an X-D to A converter 78 which positions the electron beam in the proper position to scan a character in response to input digital signals. D to A converter 76 and 78 are driven by a character entry shift register 79 which supplies character position information to the monoscope 10 from a dynamic storage memory 80 such that the cathode ray tube 70 will continuously display a raster of information based on digital information stored in the memory 80. Expansion amplifier 75 generates a signal which drives a small excursion deflection coil 81 on the display tube 70 in synchronism with similar excursions of the beam of the monoscope.

The position of the beam on the cathode ray tube 70 is determined by vertical deflection coils 82 and horizontal deflection coils 83 which are driven by a Y deflection amplifier 84 and an X deflection amplifier 85 respectively in accordance with synchronizing input signals to produce a normal television type raster scan of the face of the tube 70. A synchronizing pulse supplied to the video amplifier 72 blanks the amplifier during intercharacter deflection periods such that when the beam is scanned from one character to another noise will not be amplified and appear as bright flashes on the face of the screen. The character ramp generator produces a deflection across the face of the cathode ray tube in synchronization with the monoscope horizontal deflection across the character being scanned.

As illustrated herein successive rasters of information may be displayed on the cathode ray tube 70 by being fed from a central computer memory through an input register 86 to character entry shift register 79 and stored in the dynamic memory 80. The information which represent a raster of character positions is then continuously read by register 79 and fed to monoscope 10 to produce characters which are displayed repetitively on the face of the cathode ray tube 73. The particular details of such a data display system are described in greater detail in the previously mentioned Bryden patent application. In such a system incorporating this invention the output from the target electrode 70 may drive the cathode 71 directly without any amplification by a video amplifier 72 if a sufficiently high voltage is supplied between the cathode 12 and the target 20. For example, good results may be achieved with a monoscope of this invention voltage of 3500 volts and output character signals fed directly to a cathode ray tube will have a clarity and brillance equivalent to those produced by conventional monoscopes with amplifiers including preamplifiers may be achieved.

This completes the description of the particular embodiment of the invention illustrated herein, however, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, the system may be used as a television camera and display system, a digital radar or sonar display system, or a facsimile transmission system. Accordingly, the scope of this invention is not limited except as defined by the appended claims.

Claims

1. A signal generating device comprising:

an evacuated envelope;

a source of electrons positioned within said envelope;

a body comprising silicon semiconductor material spaced from said source within said envelope, said body having at least one junction therein providing a lower resistance to charge carrier flow in the forward direction than in the reverse direction;

said junction comprising a layer of P-type semiconductor material diffused into an N-type region of said semiconductor material and extending substantially continuously across the surface of said semiconductor body facing said source of electrons; said P-type layer having substantially less resistivity than said N-type region of said semiconductor material;

means producing charge carrier flow in said semiconductor material comprising electrodes contacting said semiconductor body on the opposite sides of said junction and means directing a beam of electrons from said source toward said junction;

means interposed between said source and said junction for varying the number of electrons of said beam striking said junction; and said means for varying said portion of electrons comprising a layer of insulating material impervious to penetration of electrons from said beam and having apertures therein through which electrons from said beam pass with sufficient velocity to produce charge carrier multiplication in said semiconductor material.

2. The signal generating device in accordance with claim 1 wherein said means for varying said number of electrons further comprises means for varying the deflection of said beam.

3. The signal generating device in accordance with claim 1 and means for producing a back bias across said junction.

4. The signal generating device in accordance with claim 1 wherein said semiconductor layer is attached to a metallic support electrode spaced from said source.