US3798451A

US3798451A - Log photodiode

Info

Publication number: US3798451A
Application number: US00282644A
Authority: US
Inventors: E Eberhardt
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1972-08-21
Filing date: 1972-08-21
Publication date: 1974-03-19
Anticipated expiration: 1991-03-19
Also published as: GB1434747A

Abstract

A light sensitive diode comprises an anode including a metal mesh and a wire probe disposed on the inner surface of a faceplate. A photocathode is disposed on a metal plate spaced opposite the anode and faceplate with the wire probe extending from the mesh close to the photocathode. Both anode and cathode have low resistances to permit application of maximum voltage across the gap. The device provides a non-saturating response with output signal current at the anode substantially proportional to the logarithm of input light radiation. Space charge limits, which normally occur at high light levels, are avoided in the area at the tip of the anode wire.

Description

United States Patent 191 Appl. No.: 282,644

Eberhardt Mar. 19, 1974 LOG PHOTODIODE Primary ExaminerArchie R. Borchelt Assistant ExaminerD. C. Nelms t d H. Ebe h dt, W [75] Inven or E ward r at ayne Ind Attorney, Agent, or Firm-John T. Ol-lalloran; Me- [73] Assignee: International Telephone and mi J, L b rdi, J Edward Goldberg Telegraph Corporation, Nutley, NJ. 1

[22] Filed: Aug. 21, 1972 [5 7] ABSTRACT A light sensitive diode comprises an anode including a metal mesh and a wire probe disposed on the inner surface of a faceplate. A photocathode is disposed on a metal plate spaced opposite the anode and faceplate with the wire probeextending from the mesh close to the photocathode. Both anode and cathode have low resistancesto permit application of maximum voltage across the gap. The device provides a non-saturating response with output signal current at the anode substantially proportional to the logarithm of input light radiation. Space charge limits, which normally occur at high light levels, are avoided in the area at the tip of the anode wire.

6 Claims, 2 Drawing Figures [56] References Cited UNITED STATES PATENTS 2,692,948 10/1954 Lion 250/211 2,034,586 3/1936 Long... 250/211 3,641,352 2/1972 Fisher..... 250/213 VT 2,646,533 7/1953 Carne 313/97 3,688,145 -8/l972 Coles 313/99 LOG PHOTODIODE BACKGROUND OF THE INVENTION This invention relates to light sensitive diodes and particularly to a photodiode which has a non-saturating or substantially logarithmic response at high light levels.

DESCRIPTION OF THE PRIOR ART The standard photodiodes used in the past incorporated a planar photocathode on the inner face of a transparent faceplate and a parallel anode spaced therefrom at a predetermined distance at the opposite end of an evacuated envelope. Fixed potentials are applied across the electrodes. For an increasing light input to the photocathode, the output current at the anode increases proportionately until a space charge accumulates between the electrodes which causes saturation and flattening of the response curve. In addition, the output signal current is generally connected to an amplifier which also saturates at high signal levels.

An attempt to avoid this condition included the use of a conical anode having the tip closely spaced from the photocathode on the input glass faceplate of the diode. ln this case, as light input increases, space charge saturation builds up at the larger gap outer peripheral areas of the full cathode diameter and progressively reduces the effective sensitive area. However, due to the high field and smallspacing at the tip of the cone, the space charge limits are theoretically not reached at that area. This configuration was subject to the problem of having a finite photocathode resistance which caused a reduced voltage to be applied across the gap so that high currents at the tip brought the cathode'to the anode potential and resulted in undesired space charge saturation at the tip. Another variation to eliminate the problem of photocathode resistance has also been suggested. This arrangement includes a reversed diode having a conical photocathode on a metal base spaced from a planar metal mesh anode on the input faceplate. Both the anode mesh and photocathode layer thus have low resistances. Input light flux penetrates the openings of the anode mesh to strike the photocathode which emits electrons for collection by the anode. The conical photocathode coating in thiscase however is somewhat difficult to apply.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide an improved simplified photodiode which has a non-saturating or logarithmic response at high light levels.

This is achieved by a novel electron tube configuration including an anode having a metal mesh positioned on the inner surface of an input faceplate and a wire probe extending'therefrom. A planar photocathode is disposed on a conductive metal base plate spaced opposite the anode and faceplate with the tip of the anode wire positioned in close proximity to the photocathode layer. At high input light levels, space charge saturation occurs in the wide gap peripheral area but does not occur at the tip which continues conducting over an extended range of light radiation andcorresponding output signal current with a substantially logarithmic response. Low resistances of the photocathode and anode permit application of maximum voltage across the gap and the planar cathode can be processed in a simplified manner. The two anode portions can also be operated in parallel with combined current or used in separate measuring or amplifier circuits to cover a wider dynamic range. Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a cross-section of a non-saturating photodiode configuration according to the present invention; and

FIG. 2 is a curve showing the improved output current response with input light radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, a source of light 10 is directed toward a transparent faceplate 12, preferably of glass, at one end of an evacuated tubular envelope 14. An anode 16 including a planar metal wire mesh 18 and a separate wire probe 20 are disposed on the inner surface of the faceplate. The mesh 18 extends laterally within the tube across the faceplate and is of sufficient transparency to permit light to pass through the openings of the mesh into the tube toward the photocathode with negligible interference. The anode probe 20 extends longitudinally from the faceplate toward a photosensitive electron emissive coating or layer 22 on a flat metal substrate or base plate 24 at the opposite end of the tube. Suitable potentials, in the order of to 500 volts from a source of direct voltage 26, are applied across the tube between the anode and cathode. The two

anode portions

18 and 20 have separate connections passing through the tube wall and faceplate respectively and are preferably connected externally in parallel circuits to the positive terminal of source 26. The photocathode layer 22 on base 24 is then connected to the negative terminal of source 26.

The spacing between the tip of the anode wire 20 and photocathode layer 22 is in the order of fractions of a millimeter, while that between the anode mesh 18 and layer 22 is in the order of centimeters. The probe may 7 typically be about 1 centimeter in length and a fraction of a millimeter in diameter, while the mesh openings may also be in the fraction of a millimeter range. The anode and photocathode may be a few centimeters in diameter. Use of a metal substrate for the photocathode minimizes resistance and avoids IR losses to permit application of larger voltages across the electrodes. In

addition, the planar cathode provides a simplified structure for use with standard coating processes.

As light flux entering the faceplate increases, the emission of electrons by the photosensitive cathode layer increases proportionately in a substantially logarithmic manner, as does output signal current in the anode which collects the electrons. The generally known Langmuir relation applies wherein the maximum current density is proportional to the three halves power of the voltage applied and inversely proportional to the square of the distance between electrodes, 1,; z (V3 2/d Thus as light and electron emission increases, in the large gap peripheral areas 28 between the mesh 18 and photocathode 22, a space charge of electrons builds up which limits output current and causes a saturating response in those areas. However, due to the relatively high voltage across the small gap between the anode probe and photocathode layer with minimum resistance losses, space charge limits are not reached even at relatively high light levels and conduction continues in a non-saturating manner in the small central area. The configuration also permits operation at much smaller voltages and output signal currents, in the order of milliamps rather than amps, as previously, and thus does not saturate the

external amplifier circuits

30, 32,

34. Non-saturating action is extended in the order of 10 ten times the usual operating range.

FIG. 2 shows an example of response curves of input light radiation in watts on a log scale versus output current in milliamps for a normal photodetector, curve 36, and the improvement provided by the present tube, curve 38. With 100 volts applied between the anode and photocathode of a normal tube, having a photocathode sensitivity of l ma/watt, a substantially linear response occurs until about watts input radiation and 100 ma output current. At this point, the tube suddenly saturates and further light produces no additional current. In the present case, with further input light applied of 100 watts, current conduction continues in a non-linear relation to provide an output current of 200 ma, while at 1000 watts there is an output of 300 ma, without saturation.

An additional advantage of the split anode configuration, having a separate mesh 18 and wire probe 20, is that the two portions can be connected in parallel through

individual amplifier circuits

30, 32 if so desired, or combined in series with a common amplifier circuit 34. The parallel operation, using two separate current measuring circuits with different currents, may be advantageous in avoiding saturation of the individual amplifiers since two amplifiers can cover a wider dynamic range than a single one.

The present invention thus provides a novel improved photodiode construction which can operate in a non-saturating manner at substantially higher light levels. While only a single embodiment has been described and illustrated, it is apparent that other variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A light sensitive diode comprising:

an evacuated envelope,

a light transparent faceplate at one end of said envelope,

a metal base positioned within said envelope at the other end,

an anode disposed at the inner surface of said faceplate, said anode including a metal mesh extending laterally across said envelope substantially parallel to said faceplate and a wire probe extending axially from said faceplate toward said other end,

a photocathode layer disposed on said metal base,

the extending end of said wire probe being positioned in close proximity to said photocathode at a predetermined spacing sufficient to avoid space charge limits therebetween, and

a source of direct voltage applying a potential between said anode and photocathode layer.

2. The device of claim 1 including means connecting said wire probe and metal mesh to one terminal of said direct voltage source, the other terminal being connected to said metal base and photocathode.

3. The device of claim 2 wherein said anode mesh and photocathode layer are disposed respectively on said faceplate and base on opposite planar parallel surfaces within said envelope and the other end of said wire probe extends externally through said faceplate.

4. The device of claim 3 including first circuit means connected between said one terminal and said wire probe and second circuit means connected between said one terminal and said metal mesh.

5. The device of claim 3 wherein said potential and spacing between said wire probe and photocathode provide continued current conduction therebetween 'with a substantially non-saturating logarithmic retenth that between said anode mesh and photocathode. i 8

Claims

1. A light sensitive diode comprising: an evacuated envelope, a light transparent faceplate at one end of said envelope, a metal base positioned within said envelope at the other end, an anode disposed at the inner surface of said faceplate, said anode including a metal mesh extending laterally across said envelope substantially parallel to said faceplate and a wire probe extending axially from said faceplate toward said other end, a photocathode layer disposed on said metal base, the extending end of said wire probe being positioned in close proximity to said photocathode at a predetermined spacing sufficient to avoid space charge limits therebetween, and a source of direct voltage applying a potential between said anode and photocathode layer.

5. The device of claim 3 wherein said potential and spacing between said wire probe and photocathode provide continued current conduction therebetween with a substantially non-saturating logarithmic response of output current over an extended range of light radiation input after space charge build up between peripheral anode and photocathode areas upon the occurrence of a predetermined light input.

6. The device of claIm 5 wherein the spacing between said wire probe and photocathode is in the order of one tenth that between said anode mesh and photocathode.