An infrared radiation detector including a cadmium sulfide platelet having a cathode formed on one of its ends and an anode formed on its other end is presented. The platelet is suitably doped such that stationary high-field domains are formed adjacent the cathode when based in the negative differential conductivity region. A negative potential is applied to the cathode such that a high-field domain is formed adjacent to the cathode. A potential measuring probe is located between the cathode and the anode at the edge of the high-field domain and means are provided for measuring the potential at the probe whereby this measurement is indicative of the infrared radiation striking the platelet

Boer, K. W.

Hadley, H. C., Jr.

Robertson, J. B.

Tyagi, R. C.

English

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https://ntrs.nasa.gov/search.jsp?R=19740009975 2020-03-17T14:38:47+00:00Z
United States Patent U9]
Fletcher et al.
[ i i ] 3,790,795
[45] Feb. 5, 1974
[ 54 ] HIGH FIELD CDS DETECTOR FOR
INFRARED RADIATION
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration, with respect to an
invention of; Ramesh C. Tyagi,
Delhi, India; James B. Robertson,
Yorktown, Va.; Karl W. Boer,
Kennett Square, Pa.; Henry C.
Hadley, Jr., Newark, Del.
[22] Filed: July 26, 1972
[21] Appl. No.: 275,118
[52] U.S. Cl 250/338, 250/370, 250/371
[51] Int. Cl GOlj 1/02
[58] Field of Search. 250/83 R, 83.3 H, 211 R, 338,
250/370,371
[56] References Cited
UNITED STATES PATENTS
3,449,705 6/1969 Chamberlin 250/211 RX
3,602,721 8/1971 Nakamura et al 250/211 RX
3,636,354 1/1972 Leheny et al 250/83.3 H
3,340,427 9/1967 Bisso 250/211 RX
3,501,638 3/1970 Compton et al 250/83.3 H
Primary Examiner—Archie R. Borchelt
Attorney, Agent, or Firm—John R. Manning; Howard
J. Osborn; William H, King
[57] ABSTRACT
An infrared radiation detector including a cadmium
sulfide (CdS) platelet having a cathode formed on one
of its ends and an anode formed on its other end. The
platelet is suitably doped such that stationary high-
field domains are formed adjacent the cathode when
biased in the negative differential conductivity region.
A negative potential is applied to the cathode such
that a high-field domain is formed adjacent to the
cathode. A potential measuring probe is located^ be-
tween the cathode and the anode at the edge of the
high-field domain and means are provided for measur-
ing the potential at the probe whereby this measure-
ment is indicative of the infrared radiation striking the
platelet.
6 Claims, 2 Drawing Figures
12
PATENTED FEB 51974 3.790.795
CATHODE ANODE
FIG. 2
3,790,795
HIGH FIELD CDS DETECTOR FOR INFRARED
RADIATION
ORIGIN OF THE INVENTION
The invention described herein was made in the per-
formance of work under a NASA contract and is sub-
ject to the provisions of Section 305 of the National
Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 USC 2457).
BACKGROUND OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a preferred embodi-
ment of the invention; and
5 FIG. 2 is a graph for the purpose of describing the
theory of operation of the embodiment of the invention
in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
10 Turning now to the embodiment of the invention se-
lected for illustration in the drawings, the number 11 in
FIG. 1 designates a cadmium sulfide platelet. Platelet
11 is prepared by a vapor transport method in an N2-
H2S atmosphere and doped by baking at 900°C. for 3
15 hours while buried in a cadmium sulfide powder con-
taining 50 ppm AG and Al as nitrates. This insures that
cathode adjacent stationary high-field domains are
formed when biased in the negative differential con-
ductivity (NDC) region. "Negative differential conduc-
The invention relates generally to radiation detectors
and more specifically concerns a sensitive infrared ra-
diation detector.
The present invention is the result of an effort to look
for a new type of more sensitive solid state infrared de-
tector near room temperature than the usual photocon-
ductor, low band gap semiconductor devices'. It is
known that cadmium sulfide is sensitive to infrared ra-
diation and that the photocurrent can either be en- 20 tivity region" is the region of electric field strength in
hanced or quenched, depending on the wavelength of which the conductivity decreases with increasing elec-
optical excitation. However, the observed effect is usu- trie field strength. To bias in the NDC region is to apply
ally not strong enough to make cadmium sulfide an at- across the platelet a voltage strong enough to produce
tractive infrared detector, since it is caused by shifting NDC in the platelet. A Ti-Al anode 12 is vacuum-
the characteristics of a cadmium sulfide crystal be- 25 deposited on one end of platelet 11 in the form of an
tween the sensitized and non-sensitized state. This shift arc 13 of a circle. At the other end of platelet 11 and
needs, for the quenching operation, a considerable
amount of infrared intensity to unmask the fast recom-
bination path before it can be observed. That is, infra-
red quenching becomes marked only above a certain
threshold intensity which is related to the intensity of
photoexcitation. Though the threshold infrared inten-
sity decreases with decreasing photo-excitation, the in-
30
frared sensitivity decreases at attractive low intensities,
since the cadmium sulfide loss its electronic sensitiza-
tion, hence infrared radiation reduces the current only
very little. This is the major reason why cadmium sul-
fide, though very sensitive to visible light, has not been
used as a sensitive infrared detector.
SUMMARY OF THE INVENTION
This invention is a highly sensitive method and appa-
ratus for detecting infrared radiation in the quenching
range by utilizing stationary cathode-adjacent high-
field domains. Here, in addition to unmasking of the
fast recombination path by partial field quenching, the
change of the width of the high-field domain caused by
infrared quenching is used to detect such infrared radi-
ation. Since the field decreases very steeply at the do-
main edge a potential probe located at this edge can de-
tect shifts in the position of the domain with a very high
sensitivity. This method can detect infrared radiation
with considerably higher sensitivity than the currently
used methods of photocurrent detection.
The apparatus for carrying out the method of this in-
vention includes a cadmium sulfide platelet, with a
cathode and an anode formed on its ends, which is suit-
ably doped such that stationary high-field domains are
formed adjacent the cathode when biased in the nega-
tive differential conductivity region. A negative poten-
tial is applied to the cathode such that a high-field do-
main is formed adjacent to the cathode. A potential
measuring probe is placed between the cathode and the
at the center of arc 13 is a tungsten point cathode 14.
A negative d.c. voltage 15 is connected between cath-
ode 14 and ground, and a current-limiting resistor 16
is connected between anode 12 and ground. Another
sharp tungsten probe 17 is placed between anode 13
and cathode 14 to measure the change in potential dis-
tribution occurring inside the platelet when it is ex-
posed to infrared radiation. A high impedance voltme-
35
 ter is connected between the probe 17 and ground to
measure the potential at probe 17.
It has been found that a negative 2,000 volts as volt-
age source 15 is adequate to provide a cathode adja-
cent high-field domain in the platelet 11. With this volt-
age applied to cathode 14 and with platelet 11 not
being exposed to any infrared radiation, probe 17 is
moved along a radius r of arc 13 from cathode 14 to the.
•anode 12, and voltages are read on voltmeter 18. When
these voltages are read they are plotted on the graph in
FIG. 2 to obtain the solid curve 19. As is noted from
curve 19, the high-field domain extends to the point 20
which is the knee of the curve. At this point, the voltage
drops to a much lower field. When platelet 11 is sub-
jected to a certain amount of infrared radiation, the
curve 21 in FIG. 2 is obtained, and when platelet 11 is
subjected to another level of infrared radiation, the
curve 22 is obtained. Consequently, as noted in FIG. 2,
if probe 17 were placed at the point 20 and if platelet
11 were subjected to any infrared radiation there would
be a sudden drop in the voltage read by voltmeter 18.
This reading on voltmeter 18 would be indicative of the
amount of radiation that platelet 11 is subjected to.
Hence, if voltmeter 18 is calibrated for different levels
of radiation, it can be used to directly read the infrared
radiation that is striking platelet 11 at any given mo-
ment.
The range of intensity measurement can be affected
by altering the position of probe 17 with respect to the
40
45
50
55
60
anode at the edge of the high-field domain, and means _ cathode and anode, and by extending the high-field do-
are provided for measuring the potential at the probe. main edge, which can be controlled by adjusting the ap-
plied voltage 15. Sensitivity can also be altered by
changing the intensity of the band edge illumination.
The potential measured at the probe is indicative of the
infrared radiation striking the platelet.
3,790,795
The advantage of this invention is that it provides a
method and apparatus of measuring infrared radiation
that is inexpensive and more sensitive than previous in-
frared radiation detecting devices.
It is to be understood that the form of the invention 5
herewith shown and described is to be taken as a pre-
ferred embodiment. Various changes may be made in
the invention without departing from the spirit and
scope of the invention as defined in the subjoined
claims. For example, the shape of the platelet 11 could 10
be an isoseles triangle with the cathode placed at the
apex instead of the rectangular shape as shown. Also,
different anode, cathode and probe arrangements
could be made without departing from the invention.
What is claimed as new and desired to be secured by 15
Letters Patent of the United States is:
1. An infrared radiation detector comprising:
a cadmium sulfide platelet;
a cathode formed on one end of said platelet;
an anode formed on the other end of said platelet; 20
said platelet being suitably doped such that stationary
high-Field domains are formed adjacent to said
cathode when biased in the negative differential
conductivity region;
a negative potential applied to said cathode such that 25
a high-field domain is formed adjacent to said cath-
ode;
a potential measuring probe between said cathode
and said anode at the edge of said high-field do-
main; and 30
means for measuring the potential at said probe
whereby said measurement is indicative of the in-
frared radiation striking said platelet.
35
40
45
50
2. An infrared radiation detector according to claim
1 wherein said anode is a Ti-Al electrode on said other
end of platelet in the form of an arc of a circle.
3. An infrared radiation detector according to claim
2 wherein said cathode- is a point electrode on said
platelet at the center of said arc.
4. An infrared radiation detector according to claim
3 wherein said measuring probe is a point electrode on
said platelet.
5. An infrared radiation detector according to claim
4 wherein said negative potential is connected between
said cathode and ground, a resistor is connected be-
tween said anode and ground to limit current, and said
measuring means is connected between said probe and
ground.
6. A method for making an infrared radiation detec-
tor comprising the steps of:
providing a cadmium sulfide platelet;
doping said platelet such that cathode adjacent sta-
tionary high-field domains are formed when biased
in the negative differential.conductivity region;
positioning a cathode at one end of said platelet;
positioning an anode at the other end of said platelet;
applying a negative potential to said cathode to form
a high-field domain adjacent said cathode while
said platelet is not subjected to any infrared radia-
tion; and
positioning an electrode at the edge of said high-field
domain whereby the potential at said electrode is
indicative of the infrared radiation striking said
platelet.
55
60
65


High field CdS detector for infrared radiation

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