FAST FIELD EFFECT AND PHOTORESPONSE STUDIES ON LEAD SULFIDE PHOTOCONDUCTORS

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• Collection: General CIA Records
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• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 50
• Document Creation Date: December 23, 2016
• Document Release Date: January 16, 2014
• Sequence Number: 11
• Publication Date: May 15, 1958
• Content Type: REPORT

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NAVOFtD REPORT
6164
STAT
r--
FAST FIELD EFFECT AND PHOTORESPONSE STUDIES ON LEAD SULFIDE
PHOTOCONDUCTORS
15 MAY 1958
L.
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NAVORD Report 6164
FAST FIELD EFFECT AND PHOTORESPONSE,
STUDIES ON LEAD SULFIDE PHOTOCONDUCTORS'
ABSTRACT: Concurrent studies of field effect and photoconduct-
ivity in thin-film lead-sulfide photoconductors have yielded a
better understanding of the mechanism of photoconduetivity
involved. Two problems were of particular- camerng; ((a.)) they
validity of the assumptions of the majority carrier model of
photoconductivityl, and (b) the role of the electronic: traps
probed by field effect in the photoconductive proe:eilf-Se
Experimental data from chemically deposited: lead sulfide': films'
confirm the existence of electronic traps on the surface and/Or
? in the space charge region and demonstrate for the first time the
identity of the field effect and photoresponse time constants.
a A beat frequency bridge technique applied to the fast field
effect measurements permitted heretofore Impossible accuracy,
particularly if the samples are of high impedance. The majority
carrier model was extended, by including explicitly the time for
charge transfer into and out of the majority carrier traps, to
describe the field effect data. Analysis of the data shows the
rate of transfer of charge into the majority carrier traps to
be the limiting process. Density and capture-cross-section of
the majority carrier traps were estimated.
Principle conclusions are that (a) the assumptions of the
majority carrier model are valid and (b) the electronic traps
probed by field effect are a representative sample of those
which cause photoconductivity. The latter conclusion - mean's'
that field effect measurements will be a useful taal for fUrther
classification of the mechanisms for photoconductivity in thin-
film photoconductors of the lead salt type.
U. S. NAVAL ORDNANCE: LABORATORY
WHITE OAK., MARYLAND
STAT
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NAVORD REPORT 6164 15 May 1958
This report describes an experimental and theoretical study of
the fundamental mechanism of photoconductivity in lead-sulfide
thia-film photoconductors. The study is a part of a research
program at the Naval Ordnance Laboratory for the purpose of
better understanding the photoconductive process in lead salt
photoconductors. Experimental data consist of a series of con-
current photoresponse and field effect measurements. The
principal achievement of the experiment was the substantial
confirmation of a majority carrier model 1 previously conceived
in this Laboratory for the description of photoconductivity in
the lead salt type thin-film photodetector. Theoretical contri-
butions include (a) the extension of the majority carrier model
to include explicitly electronic trapping on the surface and in
the space charge region and to describe field effect, and (b) an
analysis of experimental results which provides for the first
time estimates of values for such fundamental properties as
effective capture cross section and densities of the traps which
cause photoconductivity in the lead sulphide films.
ii
741,
NAVCRD Report 6164
TABLE OF CONTENTS
Chapter Page
I. INTRODUCTION 1
A. Objectives 1
B. Summary of Previous Investi-
gations 1
1. Engineering and Scien-
tific Interest 1
2. Physical Properties and
Fabrication Techniques. 2
3. Photoconductivity in the
Lead-Salt Type Detectors 2
4. Electronic Surface States
C. Approach
5
8
II. EXPERIMENT 10
A. Apparatus 10
1. Film Samples 10
2. Sample Holder 10
3. Sample Housing 10
4. Balanced Bridge 11
5. Beat-Frequency Bridge. 11
B. Procedure.,...,...,12
1. Transient Field Effect
and Photoresponse
Measurements 12
2. Steady State ac Field
Effect and Photoresponse
Measurements 12
C. Discussion of Measurement
Methods 14
Balanced Bridge 14
2. Beat-Frequency Bridge. 15
D. Experimental Data 16
1. Transient Field Effect
and Photoresponse Data. 16
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Chapter
NAVORD Report 6164
Page
2. Steady State Field Effect
and Photoresponse Data. .
17
3. Field Effect Mobility
?
Product ? ?
17
4. Summary of Experimental
Data
18
III.
THEORY
20
A. Empirical Equations Developed
by RC Circuit Analogies ? ? ?
20
1. General
20
2. Description of Experimental
Field Effect
20
3. Description of Photo-
response
22
B. Theoretical Equations Based on
Majority Carrier Model. .
24
1. Statement of Problem .
24
2. Film Conductance and
Resistance
24
3. Photoconductivity
26
At. Field Effect
30
C. Comparison of Empirical and
Theoretical Equations . ? ?
39
1. Photoresponse
39
2. Field Effect
40
IV.
INTERPRETATION OF EXPERIMENTAL
DATA
41
A.
General
41
B.
Majority versus Minority Car-
rier Lifetime
41
C.
Rate Limiting Process
42
D.
Estimates of Trap Densities
and Capture Cross-Sections. .
42
E.
Location of Photoconductive
Traps
48
V.
CONCLUSIONS
56
A.
Experimental Contributions . .
56
B.
Theoretical Contributions. . .
56
C.
Summary of Conclusions . ? ? ?
57
D.
Recommendations for Future Re-
search
58
iv
a
NAVORD heport 6164
Page
Acknowledgments 59
Appendix
1.
2.
3.
4 ?
Balanced Bridge Equation
Beat-Frequency Bridge Equations
Check on Ohmic Nature of PbS
Photosensitive Films
Exploratory Measurements Using
Balanced Bridge.
60
60
62
67
67
Bibliography 70
ILLUSTRATIONS
Figures
1.
Envisaged Microscopic Structure of
PbS Thin Film Photosensitive
Samples
73
2.
Typical Semiconductor Surface
73
3.
Energy Levels in a PbS Photo-
conductive Film
74
4,
Outer Surface of a PbS Film
74
5.
Components for Photo Response and
Field Effect Measurements
74
6a,
Assembled Sample Holder
75
6b.
Dismantled Sample Holder
75
7a.
Assembled Sample Housing
76
7b.
Dismantled Sample Housing
76
8.
Balanced Bridge Setup
77
9.
Beat Frequency Bridge Setup
77
10.
Example of Calibration of Beat-
Frequency Bridge Setup
78
11.
Universal Response Curves
78
12.
Typical Oscilloscope Pattern of
Response to Square Wave Field
Effect Voltage
79
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Page
Figures
Page
Figures
29.
Simplified Balance Bridge Circuit
86
13.
Photo and FE Response to Pulse
30.
Equivalent Circuit for Balanced
Signals
79
Bridge with Differential Amplifier
14.
Balanced Bridge Data on Time
Loading
86
Constants
79
31.
Circuit for Field Effect and Photo-
15.
Field Effect Response EK Co.
Conductive Response Measurements
Sample Cell 1-B Uncoated
80
with Beat Frequency Bridge
87
16.
Photoconductive Response-, EK
Co Cell 1-B Uncoated
80
32.
33.
Linearity Check
Check of Ohmic Property
87
87
17.
Field Effect Response EK Co.
34.
Equivalent Circuit of Beat-
Cell 1-A Plastic Coated
81
Frequency Bridge Setup
88
18.
Photoconductive Response, EK
Co Cell 1-A Coated
81
35a.
Oscilloscope Pattern Showing Ap-
parent Space Charge Layer Inversion
88
19.
Field Effect Response of
Uncoated Film
82
35b.
Oscilloscope Pattern Showing Photo-
conductive Response During Apparent
20.
Field 2ffect Response of Glass
Substrate Surface
82
Space Charge Layer Inversion
88
21.
Photo Response Time vs. Back-
Ground Illumination
82
22.
Field Effect Response for
Insensitive Film
83
23.
Dependence of Charge Trapped
in Surface States on Surface
Conditions
83
24.
Typical Relative Respons9 to
a Sinusoidal Field-Effect
Voltage
83
25.
RC Analogue of FE Response
84
26.
RC Analogue of Photo Response.
84
27.
Pictorial Representation of
Role of Electronic Surface
States in Field Effect Exper-
iments
85
28.
Transient Field Effect
Response
85
vi
4
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NAVORD Report 6164
I. INTRODUCTION
A. Objectives
The overall problem is to identify the
fundamental mechanism of photoconductivity in
thin-film infrared detectors of the lead salt
type. The specific objectives are to determine:
(a) the role of electronic states probed by
field effect in the photoconductive process and
(b) the validity of the majority carrier model'
for describing photoconductivity in the films.
B. Summary of Previous Investigations
1. and
Thin film p oto etectors, sometimes cal e
photoconductors or photocells, are of consider-
able interest to the military and to industry.
Their remarkable sensitivity, fast response
time, small size, convenient geometry, light
weight, and high gain are all desirable
properties. Therefore, thin-film detectors
are now produced in large quantity for use in
a wide variety of scientific, military, and
industrial instruments... There is also con-
siderable scientific interest in the properties
of these detectors. Extensive bibliographies
in review papers 2, 3, 4, 5 show the quantity
of experimental data and number of theories
concerning the physical, electrical, and
optical properties of the films. However, in
spite of the intense academic and industrial
interest, particularly during the past decade,
the identification of the fundamental mechanism
1
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NAVORD Report C164
for the observed photoconductivity in the
lead salt type of film remains one of the
tantalizing problems of photoconductor theory2
and the films are still fabricated by
empirically developed processes which are not
completely understood. The identification and
understanding of this mechanism has obvious
practical as well as scientific value, e.g.
novel and improved photocells can then be
engineered with properties required for
specific applications.
2. Physical Properties and Fabrication
Techniques: Physical properties and fabrica-
tion techniques are particularly important
when considering the mechanism for photo-
conductivity. Films are a composite of
microscopic crystallites formed by either
vacuum evaporation or chemical deposition of
the lead-salt to a thickness of about one
micron on a glass substrate. The chemically
deposited films are sensitized, i.e. rendered
photosensitive, during deposition. This
sensitization may be optimized after deposition
by baking the film in air or oxygen. Vacuum
evaporated films are sensitized by exposure
to oxygen either during or after the condensation.
According to 6-ray and electron microscope
observations, the crystallites have overall
dimensions of the order 0.1 micron. and are
separated by intercrystalline barriers with
thicknesses of the order of 10 A?, (see Fig. 1).
The individual crystallites are lead salts,
while the intercrystalline barriers are an
oxide of the lead salts or of lead. The
microscopic dimensions afford a relatively
large surface to volume ratio, thus emphasizing
the role which surface states might play in the
photoconductive process.
3. Photoconductivity in the Lead-Salt Type
Detectors: Although photoconductivity was 8
discovered in selenium by W. Smith in 1873,
the marked photoconductivity of the lead-salt
2
S.
NAVORD Report 0.64
type mategials was not discovered until 1917
when Case studied oxygen treated thallous
sulfide which he named thallofide. According
to Rose12 photoconductivity was not interpreted
in terms of the lifetime of a free carrier and
the capture cross section of a bound state for
free carriers before abouA 1945.h The excellent
review papers by Riqner, Moss,' t Smith15 and
Petritz and Humphrey.") reveal the numerous
theories and models proposed between 1945 and
1957 for describing photoconductivity in these
films. The experimental effort was correspond-
ingly large. Research on the electrical and
optical properties of the lead-salt type films
will now be summarized by considering the
fundamental processes involved in a manner
similar to that of Petritz and Humphrey .10,11
The principal processes of photoconductiv-
ity are the generation and the recombination
of free carriers. The generation of free
carriers by the absorption of photons was
shown to be a main band transition la the
agreement of the thermal energy gap-L. with.
optical transition energy determined from ttle
long wavelength limit of photoconductivity..0
Since the optical transitions are main band
ones, they occur in ttie crystallites rather
than in the barriers.." Having established the
process of generation, a significant remaining
problem is to establish the process of re-
combination. The prior research on this
problem will be classified and discussed under
one of three categories depending on the
carrier lifetime of importance. The first
category is the intrinsic carrier model in
which recombination is either by recombination
centers or directly between bands. Both the
number and the lifetime of free electrons and
holes are approximately equal, a condition
presumably achieved by the sensitization
process.,hThe early theory of von Hippie and
Rittner,4.4' which is typical of this category,
proposes photoresponse to be characteristic
3
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NAVORD Report 6164
of a uniform intrinsic semiconductor produced
by detailed balancing of n and p-type im-
purities. A basic deficiency of this model is
the failure to predict the measured small ratio
of film to single crystal mobilities.2 The
second category is the minority carrier model
in which photoconductivity depends on a long
minority carrier lifetime. The film is
visualized as a composite of microscopic n-p-n
junctions. In a thecry for this model
developed by Slater,-LD barrier modulation plays
an important role. Barrier modulation theory
has been successful in predicting certain
electrical,p,nd photoconductive properties of
PbS films.i ? The third category is the
majority carrier model in which photoconductivity
results from the predominance of the majority
carrier. The free minority carrier created by
the photon is thought to be trapped, leaving
the corresponding majority carrier free to
conduct. Petritzl developed a theory for this
model w4ich has been successful in predicting
noise 14 responsitivity, sensitivity0.1-0 Hall
coefficient, and resistivity19 measurements on
PbS films.
In identifying the photoconductive processes
in the lead-salt type films, history has
demonstrated that while success in describing
the experimental data at hand is necessary, it
does not lead to a unique theory. Rose2u con-
cludes that a unique deduction of the mechanism
of photoconductivity by the study of a par-
ticular substance is hardly possible, and
Slater15 points out that more than one mechanism
might be present simultaneously in PbS. In
spite.of these inherent difficulties, certain
recent experiments furnish strong evidence as
to the appropriate model. Humphrey and Scanlon21
reported studies on evaporated PbS films which
showed that although other oxidizing elements
formed acceptor sites in the film, only oxygen
produced photoconductivity at room temperature.
If either the intrinsic or minority carrier
?
a
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NAVORD Report 6164
models were applicable, any acceptor element
should produce photoconductivity. A plausible
interpretation is that oxygen was the only
acceptor element which formed the minority
carrier required by the majority carrier model.
Also, Woods19 concluded from his measurements
of resistivity and Hall coefficient of il-
luminated chemically-deposited PbS films, that
photoconductivity is entirely due to the in-
crease in the number of carriers and that
barrier modulation does not occur.
The theory' for the majority carrier model
does not specify where the minority carriers
are trapped. The mean free path of the minority
carriers is sufficiently long relative to the
dimensions of the crystallite2 to permit trap-
ping in either the surface or bulk states.
Estimates by Petritz22 show that the density
of the surface traps, i.e. bound electronic
states on the surface of the crystallite, is
adequate to account for the observed photo-
conductive time constants. If the trapping
cross section of the surface states for the
minority carriers is sufficiently large, the
surface instead of the bulk traps could pre-
dominate in the photoconductive process.
4. Electronic Surface States: Although
there is iiiTre-Iiithe literature concerning
the electronic surface states of PbS, there
is considerable relevant information on other
materials in the review article by Kingston23
and in Semiconductor Surface Physics.24 The
concept g surface states was first proposed
by Tamm,, who in 1928 showed theoretically
that an abrupt discontinuity of the crystal
at the surface allows an energy level within
the forbidden energy gap of the crystal. In
1939 Shockley2? showed theoretically that pairs
or bands of energy levels exist on the surface
of semiconductors (he consldered diamond) and
all metals. In 1947 Bardeen2f showed that
electronic states on the surface of semi-
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conductors could, under favorable conditions,
control the electrical properties 4, the
material. Numerous experiments2), 24 since the
advent of the transistor have confirmed
Bardeen's postulate as applied to germanium
and silicon. As a result of these experiments,
an electronic energy level model of the semi-
conductor surface of the type shown in Fig. 2
is now generally accepted.
In the studies on germanium and silicon
surfaces the bound states have been classed
as "slowt or "fast" depending on the time
required for net transfer of charge between the
surface states and the bulk. Slow states have
time constants in the range from milliseconds
to minutes and are visualized as resulting from
acceptor or donor sites located either on
or/and in the high resistance oxide layer which
coats the semiconductor crystal. Estimates
based on experimental data indicate that the
densities of slow surfacA states are in the
range of 101g to 1014 c111. These states are
strongly affected by ambient atmosphere. The
fast states have time constants of the order
of microseconds and are believed to be located
on or near the oxide-semiconductor interface.
They are often called interface states. These
states may result. from several sources such as
the abrupt discontinuity in crystal structure,
crystal defects, and foreign acceptor or donor
atoms or molecules at the crystal surface.
Fast states seem to be relatively insensitive
to ambients and their densities are estimated
to be in the range of 1011 to 1012 cm2.
Recent experiments have shown that
electronic surface states exist on PbS films
in much the same manner as they exist on
germanium and silicon. Zeme120 reports slow
states with time constants of the order of
several minutes. Rezewoski and Sosnowski29
found that electronic states exist on the
surface of 'PbS films which can affect the
6
NAVORD Report 6164
electrical conductance. Sorrows3? reported
fast field effect studies on a PbS film which
not only confirmed the findings of Rezewoski
and Sosnowski, but showed that the field effect
time constant and the photoresponse time con-
stants were approximately equal. Methods of
sensitizing, particularly in the case of the
vacuum evaporated films, suggest that surface
states play an important role in establishing
both the electrical and photoconductive
properties. Resistivity and thermoelectric
power measurements show that films formed by
vacuum evaporation are "n" type prior to
sensitization. Upon sensitization by exposure
to oxygen the films are "p" type. Conceivably,
the change in electrical and optical properties
Is caused by surface states formed during the
exposure to oxygen. The manner in which the
surface states could change the electrical
properties of the film is explained by use of
Fig. 2. The presence of oxygen on the
crystallite surfaces creates accepter sites
which trap electrons. The electrostatic field
from these trapped charges causes the indicated
bending of the electron energy level bands near
the surface. In fact if sufficient charge is
trapped, an inversion layer is created, i.e. .
the region near the surface is changed from
"n" to "p" type. The spacecharge regions in
PbS crystallites have not been studied experi-
mentally; however, theoretical calculations
indicate that the region extends about 0.07
micron into the crystallite.2 The space charge
region of the small crystallites of the PbS
films could control the electrical properties
of the film. In the case of chemically
deposited films of the type used in this study,
considerable sensitpation occurs during the
deposition process.' The crystallites which
compose the film are then thought to be "p"
type with an accumulation layer at the
sUrface.31. This concept is illustrated in the
energy level diagrams (Figs. 3 and 4) for a
-PbS photoconductive film.
7
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Trapping in fast surface states on
germanium has been studied by field effect
measurements. This measurement probes only
those states which are on the surface or within
the space charge layer of the surface.
According to the energy level diagram for the
PbS photoconductive film surface shown in Fig.
4, the states which exist on the surface
crystallites of the film are a representative
sample of those on the surfaces of all the
crystallites which compose the film (Fig. 1).
Field effect measurements should then provide
information concerning the nature of the sur-
face state and space charge region traps of
the crystallites.
C. Approach
In view of (a) the success of the majority
carrier model' in describing both the electrical
and photoconductive propertAss of the lead-salt
films and (b) the suggestion that electronic
surface states might be the traps required by
the majority carrier model, it was decided to
further investigate the basic assumptions of
the majority carrier model and the role of
electronic surface states. To do this a series
of concurrent field effect and photoresponse
measurements were made on PbS films. Two
fundamental differences in field effect and
photoresponse are pertinent in this study.
First, .only majority carriers are induced into
the film in field effect measurements, whereas
hole-electron pairs are created in the film by
the photons in the photoresponse measurements.
Second, only the surface and space Charge
region traps are probed in field effect,
whereas surface, space charge, and bulk traps
affect the photoresponse.
The main components for the study are
shown in Fig. 5. In field effect measurements,
a voltage on the field effect plate causes an
8
NAVORD Report E164
external electric field transverse to the film
surface. This field is superimposed on that
from the surface state charge. Tha external
field then affects the electrical properties
of the space charge layer in much the same
manner as do the trapped surface charges. The
field effect measurements can provide the time
constant V; associated with the charge transfer
between the space charge region and the surface
traps, the fraction oc. of the total external
field lines terminating on immobile charges
at quasi-equilibrium, and the mobility ,a4
of the excess charges induced into the film.
In measuring photoresponse, the change in
electrical conductance caused by incident light
pulses is determined as a function of either
time or light pulse repetition rate. The
measurement yields D, 0 the photoresponse time
constant. Transient and steady state field
effect and photoresponse were measured as a
function of background illumination, film
surface condition, and sensitivity of the film.
PbS films were chosen for the study be-
cause (a) they are typical of the lead-salt
type photodetector, (b) the art of fabrication
is far more advanced than for other members of
this class of photodetector, and. (c) there is
already a considerable accumulation of informa-
tion concerning the electrical and photo-
conductive properties of PbS in both the film
and bulk form which aids in interpretation of
experimental data.
9
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NAVORD Report 6164
II. EXPERIMENT
A. Apparatus
1. Film Samples: With the exception of
one sample furnished by the Naval Ordnance
Laboratory, samples (similar to that shown as
an insert in Fig. 5) were furnished by Eastman
Kodak Company. The films were formed by
chemical deposition on a glass substrate and
some of the samples had the free surface
coated with varnish. The Eastman Kodak Company
films were sensitized when received. A film
furnished by NOL had purposely not been sen-
sitized and the surface was uncoated.
2. Sample Holder: The holder, Figs.
6a and b, was made from teflon stock. When
assembled with the sample properly mounted,
one surface of the film surface was parallel
to the solid brass field-effect plate. The
other surface was exposed to both the controlled
background illumination and to the pulsed light.
Most of the measurements were made with the field
effect plate on one side of the film and the
light sources on the opposite side'. A perforated
field-effect plate, fabricated by pressing a
fine mesh wire into a heated transparent sheet,
Permitted a series of measurements with the
field effect plate andthe light sources on the
same side of the film.
3. Sample Housing: The housing shown in
Figs. 7a and b was made of heavy brass stock to
afford a large heat capacity and could be sealed
off from the atmosphere. This design was chosen
10
ft ?
/MORD Report 6164
to prevent rapid fluctuations in both film
temperature and ambient during measurements.
The housing consisted of two sections separated
by a fine mesh screen; the sample holder was
firmly mounted in the bottom section, and the
top section contained the background illumination
and the pulsed light sources. A standard
incadescent lamp (6-8 volt Mazda 82) was used (g5)
and its intensity was selected by adjusting
the de voltage across the lamp terminals. The
source for the light pulses was a 0.4 watt
neon glow tube (NE-2).: Electromagnetic
coupling between the circuitry associatedwith
the sample and that associated with the light
sources was prevented by use of the fine mesh
screen.
4. Balanced Bridge: The commercial titles
of the principal electrical instruments used in
the balanced bridge ihown in Fig. 8 are is
follows:
Function
Stabilized Voltage Supply
Voltage Pulse Generator
Differential Amplifier
Cathode Ray Oscilloscope
Micro Amp Meter
Commercial Title
Lee Model R-10
Tektronix Type 105
Tektronix Type 122
Tektronix Type 512
Weston Model 625
5. Beat-Frequency Bridge: The commercial
titles of the principal electrical instruments
used in the beat-frequency bridge shbwn in Fig.
9 are as follows:*
Function
Sq. Wave Voltage Gen.
Audio Signal Gen., Freq. b
Audio Signal Gen., Freq. f
Vacuum Tube Voltmeter, VID"
Vacuum Tube-Voltmeterl,
Wave Analyzer
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11
Commercial Title
Tektronix 105
(Hewlett Packard
Model 205 AG'
General Radio 1800 A
Ballentine
General -RadicW736A or
H.P. 300A
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NAVORD Report 6164
B. Procedure
1. Transient Field Effect and Photo-
response Measurements: Transient changes in
film conductance caused by both square wave
field effect voltages and pulsed light signals
were observed with the balanced bridge shown in
Fig. 8. The square wave voltages used to drive
the field effect plate and the neon glow tube
were purposely made asymmetrical in time in
order to establish the relative signs of the
respective bridge responses. The initial step
for field effect measurement was to balance the
bridge with no film current Is and a square
wave voltage Vf applied to the field effect
plate. This was done by alternately adjusting
R1 or R2 and the trimmer capacitors (labeled
1.5 - 7.5"gdY) until the differential output
voltage across the film terminals was a minimum.
When the bridge was balanced and a current
was in the film, the field induced transient
conductance change AG caused a transient
voltage Z0/0 across the film terminals. PY0
was amplified by the differential amplifier
and displayed on the cathode ray oscilloscope.
LiVn and 4NG are related as follows
(derivation in Appendix A).
- ?Ave, Mi.+ Rz+
G
Is R102,4- Rz) 1
where R is the film resistance. Equation 1 is
valid for photoresponse as well as field effect
measurements. It is not necessary to balance
the bridge for the photoresponse measurements.
Since AGis a linear function of EA/0, the
field effect and photoresponse time constants
Z7f and t",. respectively were estimated
directly from the-oscilloscope patterns.
(1)
2. Steady State Field Effect and Photo-
response Measurements: Steady state ac field
effect and photoresponse were both measured on
12
NAVORD Report ?164
the beat-frequency bridge shown in Fig. 9. The
bridge was biased with a sinusoidal voltage
Vb of frequency b and then balanced by alternatelir
adjusting R and C until the wave analyzer tuned
to b indicated a voltage minimum. Application
of either a sinusoidal field effect voltage
Vf at frequency f or a pulsed light at frequency
I caused a resistance modulation A R at frequency
f which beat with the bias voltage. The wave
analyzer measured the beat frequency voltage
Vw when tuned to either f+b or f-b, where
w= f+b or f-b. In order to compare readily the
field effect measurements on the various films
studied, a normalized factor Ff, the fractional
change in film resistance per field-effect volt
at frequency f, was calculated for each measure-
ment from the following equation:
F - ? (2)
4 fa V V V
; b
In photoresponse measurements, the fractional
change in film resistance for a pulsed-light
of given intensity was calculated from
62 L. V
_
R Vb
(3)
Equations 2 and 3 are derived in. Appendix2;
Vw, Vf and Vh are rms values. The loading of
the film resistance by the bridge and wave
analyzer circuits is accounted for by the factor
14 and had to be determined for each film and
for each selected background illumination
intensity by an experimental technique
described in Appendix 2. Because of its .
frequency dispersion, see Fig. 10, the value
of Lw was chosen in each measurement for the
frequency corresponding to VW.
The selected background illumination
intensity and bridge bias frequency were held
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constant while both the field effect and photo-
response were measured at a number of selected
frequencies covering the range from 20 to
15,000 cps. The response Ff and AR/R of each
frequency run were plotted on log-log paper and
compared directly with-the corresponding
universal curves of Fig. 11. From this com-
parison, a, IT, and Tn were obtained directly;
jut was calculated fr6m the experimental data.
C. Discussion of Measurement Methods
1. Balanced Bridge: Although the
balanced bridge method hasbeen widely used in
surface state stUdiesfor.both transient and
steady state field effect measurements, the
method is not entirely satisfactory for
quantitative fielcr=effebt-measurements. The
inherent difficulty is that a small response
voltage must be measured in the presence of a
large capacitively induced voltage at the same
frequency or frequencies. In this particular
experiment, the induced voltage was of the
order of 125 volts and the response voltages
were only a few millivolts. This large ratio
of voltages necessitates a very precise balance.
In spite of the care taken in mounting the
sample securely, in the prevention of thermal
and ambient changes, and in wiring the bridge
circuit for minimum unbalanced stray capacitance
and lead inductance; the balance was both tedious
to achieve and subject to small but disconcerting
drifts. The large resistance of the films and
the lack of common mode range of the commerciaDy
available differential amplifier also contributed
to the inherent difficulties of the measurement
method. A differential amplifier with in-
creased common mode range could have been con-
structed using already established techniques,
but at the sacrifice of the high frequency
amplification necessary to measure the field
effect transients.
11E
NAVORD Report 6164
In the transient measurements, the mag-
nitude of the resistance of the bridge arms
(111 and R2) was relatively small, of the order
of 10K ohms or lessyto permit charging of the
condenser formed by the field-effect plate and
film in less than a microsecond. This relatively
short charging time was necessary in order to
avoid masking field-effect transient responses,
the time constants of which were as short as
100 microseconds. The condenser charging time
was that required for the charge to flow
through the parallel resistance of R1 and Rp.
The relatively small resistance in the bridge
arms reduced the bridge sensitivity consider-
ably below optimum.
The balanced bridge is also often used in
steady-state ac field-effect measurements.
All of the information contained in the
transient response can be obtained from a
measurement of steady-state response versus
field-effect frequency. To obtain the informa-
tion, response must be measured at each of a
number of frequencies coveringthe4requency
band of interest. Several such frequency runs
were made. The balancing at each frequency
was found tedious and the small higher
harmonics of the field effect frequency
source were annoying.
Both transient and steady state photo-
response measurements can be made with the
balanced bridge setup without balancing the
bridge: These measurements are thus simple
and reliable.
2. Beat-Frequency Bridge: The beat-
frequency bridge, which was applied to field-
effect measurements for the first time in this
study, is well suited for quantitative steady
state ac field-effect measurements. This
bridge circumvents the basic difficulty of
balanced bridges by measuring the field effect
response voltage at a frequency different from
the applied field effect or bias voltages.
15
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The beat-frequency bridge operation is based
on the field-effect modulation of the film
resistance which beats or mixes with the bias
voltage to produce sum and difference frequency
voltages. There is no inherent need for bridge
balancing. However, in this experiment, the
bridge was sufficiently balanced to prevent
the bias voltage from saturating the wave
analyzer. The degree of balance required was
easy to achieve and maintain. Small drifts
off balance, which were disastrous in the
balanced bridge measurements, caused negligible
difficulty in the beat-frequency bridge. The
principal requirements in the beat-frequency
bridge measurements were that the field effect
and bias frequencies be sufficiently stable to
permit the sum or difference frequencies to
remain within the pass band of the wave
analyzer, and that the harmonic content of the
field effect and bias voltages be sufficiently
low to permit accurate measurement of these
voltages with vacuum tube voltmeters. These
requirements were not stringent. The ease of
operation of the beat-frequency bridge is
important; however, its principal virtue is
the precise measurement of small field effect
or photoresponse voltages.
D. Experimental Data
1. Transient Field Effect and Photo-
response Data: A large number of oscilloscope
patterns of field effect and photoresponse
were observed using the balanced bridge.
Figure 12 is typical of the oscilloscope
patterns of the transient response to a square
wave field effect voltage. Figure 13 indicates
the sign and time relation between transient
field effect, transient photoresponse, and
driving voltage. A negative deflection in the
oscilloscope pattern indicates an increase in
film conductance. Both the photoresponse
time constant Tp and the field effect time
16
NAVORD Report 0_64
constant Tf were estimated from oscilloscope
patterns for selected intensities of background
illumination. The qualitative correspondence
of T and Tf is shown in Fig. 14.
A number of exploratory type experiments
were made using the balanced bridge. Those
experiments, which contribute to the general
understanding of fast surface states but are
not relevant to the objectives of this study,
are discussed in Appendix 4.
2. Steady State Field Effect and Photo-
response Data: The experimental data are
presented for the most part in pairs of figures
which present field effect and photoresponse
measurements for a given surface condition and
selected intensities of background illumination.
The circles represent the experimental data
and the solid line the universal curve which
best fits the data. Figures 15 and 16 are the
data for Eastman Kodak Company sample 1-B which
had an uncoated surface. The field effect
plate faced the uncoated surface of the film.
Figures 17 and 18 are data of a film, EK Co.
sample 1-A, which was of the same type as
sample 1-B except for a coating on the free
surface of the film. The electric field was
Induced through the coating. Figures 19, 20
and 21 are concerned with EK Co. sample 16, an
uncoated film: in Fig. 19, the field effect
plate faces the free side of the film surface;
in Fig. 20, the electric field is induced
through the glass substrate; and Fig. 21 is the
photoresponse data. Figure 22 is the field
effect response on a film, NOL 5-11-9, purposely
made to be approximately 20 times less sensitive
than the above films. Figure 23 compares the
field effect on different types of surface and
degree of sensitization.
3. Field Effect Mobility Product: The
mobility product (1+ Bf)Alf was calculated from
Eq.(76) for several samples. A typical set of
17
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L.
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NAVORD -Report 6164
results for sample EK Co. 1,B as a function of
background illumination is
Lamp Voltage
(1-s-Bait
0 ' 4.0 cm2/volt-second
1.5 4.4 " " It
2.5 3.0 " ? "
The product is approximately L. cm2//volt-
second with no definite trend as a function of
background illumination.
4. Summary of Experimental Data: In the
transient measurements, a negative square wave
field effect voltage caused an instantaneous
increase followed by an exponential decrease
in film conductance. The decrease was a
fraction a of the initial increase. When the
negative field effect voltage was removed, the
conductance decreased by the same increment as
the initial increase and then increased ex-
ponentially with time to the equilibrium value.
The time constants associated with the ex-
ponential increase and decrease of conductance
were equal within experimental error and were
both a function of background illumination.
In the photoresponse measurements, upon
exposure of the film to a light pulse of con-
stant intensity, the conductance increased '
exponentially to an equilibrium value. When
the light pulse was removed-, the conductance
decreased exponentially with time to the
original equilibrium value. Time constants
for increase and Oecrease in conductance were
approximately equal and were a function of
background illumination intensity. The
qualitative data indicate a correlation of'
field effect.and photoresponse time constants
as a function of background illumination (See
Fig. 14).
18
NAVORD Report 6164
Steady state ac field effect and photo-
response measurements with the beat-frequency
bridge were a quantitative verification of the
qualitative results of the transient measure-
ments. The field effect response was a
maximum at high frequency and decreased at
low frequencies to a fraction (1 - a) of the
maximum (See Fig. 24). Photoresponse was
maximum at low frequencies and decreased at
high frequencies. The concurrent field effect
and photoresponse measurements showed that
a, Tf and To were inversely dependent on back-
ground illukination intensity. There was a
one-to-one correlation between Tr and To as a
function of background illumination intbnsity.
a was a function of the side of the film from
which the field was induced but Tf was not.
Tp was independent of the side " film
from which the pulsed light was incident, a
was a function of the surface conditions. An
insensitive film showed a negligible value of
a. A comparison of the experimental data with
the theoretical universal curves indicates
single effective time constants Tr and Tp for
both field effect and photoresponse respectively.
The mobility product (14-Bf)I2f for field
effect was found to be approximately equal to
the mobility 4all as determined by Hall effect
measurements.19
19
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NAVORD Report 6164
III. THEORY
A. Empirical Equations Developed
By RC Circuit Analogies
1. General: In the initial stages of the
study reported herein, the empirical equations
developed by RC circuit analogies were quite
useful in describing and analyzing the experi-
mental data. Later, these equations served as
a guide in developing the theory. Because of
the success of the empirical equations in
describing the experimental data, any models
or theories for photoconductivity or field
effect in PbS must give the same equations.
2. Description of Experimental Field
Effect: The overall appearance of the
oscilloscope patterns of the transient field
effect response AV0 (see Figs. 12 and 13),
was similar to the transient voltage response
of the RC circuit shown in Fig. 25. For this
circuit the transient voltage response Vr
to a step function input voltage Vi is
_*
121, R?
V = -+ e (4)
20
4,
NAVORD Report 6164
where T is the RC time constant:
"C" C
t RI)
?
(5)
The similarity of the field effect and circuit
responses leads to the following empirical
equation for the voltage AVID across the film
terminals in response to a step function
field-effect voltage
v
AV? C4t Vf
(6)
The parameters Gft, ap and Tf are uniquely
determined by a single observation of the
transient field effect response to a step
function field effect voltage. Gft is the
ratio AVoilif at the start of the field effect
pulse, a is the fractional decrease in response
from t5.0 to quasi-equilibrium and is analogous
to Raie(Ra+Rb), and IT is the time constant
of the exponential decay and is analogous to
T. a and Tf are determined solely by the
properties of the film. Gft is determined by
the properties of the bridge and the film to
field effect plate capacity, as well as the
properties of the film.
The RG circuit steady state response Vr
to a sinusoidal input voltage Vi of angular
frequency co is 1
4 t zo)214- (1???`")
+ R6
col't
The corresponding field effect response V.pf
is then
. (7)
V - G
Vsl _Fs
J.
t ,
( I (-+ 0-) ) ( "2.
. (8)
I + (A.) Z4t
21
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NAVORD Report 6164
Referring to Fig. 24, a is the fractional de-
crease in response from maximum to minimum
and Tf can be calculated fromwaTr=1, where
wa is the angular frequency at which the re-
sponse is (1-a4ia2)1of maximum. Therefore
a and Tr can be determined from the relative
steady state ac field effect response as a
function of frequency while an absolute cal-
ibration is needed to obtain Gra. The cal-
culations of universal curves, Fig. 11, for
relative field effect were based on Eq. (8),
in which
relative response ?
I -I- Gol't
'(9)
Since the frequency dependent part of Eq. (8)
becomes unity and VII/ is maximum at high
frequency
V /
????,
f-s
V
The fractional change in resistance per field
effect volt as obtained from Eqs. (2) and (8)
(a)
[(1-0t+to'rcz)t-i- cec:V]
I + coat' 1-
*
3. Description of Photoresponse: The
transient response to a pulsed light signal
(see Fig. 13) was similar to that of the RC
circuit shown in Fig. 26. The transient
response Vr to a step function input voltage
22
NAVORD Report 6164
Vi for this RC circuit is
V=V
p.
+ 5
where the RC time constant T is
/- - EL C
Pb
?
(12)
(13)
Using the RC analogy, the response 1V0 to a
step function light pulse is
_et
ZW0 ?=.* Get ( I )
- e
where g is the rate at which majority carriers
are created in the film when the light is on,
Gpt is a proportionality constant involving
both film and balanced bridge properties, and
Tio is the photoresponse time constant. The
steady state response at a frequency w=(f?b) is
V = GPs/(
+ 0.) a
e (15)
Gps is the steady state proportionality constant.
To, the parameter of primary interest in this.
woTp=1 aso
study, can be calculated from the equation
where
is the angular frequency at
which the response is 1/1T of maximum. The
universal curve for photoresponse, the dotted
line in Fig. 11, was calculated using the
frequency dependent term of Eq. (15), i.e.
Pt ?
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NAVORD Report 6164
relative response=
tari)t
B. Theoretical Equations Based
On Majority Carrier Model
(16)
1. Statement of Problem: As stated pre-
viously, the majority carrier model has been
developed and applied to the calculation of
the electrical and photoconductive properties
of lead salt type films.1 The objective here
is to extend the model to describe the field
effect by explicitly including the electronic
surface states. Later the theoretical equations
obtained from the extension will be compared
with the experimental data, i.e. the equations
developed from RC analogies. Certain concepts
of Petritz' paper are restated as background
material for extending the model, particularly
those pertinent to the interpretation of the
experimental data. The procedure is to develop
the equations for the macroscopic conductance
and resistance of the film from a consideration
of the microscopic properties of the crystallites
and intercrystalline barriers. The ideas and
equations developed are then used both in the
description of photoconductance and in the
extension.of the majority carrier model to
describe field effect.
2. Film Conductance and Resistance: The
microscopic semiconductor crystallites which
compose the film have a resistivity,?c. These
crystallites are separated by thin dielectric
partitions which provide an effective resistivity
fob to electron current. The total series
resistivity /0 of.the crystallite and barrier is
/4) = /476* ?
24
4
NAVORD Report 6164
The film resistivity is much larger than that
of the crystallites, ">>A p therefore /1?0
can be neglected in comparison with /010 in
the expression for the macroscopic conductivity
of the photosensitive film. The current voltage
relationship for the barrier is
wg4
/Y1 -rb kl-(e
(18)
where is the current density, q the charge
of a carrier, lob the mean density of majority
carriers in the bulk, i.e. the crystallites,
is the effective potential height of the
barriers relative to the valence band edge,
k Boltzman's constant, T the absolute tempera-
ture, AVb the voltage drop across the barrier,
and M a parameter dependent on the specific
nature of the barrier but independent of 4)
Because of the microscopic dimensions of the
crystallites the voltage per crystallite is
so small that Alerb>10), then the data of
Tf = Tp indicates that bulk traps are dominant.
This is shown in line two of the table where
we also assume Tsc
Tbe.
F. Evidence Concerning Barrier Modulation
The field effect measurements give support-
ing evidence that the majority carrier model
can be based entirely on changes in the number
of majority carriers in the crystallites, i.e.
barrier modulation of the effective mobility by
small densities of excess carriers is negligible.
This rpult was previously established by
Woods.L. who compared the change in resistivity
with the change in Hall coefficient under
illumination. Confirmation of Wood's results
is based on the agreement between the measured
field effect mobility and the Hall mobility in
these films (See Section II-D.3 where Bf = 0).
Our experiment is not entirely equivalent
to Woods' since the field effect induces only
majority carriers, while Woods used radiation,
which induces both majority and minority carriem.
Stated more precisely, the field effect shows
that there is no barrier modulation resulting
from changes in the density of majority carriers.
Woods' experiment showed that there is no barrier
modulation resulting when both minority and
majority carriers are generated by light. Taken
together the two experiments strongly support
the model of photoconductivity. based purely on
-a change in the density of majority carriers;
no barrier modulation or mobility modulation
occurs.
54-
NAVORD Report 6164
The homogeneous model also suggests that
the field effect mobility product (1+Bf)/af
should be approximately equal to the Hall
mobility. The point being that the space
charge region adjacent to the outer surface
of the film is representative of the crystal-
lites throughout the film. Thus one expects
the same mobility from field effect as from
Hall effect as was found to be the case ex-
perimentally (see p. 17, Chapter II-D-3).
Ii
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V. CONCLUSIONS
A. Experimental Contributions
Four specific contributions to the field
of experimental photoconductivity and surface
physics were made:
1. A beat frequency bridge method was
conceived and developed for field effect studies
which allows for an accurate determination of
field effect time constant, the combined trap-
ping efficiency of the surface and space charge
states, and the effective mobility.
2. The identity of the field effect and
photoconductive time constants, independent of
background illumination, film surface condition,
and side of film probed, was established for
chemically deposited PbS films.
3. Field effect mobility was determined
to be equal to the Hall mobility.
4. One insensitive film was also studied.
The results indicated no clear field effect
time constant and that a sl O.
B. Theoretical Contributions
1. The majority carrier model was extended
so asto.describe field effect measurements by
including explicitly the time constants for
charge transfer into and out of the majority
carrier traps.
56
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NAVORD Report 6164
2. Equations were developed for estimating
the densities and capture cross-sections of the
majority carrier traps. These equations demon-
strate the usefulness of field effect in further
identifying the fundamental mechanism of photo-
conductivity in thin films.
3. An analysis was made of homogeneouspoly-
crystalline films for crystallite dimensions both
larger and smaller than a Debye length. The
analysis yields both photoresponse and field
effect time constants in terms of the time
constants which characterize the bulk, space
charge, and surface state trapping.
C. Summary of Conclusions
Interpretation of the experimental data
with due consideration of the theory developed
yields the following principal conclusions
concerning the PbS films studied:
1. The majority carrier lifetime is
predominant in the photoconductive process.
2. The rate limiting process in a
sensitized film is the rate of transfer of
charge into the traps.
3. The density of majority carrier traps
is high while the capture-cross-section is very
small, conditions required for the long
majority time constants measured. The estimated
density of surface traps required to provide
the time constants measured is of the order
expected on crystallites with dimensions of
the order of 0.1 microns. The density of bulk
traps estimated to provide the time constants
is larger than would be expected.
4. The film is a homogeneous poly-
crystalline structure composed of crystallites
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NAVORD Report 6164
with dimensions of the order of a Debye length
or less.
5. The traps probed by field effect
measurements are a representative sample of
those which cause photoconductivity. Field
effect measurements will provide a useful tool
for the further clarification of the mechanism
of photoconductivity in thin films, and its
relation to the sensitization process.
D. Recommendations for Future Research
1. Additional surface state studies
should be made on sensitive film to further
determine the effective density, energy relative
to the main bands, and effective cross-
sectional area of the traps. This pertinent
information should be obtainable from a series
of measurements of the time constant and a as
a function of temperature and ambients.
2. Surface studies should be made on a
series of films covering a range of sensitivity
along with other fundamental measurements of
serisitivity, time constant, responsitivity and
noise. The measurement of a and Tf offer a
direct means of evaluating the density and
cross-section of the photoconductive traps.
Such measurements, when correlated with the
sensitization conditions should lead to an
improved understanding of the sensitization
.process.
3. A series of precision absolute field
effect measurements should be made to determine
accurately the extent of barrier modulation and
the space charge region mobility.
58
NAVORD Report 6164
ACKNOWLEDGMENTS
Catholic University of America kindly
permitted the research reported herein to be
performed off campus under a program jointly
sponsored by the Naval Ordnance Laboratory,
White Oak, Maryland and the Office of Naval
Research, Washington, D. C. The sincere
Interest of the Navy in science is shown by
the policy of ONR which encourages its
scientific administrators to perform basic
research studies. In particular, I wish to
thank Drs. Robertson, Silverman, and Shostak
of ONR for their personal interest and en-
couragement. I am indebted to the NOL for
space, facilities and the opportunity of
performing research under their Foundational
Research Funds. Discussions with Drs. Jay N.
Zemel, James N. Humphrey, Joseph F. Woods,
and Miss Frances L. Lummis concerning both
the theory and apparatus were beneficial.
Suggestions concerning both the analysis of
data and organization of the report by readers
Drs. F. T. Byrne and Jay N. Zemel contributed
much to the report. Finally, the guidance
and contributions of Dr. Richard L. Petritz,
the Major Professor, were invaluable.
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APPENDIX
1. Balanced Bridge Equations
Refer to Fig. 29 for nomenclature and
assume that either the FE or a light pulse
causes a change AR in the resistance R of the
film. The resulting voltage change AV0 across
the film terminals is then
6V0 III1 4- 2 6I? (A-1,1)
where Is is the dc current through the film
and EI s is the change in current in the film
produced by AR. The equation for Is is
and
Vb R. R)
(A-1,2)
5 ( R1 RX + fez t'e +6E) (A-3"3)
where Vb is the bias provided by a stabilized
voltage, i.e. low impedance source. We may
now write:
A-2 121+211-6R \
A Vo
(A-1,4)
In this particular bridge R1 = R2 = 10k ohms,
Rrv1000k ohms, and AR < 41c ohms even for the
maximum resistance change; therefore
60
NAVORD Report 6164
AR
w
z 8
Ow 6
17-
cjOlO
0 z 4
a
tn
?I 2
0 CC
>
I
?QICr 10 20 40 60 100 200 400 600 11< 2K 4K 61(81(10K
FREQUENCY (CPS)
FIG 19 FIELD EFFECT RESPONSE OF UNCOATED FILM
? THEORETICAL
? MEASURED BY BEAT-FREQUENCY BRIDGE
_ EK Co FILM NO 16
OF CELL
FE PLATE ON FILM SIDE 0 VOLTS LIGHT BIAS-
a 1:0?8280,u S
_ FE PLATE TO FILM
_ CAPACITY 9pjuf I 5 VOLTS LIGHT BIAS-
TF?220,uS
2 5 VOLT
LIGHT BIAS
ID 40-
o 0 VOLT LIGHT BIAS. .1
-
w x Tr?280pS
w z t20 _
.4
15 VOLTS
z 0 a'062
0 >LIGHT BIAS
_., cc 1C) YF?220pS
4w 8-
TellOpS 2 5 VOLTS_
z a. 6- ta ?018 LIGHT BIAS,
ow
I II Ili I
- INSENSITIVE FILM CELL NOL 5-11-9
FE PLATE ON FILM SIDE OF CELL
- ? MEASURED
Tp. 107,uS
FIG 22 FIELD EFFECT RESPONSE FOR INSENSITIVE FILM
0 z
?THEORETICAL
sr to ? MEASURED USING BEAT-FREQUENCY BRIDGE
2- EK Ca FILM NO 16
-I W
0 cc FE PLATE ON GLASS SUBSTRATE SIDE OF FILM
/3 20 40 6080100 200 400600 IK 2K 4K 6K 10K
FREQUENCY (CPS)
FIG 20 FIELD EFFECT RESPONSE OF GLASS SUBSTRATE SURFACE
10
8- 1 1 1 1 1 1 1 1 I_
EK Co CELL NO.16 UNCOATED
o 6-
?THEORETICAL
? MEASURED
W? 4-
LI
0 VOLTS LIGHT BIAS
? ? ? ?
/-
in 2- rp?280pS
in
1.5 VOLTS LIGHT BIAS
E ? I -
- 2.5 VOLTS LIGHT BIAS -
'10.6
-3
00.2- ?II0 AS
ccI I 1 1 1 I I III I
54. 10 20 40 60 80 100 200 400 IK 2K 4K 6K 81(10K
FREQUENCY (CPS)
FIG.21 PHOTO RESPONSE TIME VS BACKGROUND ILLUMINATION
ALL INDUCED CHARGES
ARE MOBILE
20 40 100 200 400 IIC 2K 4K 6K 10K
FREQUENCY (CPS)
FIG. 24 TYPICAL RELATIVE RESPONSE TO
A SINUSOIDAL FIELD-EFFECT VOLTAGE
Declassified in Part - Sanitized Copy Approved for Release ? 50-Yr 2014/01/16: CIA-RDP81-01043R002800140011-0
Declassified in Part - Sanitized Copy Approved for Release ? 50-Yr 2014/01/16: CIA-RDP81-01043R002800140011-0
NAVORD Report ?164
NAVORD Report 6161+
INPUT
IVT
RESPONSE
r 110 Abc
Rai Rb
fib+Roe-7-
FOR A STEP FUNCTION VI ;V,. Rb+R.
Rb fr2.,2)2 (11-24-)2i
?Re+Rb
FOR A SINUSOIDAL Vi; Vr? VI R?+Rb 01
I +r2m2
FIG.25 RC ANALOGUE OF FE RESPONSE
LINES OF ELECTROSTATIC
FORCE
EXCESS HOLES
SURFACE STATES
SURFACE STATES IN
EQUILIBRIUM WITH BULK
POSITIVE CHARGE
NEGATIVE CHARGE
FIG.27 PICTORIAL REPRESENTATION OF ROLE OF ELECTRONIC
SURFACE STATES IN FIELD EFFECT EXPERIMENTS
FIELD EFFECT PLATE
VOLTAGE
FOR A STEP FUNCTION V1:
FOR A SINUSOIDAL Vi ;
[R.+ lit,
I/1+
8 ? TAN-I
i Ft p_stk,..,c(6,,,:fibr.
k R.+ Ftb )
FIG:26 RC ANALOGUE OF PHOTO RESPONSE
FIG. 2B TRANSIENT FIELD EFFECT RESPONSE
Declassified in Part - Sanitized Copy Approved for Release ? 50-Yr 2014/01/16: CIA-RDP81-01043R002800140011-0
Declassified in Part - Sanitized Copy Approved for Release ? 50-Yr 2014/01/16: CIA-RDP81-01043R002800140011-0
NAVORD Report 6164
FIG.29 SIMPLIFIED BALANCE BRIDGE CIRCUIT
R E
BALANCED BRIDGE
LI
JO Jd y?,1
1
DIFFERENTIAL AMPLIFIER
FIG 30 EQUIVALENT CIRCUIT FOR BALANCED BRIDGE
WITH DIFFERENTIAL AMPLIFIER LOADING
86
NAVORD Report 6164
PbS
FE PLATE
vF,FRE0.? ICPS
4.
FIG.31 CIRCUIT FOR FIELD EFFECT AND PHOTO-
CONDUCTIVE RESPONSE MEASUREMENTS WITH
BEAT FREQUENCY BRIDGE
o ?? 0
V. ?100 VOLTS RMS
EK Ca CELL IA
?I000 CPS
b ? 90 CPS
20 40 60 80 100
V, ,BRIDGE BIAS VOLTAGE IN VOLTS
V1 ? 30 VOLTS
EK Co,CELL IA
b.90 CPS
? 1000 CPS
20 40 60 80 100 120
V, ,FE PLATE VOLTAGE IN VOLTS
FIG 32 LINEARITY CHECK
100
140
160
R.?247 bA; 3 VOLTS, LIGHT WAS
It, 377 60; 2 VOLTS, LIGHT BIAS
K, 657 102; 1 VOLT, LIGHT BIAS
R. 1160 kg); 0 VOLT, LIGHT BIAS
FIG. 33 CHECK OF OHMIC PROPERTY
87
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R
STAT
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