A phototransistor is described in which there is included as a part of its integral structure an auxiliary diode in the form of an added base-collector junction. This junction is formed in the surface of the base element and thus out of the direct path between the emitter and collector regions of the phototransistor

Mccann, D. H.

English

NASA Technical Reports Server

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REPLY TO
ATTN OF:
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
^NASA-Case -BFS-20U07)
Patent
V .
(Westinghouse
WASHINGTON, D.C. 20546
-- PHOTOTRANSISTOR
Electric Corp . ) 5 p .
CSCL 09A
00/09
N73-1923
Unclas
64808
5
TO: KSI/Scientific & Technical Information Division
Attention: Miss Winnie M. Morgan
FROM? GP/Office of Assistant General Counsel for
Patent Matters
SUBJECT* Announcement of NASA-Owned U.S. Patents in STAR
In accordance with the procedures agreed upon by Code GP
and Code KSI, the attached NASA-owned U.S. Patent is being
forwarded for abstracting and announcement in NASA STAR.
The following information is provided?
U0S0 Patent No. :
Government or
Corporate Employee
Supplementary Corporate
Source (if applicable)
NASA patent Case No.
NOTE - If this patent covers an invention made by a corporate
employee of a NASA Contractor, the following is applicable:
Yes /2</ No / /
Pursuant to Section 305(a) of the National Aeronautics and
Space Act, the name of the Administrator of NASA appears on
the first page of the patent; however, the name of the actual
inventor (author) appears at the heading of column No. 1 of
the Specification, following the words
an invention of . . .H
Elizabeth A. Carter
Enclosure
Copy of Patent cited above
NASA-HQ
https://ntrs.nasa.gov/search.jsp?R=19730010508 2020-03-17T08:24:45+00:00Z
United States Patent
Low et al.
[MI 3,714,526
[45] Jan. 30, 1973
|54] PHOTOTRANSISTOR
176] Inventors: George M. Low, Acting Administra-
tor of the National Aeronautics and
Space Administration with respect
to an invention of; David H. Mc-
Cann, Linthicum, Md.
( 2 2 ) Filed: Feb. 19, 1971
[ 2 1 ] Appl. No.: 116,777
|52| U.S. Cl 317/235 R, 317/235 N, 317/235 AM,
317/235 UA, 317/235 T
|51 ] Int. Cl H01I 15/00
|58| Field of Search....3 I 7/235 N, 235 AM, 234 UA,
317/235 T
[56] References Cited
UNITED STATES PATENTS
3,244,949 4/1966 Hilbiber 317/235
3,546,542 12/1970 Riel et al 317/234
3,532,945 10/1970 Weckler 317/235
Primary Examiner—Martin H. Edlow
Attorney— L. D. Wofford, Jr., G. J. Porter and John R.
Manning
[57 ] ABSTRACT
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 U.S.C. 2457).
A phototransistor in which there is included as a part
of its integral structure an auxi l iary diode in the form
of an added base-collector junction which is formed in
the surface of the base element and thus out of the
direct path between the emitter and collector regions
of the phototransistor.
1 Claim, 8 Drawing Figures
,40 /30/N-f/-34
PATENTED JAN 3 01973 3.714.526
P
16
FIG. I
(PRIOR ART)
2 5
,40
j_r
i
i
I N +
1
L
34N
N-t-
P
v
N
SA
-30
-28
X
FIG. 4 25
I4N
FIG. 2
(PRIOR ART)
24- — 22
FIG. 3
(PRIOR ART)
48'
-12 ,•*• \ ,' ,x—a/ / / / J
/
36
FIG. 5
FIG. 6
LIGHT FLUX
VOUT
\ \ X PX \ 'V^ f<
-26
28
--«
r~
L
,44
. — - ( — —
P
,46
/ N
N
— — _
56X
\
N-t-
+
~~\
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i1
1
1
i
N1
J
-41
David H.MCCann,
INVENTOR
BY
-A-/:
3,714,526
1 2
PHOTOTRANSISTOR BVC8o = collector-base breakdown voltage
hfe = the direct current transistor forward gainBACKGROUND OF THE INVENTION
 Thjs place a severe limit on both the ma)linlum voit.
This invention relates to Phototransistors and more a8e at which known devices may be operated and the
specifically to a phototransistor with improved dynam- 5 maximum base-collector capacitance which can be ob-
ic range when operated in a charge-storage mode. tained from them.
Accordingly, it is an object of the present invention
GENERAL DESCRIPTION OF THE PRIOR ART to provide an improved phototransistor capable of ex-
A phototransistor or phototransistor sensor essen- ,„ hibiting increased base-collector capacitance and thus
tiaily consists of a semiconductor comprising emitter, 10 Pr°vldln8 increased dynamic range when operated ,n a
base and collector regions and wherein there are ap- charge-storage mode.
u j u ii . • .- Another object of this invention is to provide apropnate emitter-base and base-collector junctions . . J .. . . ,. ... r. .r r
 . . . , ' • photo-transistor which embodies an auxiliary base-col-
separating these regions. lector diode which increases the overall base-collector
Two alternative readout modes of operation are ob- ., . , , ., . . ,
. . . . . . . - , r. .-, '5 capacitance but does not contribute to a reduction in
tamable with phototransistor sensors: the D.C. or con-
 th;base.collector breakdown voltage.
tinuous mode and the integration or charge-storage
mode. The D.C. mode of operation is the basic mode BRIEF DESCRIPTION OF THE DRAWINGS
for continuous operation of a phototransistor whereby
photon-generated carriers in or near the base region
 20
 FIG l
 .
is a toP vlew of an exlstl"8 tvPe of
are beta multiplied, resulting in an output collector cur- phototransistor; , , , , , ,- ,
rent proportional to the instantaneous photon flux on F ° 2's a "ctiona view along lines 2-2 of FIG. 1;
the sensor. When employed in large scale photo-sensor . FIG .3 1s a° e*™f** circuit of the phototransistor
arrays, this mode of operation offers several draw- shown in MOb. l ana 2
, / ' . . . K .. .. ,. , . . .. FIG. 4 is a cop view of an embodiment of abacks. First, output current is highly dependent on the js i_ • j- j • u u- •
, . . , . • . u - u - i phototransistor constructed in accordance with this m-beta of each phototransistor sensor which, in a large ar- v .
. . • -f ,
 f - - , • vention;ray, might vary significantly from unit to unit resulting
 sectiona, yjew taken a, ^ ,mes 5_g
in uneven response. Second, output signal levels for f piG 4-
normally encountered light levels are in the microvolt _.„ '> ' . . . ,. .. . , ,. . ,
' LI i_- t i ^i • o L FIG. 6 is a top view of an alternate embodiment of
range for reasonably high values of load resistors. Such 30 .u •'v ' t- .
low-output levels, coupled with an inherently high out-
 F,G ? / a";ectiona, view along the lines 7-7 of FIG.
put impedance, normally requires that some amphfica-
 6 ^t whh an exam ,e of electrical circuit for
tion prior to commutation be provided, a distinct disad-
 o ration of the phototransistor in a charge-storage
Vanta8e- . . .
 u mode; a n dThe phototransistor integration or charge-storage 35
 RG „ jg a series of waveforms iilustrating operation
mode, a recent development, is a dynamic mode of
 of a phototransistor in the charge-storage mode,
operation in which a transient output pulse is obtained.
This pulse is a measure of the total number of photons DESCRIPTION OF EXAMPLE OF PRIOR ART
incident upon the light sensitive regions during the time .-•,-.<. . -. ••• • • \.
between samplings. Between samplings light continu- 40 _ FIGS' 1-3 illustrate an existing type phototransistor..
ously generates carriers which are stored on the June- FrO1" an *na{\sis °f "'. lluhas been dlscove;ed that the
tion capacitances of the phototransistor. Sampling typi- breakdown V^a8e wh'ckh occurs » controlled not by a
„
 K
 . ,. . .. .. .- . ,.. ., . general breakdown of the col ector-base junction but
cally consists of detecting the discharge of the collector f , . . ... , • .. , ,
•
 J
 . . .. ..
 u i j • . by a breakdown which occurs only immediately underbase mnction capacitance through a load resistance. * . . ., . • . " TU . • * • < • L
_.
 J
 . , , . ° . , . ., the emitter of the transistor. That is, for example, the
" The magnitude of the output transient depends upon 45 . . . . , ... ,„ - . . .
* . ri- u. . j u i.- u areadirectly under emitter region 10 of phototransistorthe tola amount of light-generated charges which were ,« _ ., . . . j . • j ... . •
.. . . - ,. JL . u 12- Further, it has been determined that increases in
stored since the previous sampling. The output ob-
 thig afea whjch mj ht jncrease itance have the ef.
tamed ,n this manner greatly exceeds that which would
 fcct of ,oweri br:akdown voltag
H
e. There •„ some base
be obtained by reading out the static, light-generated
 area of base r« jon |4 afea ,6 whjch .g ^ ^
photocurrent as per the D.C. mode of operation which 50
 lectof n Jg ^ ^ ^  d undef ^.^ f
is a function of the light flux on the element only during
 10 md whjch does not by Us expanse ten(J to CQn
the sample time. . .
 t . tribute to collector-emitter breakdown and thus break-
The dynamic range of operation in the charge- down of base-collector junction 20. The equivalent cir-
storage mode basically depends for its effectiveness
 cuit (FI(J 3) may be vjewed as an intrinsic
upon the capacitance of the base-collector junct.on phototransistor 22, a phototransistor having equal
and the maximum voltage at which this junction can be emitter-base and base-collector junction areas, plus an
operated. In turn, this maximum operating voltage is
 added reverse-biased parasitic diode 24 is parallel with
limited by the punch-through or breakdown voltage of
 the base.collector junction of phototransistor 22.
the base-collector junction. Parasitic diode 24 is representative of the junction por-
The maximum base-collector voltage before break- ou
 tion of base-collector junction 20 just opposite base
down is limited to the open-base collector-emitter area 16 and the capacitance added by it is not at the ex-
breakdown of a transistor or phototransistor and this
 pense of a reduced breakdown voltage. As will be ap-
voltage is given by the equation: preciated from the configuration of the prior art device
BV = BV 14 \r~h— 65 snown 'n FIGS. 1 and 2, the parasitic diode junction
FE
 area is inherently limited to the horizontal planar area
where of the device which exceeds the planar area of the base-
BVcEo — collector-emitter breakdown voltage collector junction directly under the emitter.
3,714,526
3 4
BRIEF DESCRIPTION OF THE INVENTION As shown, phototransistor 41 includes an additional
. . ... ... . .. , ,.-~ ... base-collector junction 42 which is provided by diffus-In accordance with this invention, the difficulties ex- . . . . . , * . • • * * • i
. . . . . . . . . . . . . . ing into the periphery of base region 44 a supplementalpenenced with prior art type phototransistors as to in- * „ r . J , B ^f _.. Yv. . . ,
a- • • j u i - j j j • N+ collector region or layer outer 46. This added col-
sufficient capacitance, and thus by reduced dynamic , , ' 3 . .
 A0
range, is overcome by an added base-collector junction 5 'ector re«10" 46 connects to basl<
 u
collefor «f°n 48
which is not in the operational path between the by means of a common peripheral boundary 50^ Being
emitter and collector. This added junction area is posi- out of the Path of the Bitter-base junction 52 and
tioned on a top surface of the base element. In this base-collector junction 54 between emitter region 56
manner base-collector capacitance is greatly increased
 10
and collector re
»°
n 48; added base-collector
without causing a decreased in the emitter-collector capacitance ,s obtained without a decrease in max-
breakdown voltage. This then greatly increases the ""um base-collector operating voltage,
dynamic range of operation of a phototransistor when *" e*amPle of charge-storage mode of operation of
operated in a charge storage mode NPN Phototransistor 41 » illustrated by the electrical
interconnections to phototransistor 41 in FIG. 6 and in
DESCRIPTION OF THE PREFERRED FIG. 7 and the wave forms shown in FIG. 8. The base or
EMBODIMENT base region 44 of phototransistor 41 is left floating and
n , . r-t/-o A j f t. • te • pulse source 57 is connected in series with output loadReferring now to FIGS. 4 and 5, phototransistor 25 is . -„ . „
 io . - ,.,
. . j , c •%* c • -i . .-i resistor 58 between collector 48 and emitter 56, one
constructed of a wafer 26 of semiconductor material . , , , „ _ , • , . , . . . « , , •
such as silicon or germanium. The lower and most sub- 20 terminal of pulse source 57 and load resistor 58 being
stantial portion of wafer 26 forms a collector region 28, f °U"ded- Pu'SC SOUrCe 5? pr°V'deS a Mn.e? °\ ""*"
it being a substrate appropriately doped with impurities 60 which "se from a zero Pot,e"tlal to a Positlve level °\
to provide an N-type conductivity for an NPN type operating bias, shown in FIG. Sa. The proport.onal
phototransistor and P-type for a PNP type of response or output of the circuit as appearing across
phototransistor. Above the collector region is a base re- 25 load resistor 58 to three diff«ent levels of illumination
gion 30 which is formed by the diffusion into the sur- is shown bv waveforms 62, 64 and 66 in FIGS. 9b, 8c,
face of impurities to provide a substrate exhibiting a P- and *d- respectively. A feature of the charge-storage
type conductivity for an NPN N-type of mode of operation is that the peak of the output signal
phototransistor, N-type for a PNP type
 of >s essentially independent of both the current gain and
phototransistor. There is thus formed a base-collector 3° the area of the transistor.
semiconductor junction 32 between the base and col- Durin« the application of a pulse 60 the collector
lector regions which extends over the greater portion of depletion capacitance, as represented by the
the horizontal cross-section of the device. A third capacitance of base-collector junctions 42 and 54,
distinct semiconductor substrate or region, emitter re- „ Cnar8es to a Volta8e value which is e<»ual to the P"!«
gion 34, is formed in what otherwise would be the sur- amplitude voltage less the voltage drops across base
face of base region 30 and it is the result of appropriate emitter Junction 52 and output resistor 58, resulting
diffusion of impurities to create a N+ conductivity re- from the illumination ionization current flow produced
gion, (P+ for PNP type phototransistors), that is it has a by an applied light flux. When pulse 60 falls to zero the
higher impurity concentration than the N region form-
 40 light nux causes the emi«er-base junction to be con-
ing the collector. The emitter region 34 is exposed to nected ln parallel with the base-collector junctions 42
receive a light flux as illustrated in FIG. 5 or as also il- and 54- Thus the charge across the two base-collector
lustrated by emitter region 56 in FIG. 7. With the addi- junctions is applied across the base-emitter junction,
tion of the emitter region 34 there is created a second This then has the effect of causing a reverse bias to be
junction, emitter junction 36, between the emitter and 45 applied across all junctions and a small negative signal
base regions. Up to this point the construction to appear across load resistor 58 corresponding in time
generally follows the prior art as illustrated in FIGS. 1 to the trailing edge of pulse 60. With all junctions
and 2. The departure is that of the addition of a second reverse biased, and because of the photo-sensitivity of
base-collector junction 38 provided by a thin N+ con- the device, the amount of discharge is proportional to
ductivity outer layer 40 as in the case of emitter region 50 the integral of the illumination ionization current which
34. As shown in FIG. 5, layer 40 covers a large region flows during the period in which the source is
of the surface of phototransistor 25 and thus substan- grounded. In other words, during the period between
tially extends the effective base-collector junction area, pulses, base-collector junctions 42 and 54 are
extending nearly to emitter region 34. Thus, there is discharged by an amount proportional to the light
achieved a folded configuration which results in a max- 55 received between two pulses. Upon the application of
imum area base-collector junction for the device the latter of the pulses there is an initial charging cur-
wherein a large area base region is sandwiched between rent, and thus output positive pulse across resistor 58,
collector regions. The added layer 40 and junction 38, of a magnitude proportional to the state of discharge
being near the surface of base region 30 is out of the which has been produced by the light flux received
emitter-collector path, and, as described above, does 60 between the pulses. For example, waveform 62 in FIG.
not materially contribute to base-collector junction Sb illustrates an essentially zero received light flux con-
breakdown of the device. At the same time, this con- dition, waveform 64 in FIG. 8c illustrates a received
struction materially increases the base-collector light flux of intermediate value, and waveform 66 in
capacitance, a desired feature for phototransistor ,. FIG. Bd illustrates a received light flux of a fairly large
operation in the charge-storage mode. value.
FIGS. 6 and 7 illustrate a second type construction As discussed above, the ability to accommodate
for achieving the increased base-collector capacitance. larger light values, and thus a wider dynamic range is
3,714,526
enhanced by the increased base-collector capacitance
provided by the added or auxiliary base-collector junc-
tion of this invention. In fact, tests have shown that by
the application of a thin diffused layer to form the
added junction, layer 40 in FIG. 5 and layer 46 in FIG. 5
7, increases in base-collector capacitance by a figure of
five have been achieved over known previous types of
phototransistors.
While two arrangements have been shown for added
base-collector junction area in the surface of an other- 10
wise base region of a phototransistor, it is to be ap-
preciated that other configurations are possible de-
pending upon the arrangement of the particular ele-
ment of a phototransistor. The significant feature is
that the added base-collector junction is provided by IS
means of an added collector layer in the surface of the
base of the transistor and thus remote from a direct
path between the emitter and collector regions.
What is claimed is:
1. A phototransistor comprising: 20
a collector element comprising a semiconductor re-
gion of a first conductivity type material;
a base element comprising a semiconductor sub-
strate of a second and opposite conductivity type
material integrally formed in a surface of said col- 25
lector element:
an emitter element comprising a semiconductor sub-
strate of said first conductivity type material
formed in a portion of the surface of said base ele-
ment and providing an exposed surface adapted to
receive a light flux;
an auxiliary collector element comprising a semicon-
ductor of said first conductivity type of higher con-
centration than said collector element, said aux-
iliary element diffused in the principal portion of
the otherwise free surface of said base element and
said auxiliary element extending into contact with
said collector element;
whereby the effective base-collector junction is ex-
tended by a configuration wherein a substantial re-
gion of the base of the phototransistor is sand-
wiched between effective collector regions provid-
ing an effective extension of the base-collector re-
gions remote from the path between the emitter
and collector of the phototransistor and thereby
substantially increasing the collector-base junction
capacitance without making the phototransistor
more susceptible to emitter-collector breakdown.
30
35
40
45
50
55
60
65


Phototransistor

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