Yj a FIG. 2
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a 46 INVENTOR
- ay R.L. WALLACE JR.

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ATTOANEY

Patented Aug. 7, 1951

2,563,503.

UNITED STATES PATENT OFFICE

TRANSISTOR >

_ Robert L. Wallace, Jr., Plainfield, N. J., assignor
_ to Bell Telephone Laboratories, Incorporated,
New York, N. Y¥., a corporation of New York —

Application April 29, 1949, Serial No. 90,533

|
This invention relates to the translation of
electric signals and particularly to semiconduc-
ror translating devices of improved construc-
ion. |
A primary object of the invention is to improve
the stability of a semiconductor translator. .
Another primary object is to increase the in-
put impedance of a semiconductor translating de-
vice. | — 7
A related object is to improve the operation of
a semiconductor translator at high frequencies.
The invention provides specific improvements
in the construction and mode of operation of a

three electrode semiconductor amplifier of the

type which forms part of the subject-matter of

an application of John Bardeen and W. H. Brat-

tain, Serial No. 33,466, filed June 17, 1948: now
Patent No. 2,524,035, which is a continuation in
part of an earlier application of the same inven-

tors Serial No. 11,165, filed February 26, 1948, and

later abandoned. This central element comprises

19 Claims. (Cl. 175—366)

10

15

sions of the voltage, current, and power of: the
original signal appear in the load. The device,
Which may take various forms has received the
appellation “Transistor” and will be so designated
in the present specification. oe |
_In a modified transistor described in an applica- |
tion of W. E. Kock and R. L. Wallace, Jr., filed
August 19, 1948, Serial No. 45,023, now Patent No.
2,960,579, the block takes the form of a disc into
opposite faces of which hemispherical depressions
have been ground to depth such that the block
thickness, at its thinnest part, is only a few thou-

sandths of aninch. The emitter makes point con- -

tact at the low point of one of the depressions
and the collector makes point contact substan-

_ tially colinearly with the emitter, at the low

PAL

point of the opposite depression. The base elec-
trode makes low resistance contact with the pe-
ripheral surface of the disc. ‘The base is thus
separated from the other electrodes by the full —

Yadiusofthedise. |

a small block of semiconductor material such as. .

Bermanium having in its original form at least
three electrodes electrically coupled thereto,
which are termed the emitter, the collector and
the base electrode.
tor are point contact electrodes making rectifier
contact with one face of the block and spaced
apart by a few thousandths of an inch, while the
base electrode may be a plated metal film making
_ @ low resistance contact with the opposite face
of the block. The base electrode is thus sepa-
rated from the emitter and the collector by the
full thickness of the block. The emitter may
be biased for conduction in the forward direc-
tion and in this condition its impedance is a few

hundred ohms. On the other hand the collector |

Is biased for conduction in the reverse direc-

tion in which case its impedance is several thou-

sand ohms. The forward emitter bias results in
the injection of charges into the block through
the comparatively low emitter impedance and
these charges are transported under the influence
of electric fields within the block to the collector
where they are withdrawn through the compara-
tively high collector impedance. The application
of 2 Signal current or voltage to the emitter re-
sults in variation of the injected charges and so
of the charges transported to the collector. The
high ratio of the collector impedance to the emit-
ter impedance results in voltage amplification. In
addition the transported charges serve to modify
the currents flowing from the base to the collec-
tor so that current amplification results as well.

Due to either or both phenomena amplified ver-—

The emitter and the collec-

25

For some purposes it is desirable to introduce
a time delay between the introduction of signal
at the emitter and its recovery from the collector.
Yo this end the transistor may be drawn out an

elongated filament on which the emitter and the

Bt

35

40

45

50

55

collector make point contact at widely spaced
points. In place of a single base electrode there
are now two low resistance electrodes, one at each
end, the emitter making contact with the fila-_
ment near the’ one and the collector making con-~
tact with the filament near the other. In par-
ticular, the low resistance end electrodes may
be metal annuli on the end faces of the filament,
the point electrodes protruding through the holes
in these annuli. Certain features of this elon-
gated transistor form the subject-matter of an
application of G. L. Pearson and W. Shockley,
Serial No. 50,897, filed September 24, 1948 now
Patent No. 2,502,479, while certain other features
form the subject-matter of an application of J. R.
Haynes and W. Shockley, Serial No. 50,894, filed
September 23, 1948. | :

_ The performance of transistors is conveniently
described in terms of an equivalent four terminal
network comprising an emitter resistance re, a
base resistance rp, a collector resistance 7c, and
an internal electromotive force which is propor-

tional to the emitter current. In this network

the base resistance is common to the input cir-
cuit and the output circuit and is therefore a
source of feedback which, if sufficiently great,
makes for instability and in any event reduces
the effective input impedance of the device. To
this extent the base resistance is objectionable.

Transistors of the close-spaced electrode variety
are characterized. in general. by base resistances
of the order of 100-500 ohms, which, in compari-
son with the usual values of emitter resistance,
which are of the order of 300-800 chms, is suf-
ficiently high to be objectionable. <n transistors

of the elongated filament variety, it has been pos- -

sible to reduce this base resistance somewhat by
increasing the proximity of the emitter to the
base: but such elongated transistors are not of
general application because, especially at high

frequencies, their long transit times impose an

10

upper frequency limit to successful operation |

which is objectionably low.
Accordingly, it is a subsidiary or intermediate

1b

object of the presen* invention to reduce the base —

resistance of transistors of the close-spaced elec-

trode variety. ‘This object is attained, in accord-
ance with the invention, by a departure from the
conventional transistor construction in which,
while the collector and the emitter are located
in close proximity to each other. each of these
electrodes is separated by a relatively large dis-
tance from the base electrode. In contradistinc-
tion to this conventional construction, the pres-
ent invention brines the emitter as close to the
base electrode as it is possible to do without mak-
ing direct contact therewith. The new construc-

tion has resulted in base resistances of the order
of 10 to 20 ohms and consequently greatly im-
proved stability and input impedance. With this
construction the high resistance barrier, whose
existence between the base electrode and the
emitter electrode is believed. to characterize

transistors of all configurations, may no longer.

20

(McGraw-Hill, 1948).

9,563,508

4

Fig. 9 is a diagram illustrating a step in the
fabrication of the transistor of Fig. 7;

Figs. 10 and 11 are a side view and a plan view,
respectively, of another modification; and

Fig. 12 is an enlarged sectional view of 2 part
of Fig. 10.

Referring now to the drawings, Fig. 1 shows

a transistor comprising a block { of semiconduc-

tor material such as high back voltage germanium
which may be prepared as described in “Crystal
Rectifiers” by H. C. Torrey and C. A. Whitmer
On one part of the upper
face of this block there is provided, for example
by evaporation of an inert metal such as rhodium
in a vacuum, a metal film 2 which makes low
resistance contact with that part of the semi~
conductor block which lies immediately below it.
In the complete transistor, this film serves as_
the base electrode. The remainder of the upper
surface of the block, or 9t least a suitable frac-

- tional part thereof, is otherwise treated in such

20

30

merely lie below one surface of the semiconduc- ©

tor block and parallel to it, as described in the
aforementioned application of J. Bardeen and
W. H. Brattain, but should rather intersect the
block surface between the emitter and the base

electrode. ‘This barrier, located in this fashion,

operates to maintain a high value of the emitter
resistance of which the principal component part
resides in the spreading resistance immediately
under the emitter contact, and so enables the

a way that when metallic point contact electrodes
are engaged with it and suitable electroforming
processes are applied to them, these contacts —
will exhibit rectifier characteristics. A suitable
surface treatment for this purpose is to etch the
upper face of the block with a solution which
contains 10 parts by volume of concentrated nitric
acid, 5 parts of commercial standard (50 per
cent) hydrofluoric acid and 10 parts of water, in
which a small amount, e. g. 0.2 gram of copper:
sulphate has been dissolved. The proportions of
the etchant are not particularly critical and may
be varied over a considerable range. The ger-
manium material is etched with this solution by

immersion therein for a period of about 1 minute,

although the etching time is not particularly crit-

-- ical, but lies in the range from 15 seconds to 10.

40

minutes.

by the etchant by a, coating of wax or otherwise

as desired. Immediately upon removal from the

etching solution the semiconductor block is

45

emitter electrode to support the applied signal

voltage while still controlling the flow of collector
current. In other words, the construction of the

invention makes for 2 low base resistance with- —

out a corresponding or proportional reduction in
the emitter resistance whose magnitude must not
be too small.

The invention will be fully apprehended from
the following detailed description of preferred
embodiments thereof, taken in conjunction with
the appended drawings in which:

Fiz. 1 is a schematic diagram, partly in section,
of an amplifier embodying a transistor in ac-
cordance with the invention;

Fig, 2 shows two mutually perpendicular side

views of a point contact electrode of preferred
form;

Fig. 3 is a plan view of the upper face of the
transistor of Fig. 1, with current streamlines in-
dicated; |

Fig. 4 is a schematic diagram of the equivalent
network of a transistor; . |

Fig. 5 is a schematic diagram of a modifica-
tion of Fig, 1; —

. Figs. 6 and 7 are perspective diagrams illus-
trating modified forms of the invention;

washed in a brisk flow of cold tap or distilled
water for a period from several seconds to 2
minutes. ‘The block is then immediately dried

in a strong air blast. An alternative drying pro-

cedure is to rinse the block, after washing, in

: methyl alcohol and then in acetone and to dry

o0

it in still air or an air blast. It is important that
the step of washing in water be carried out as

-. described because it results in a high reverse im-= |

oo

60

pedance and a high maximum reverse voltage at
the rectifying junctions which are formed when
the point contact electrodes are engaged with
the block. These characteristics are particularly
important at the collector electrode.

The treated surface of the block, prior to treat-
ment, is preferably ground or even polished. It
has been found that the greatest sensitivity of

the treated surface is. achieved by thoroughly

69

70°

Fig. 8 is an enlarged sectional view of a portion |

of 2 transistor embodying the invention in an-
other form: |

15

into contact with the sensitized surface.

smoothing the surface before etching. The
roughness of the unetched surface is a factor in
determining the sensitivity which results from
etching, a smooth surface becoming more sensi-
tive in the course of the etching process than a
rough one.

When the perinding, etching, washing and dry-
ing have been completed, point electrodes of
metal such as Phosphor bronze may be brought —
One of
these, 3, termed the emitter, is now ready for
action. ‘The other, 4, termed the collector, is
preferably subjected to an electric forming proc-
ess in order to reduce its resistance to reverse

Throughout the etching process the
base electrode may be protected from corrosion

"current. | This electric forming process involves .

the application of a voltage to the collector elec-
trode in the reverse direction and of a magnitude
sufficient to exceed the peak value of the rectifier

back voltage, the electrode meanwhile being pro-.

tected from injury by the
in circuit. | | | |

In accordance with the present invention, the
emitter electrode 3 is placed as close to the base
electrode 2 as is mechanically possible. To this
end the emitter electrode may be a wire of cir-
cular cross-section and about 1 mil in diameter
and if may be sharpened to a chisel edge having
a configuration which in one aspect is shown at
the left in Pig. 2 and in a perpendicular aspect
is shown at the right. The resulting point lies in
the plane of one side of its shank as also shown
in Fig. 1, and is of such a shape that when it is
brought into engagement with the sensitized
semiconductor surface the resulting minute area
of contact is elliptical in form. This permits the
contact to be made with the bevelled portion of
the chisel edge of the wire electrodes 3 overhang-
ing the edge of the base electrode 2, so that the
major axis of the minute contact area lies par-
allel with the edge of the base electrode, as illus-
_ trated in Fig. 3. When the collector electrode 4

is similarly shaped, the two point contact elec-
trodes 3, 4, may be brought as close together as

inclusion of a resistor

desired without making mutual contact, while

the minor axis of the minute area over which
the emitter 3 makes contact with the sensitized

surface and the separation distance between this |

area and the base electrode may be held as low
as a' few tenths of a mil; i. e., less than the
diameter of the shank of the point: contact elec-
trode 3. Specifically, while the major axis of the
contact area may be one mil in length, its minor
axis may be about 0.1 mil and the distance sep-
arating this area from the base may be about 0.2
mil. 7

An electric connection is now made to each of
the three electrodes, as by soldering. A work
bias of a fraction of a volt, derived from a source
such as a battery 5, is applied to the emitter and
a bias of 40 to 100 volts derived from a source
such as a battery 6 is applied to the collector 4.
When the material of the semiconductor block |
is N-type germanium the sign of the emitter
bias is positive and that of the collector bias is
negative. With material of opposite type, such

as P-type silicon, the signs of these bias voltages.

are to be reversed. | |
When a signal source 7 is connected between
the base electrode and the emitter and a load is
connected between the base electrode 2 and the
emitter 3, it is found that amplified versions of
the signal of the source appear in a load 8 con-
nected between the collector 4 and the base 2;
and that, furthermore, the problems of instabil~
ity and of low effective input impedance which
have characterized other. transistors are absent
with the transistor of the invention. _ |
Fig. 3 is a plan view of the surface of the semi-
_ conductor block i of Fig. 1 in the plane of its
surface, showing the minute elliptical areas 9,
10, over which the emitter and the collector make
contact with the surface of the block, and show-
ing the base electrode 2 which lies exceedingly
close to the emitter 3. When current flows be-
tween the base electrode 2 and the collector d,
the emitter being open-circuited, the current
‘Streamlines follow paths such that their traces
on the surface of the block are approximately
_ as indicated in the figure by broken lines. The

10

15

25

30

2,568,508

total voltage drop along any one of these stream~-

lines is evidently equal to the voltage difference

between the base electrode 2 and the collector 4,
As the collector current is changed, the potential |

Of each part of the block surface changes, but

the potential of that part at which the emitter
electrode 3 makes contact undergoes only a very

Small change. This change is very small for two

reasons, First, it is spaced along the central
streamline from the base electrode to the collec-
tor only by a fraction of the length of this line:
and second, the voltage drop per unit length of
any streamline is less near the base where the
Streamlines are spread apart and greatest near
the collector where they are gathered close to-
gether. For both reasons, therefore, a change in
the collector current results in a. comparatively
small change in the open circuit emitter voltage.

From the foregoing explanation it is clear that
the major axes of the elliptical areas 9, 10. may
be extended laterally as far as desired: until, in-
deed, their lengths become equal to the width of
the block {. _ |

Fig. 4 shows an equivalent network for a tran-
sistor. As fully explained in H. L. Barney Pat~
ent 2,950,518, this equivalent network has been
found to be of assistance in describing and ana-~
lyzing the operation of transistors. Here, re rep-
resents the emitter resistance, re represents the
collector resistance and rp represents the base re-
sistance, while the amplification features of the

transistor are represented by the inclusion of

40

O8

60

65

70

75

tance rp be. small.

a fictitious generator of electromotive force
‘==Zmle, Where ie is the emitter current and 2m.
is termed the mutual impedance of the tran-

, Sistor.

When, as indicated in Fig. 4, an external signal
Source (2 is applied between the base 2 and the
collector 4, the emitter 3 being left on open cir-

cuit, the only current which flows is the collector

current te. It is also apparent that under these
conditions the open circuit voltage Ve measured
between the base and the emitter is equal to the

‘voltage drop across the base resistance rv. Hence,
the condition that the potential of the emitter

Shall remain nearly equal to the potential of the
base despite changes in the collector current as

described above in connection with Fig. 2, is —

equivalent to. the condition that the base resis-
This is the result produced
by the structure of the invention. |

At the same time, in order that the device
shall operate efficiently as a transistor, it is nec-
essary that the emitter resistance re in Fig. 4
be of substantial magnitude. The fabrication
technique described above gives this result.
Without necessary subscription to any particular
theory of operation, it is believed that the rea=
son for this is that there exists, immediately
below the minute area of contact of each of the
point contact electrodes, a high resistance bar-
rier indicated by the broken lines 13, {4 in Fig.
1, and that the emitter barrier {3 reaches the
exposed surface of the block i somewhere in
the minute region between the edge of the base
electrode 2 and the emitter contact area 9, as
likwise indicated on the figure. On one side of
this barrier the material is believed to be of
one conductivity type, for example, excess or
N-type conductivity, while on the other side of
the barrier the material is believed to be of the

opposite type, namely, deflect or P-type conduc-

tivity. In any event it is clear that the principal
Part of the emitter resistance resides in the

Spreading resistance in a minute hemispherical

2,563,508

7

volume of the semiconductor material immedi-

ately under the emitter contact. oy
It is of minor importance whether or not the

barrier extends unbroken from some point to the
left of the emitter electrode 3 to some other point
to the right of the collector electrode 4, or wheth-
er, as indicated on the figure, the emitter and
the collector make contact with individual “is-
lands” of P-type material, each of which is indi-

vidually separated by a barrier from the body.

of the block of N-type material. Advantageously
low values of base resistance are achieved by the
close spacing between the emitter 3 and the base
2 without a corresponding or proportionate dim-
inution of the emitter resistance re.

Fig, 5 shows a modification in which the base
electrode 2’ is fitted into a space provided for
it in the upper face of the semiconductor block

tected from the blast by a mask of plastic or the

‘jike. Lastly, the base electrode 2’ may be applied

to the ground or etched surface by evaporation of
a metal such as rhodium. | — |
Fig. 6 is a perspective diagram, partly in sec-
tion, of a transistor of the coaxial type which is
modified in accordance with the present inven-

tion. Here the transistor takes the form of a.

10

15

f so that its surface lies flush with that of the |

block. With this construction the beveled edge
of the emitter electrode 3’ may be more oblique,
and so mechanically stronger. Fig. 5 also shows
a barrier 15 which extends unbroken from below
the emitter 3’ to below the collector 4’ separat-
ing a surface layer {6 of material of one con-
ductivity type, for example P-type,.from the main.
body of the block which may be of opposite con-,
ductivity type, and from the base electrode 2’.
Such a block may be constructed by first treat-
ing a part of the upper face of a block of N-con-

ductivity type material to convert a surface layer
- to P-conductivity type material, separated from
the main body of the block by a high resistance

barrier, another part of the face meanwhile being
masked against the treatment, then removing
the material of the block from the masked part
of its face and finally inserting the base electrode
- in the space so formed.

- The P-conductivity type layer and the barrier
below it may be formed over the required part
of the block surface in any desired manner, the

remainder of the surface being protected by a.

suitable mask. One suitable process which forms
the subject-matter of an application of R. B.
Gibney, Serial No. 11,167, filed February 26, 1948,
now Patent No. 2,560,792, comprises the anodic
oxidation of the surface in an electrolyte such as

polymerized glycol borate, the surface having —

previously been prepared by etching. The ap-
plied voltage may be increased up to about 100
volts over a period of an hour or so. The face
of the block is then cleansed of oxides which may
have formed. The barrier-layer depth resulting
from this process appears to be about 10-* centi-
meters. Another process comprises bombarding
the face of the block with nuclear particles such
as deuterons or alpha particles.
such bombardment are described in an applica-
tion of G. L. Pearson and W. Shockley, Serial No.
87,618, filed April 15, 1949. This process results
in the formation of a well-defined barrier at a
depth below the surface which depends on the
energy imparted to the impinging particles.
With alpha particles of 5.3 m. e, v. energy de-

-. tived from a radioactive source, barriers have

been formed at a depth of 0.8 mil. With 8.5

20

25

30

3D

40

50

55

The effects of -

60

65

m. e. vy. deuterons accelerated by a cyclotron, -

barriers have been formed at a depth of 5 mils.
After formation of the surface layer and the

barrier, the semiconductor material is removed

from a part of the surface and to a depth in

70

excess of the depth of the barrier. This removal |

may be carried out by grinding, sandblasting, or
otherwise. In the case of sandblasting, the
Pp-conductivity layer to be preserved May be pro-

75

disc 20 of semiconductor material into opposite
faces of which hemispherical depressions are
formed as by grinding, and of a depth such that
the bases of these depressions approach within
afew mils of each other. Metal electrodes 21,
22 serving as emitter and collector, respectively, |
make contact in alignment with each other at
the bases of these depressions. Transistors of.
this modified configuration form the subject- .
matter of the aforementioned application of W.
E. Kock and R. L. Wallace, Jr. In that applica-
tion the base electrode was provided by a band >
or film of metal which encircled the cylindri-
eal surface of the disc. In accordance with the
present invention, however, a substantial area

of one face of the disc is provided with a metal

film 23 which constitutes the base electrode and
the remaining portion of the surface of this face
is suitably treated as with the etching and wash- |
ing process described above. The emitter 2l
may be a point contact electrode which engages
the treated surface close to the edge of the metal

film 23, e. g., spaced from this ed&e by not more
than twice the diameter of the emitter contact
area, while the collector electrode 22 engages the

disc in the center of the oppositely located de-
pression and in collinear alignment with the
emitter. | |

Fig. 7 shows another modification in which the
entire surface of one face of the disc 29 is pro-
vided with a metal film 24 serving as the base
electrode with the exception of a minute hole
25 at the base of the depression through which
the surface of the semiconductor material is ex-
posed. This exposed surface is then suitably
treated as by the etching and washing process

and a metal point electrode 2! serving as the

emitter makes point contact directly with the
sensitized semiconductor surface exposed through |
this hole. The diameter of. the hole 1s prefer-
ably not more than twice the diameter of the >
emitter contact area. As before, the collector
electrode 22 makes contact at the center of the
oppositely located depression, and preferably in
axial alignment with the emitter electrode.
As an alternative to the fabrication technique
descrised in connection with Fig. 1, a modified
technique may be employed to produce the struc-
ture of Fig. 7, as follows: The metal point con-
tact electrode which is to serve as the emitter
is first covered with a thin coat of insulating
material such as wax. The surface of the semi-
conductor block is then treated with the etch-
ing, washing and’ drying process to sensitize it. |
Next, the wax-coated electrode is pressed into
contact with this sensitized surface. The sur-
face is then sandblasted and plated with an
inert metal such as rhodium to serve as the base
electrode. The wax coating of the emitter elec- _
trode, and. especially that portion of it which at
first covered the metal point and which, in the

course of pressing the point into engagement

with the semiconductor surface is forced radi-
ally outward, operates to protect a minute area
surrounding the emitter, both from the sand-
blast and from the deposition of the metal in the
plating process. As before, a barrier surround-

Transistor