website, "The Philco Surface Barrier Transistor
(SBT) was the 'hottest' transistor around until the late 1950s.
This device performed very well at high frequencies and was used
extensively in radio and computer circuits. Hobbyists were delighted
to find such an inexpensive high frequency device... [Edwin] Bohr
authored many well-remembered transistor construction projects in
the 1950s/60s." Many of Bohr's construction articles featuring SBTs
were published in Radio Electronics
magazine, and this
was one of them from 1957. The manufacturing process is described
where jet streams of an electrolyte were shot at both sides of the
germanium crystal to etch it as required - Neanderthal in nature
compared to today's etching processes.
The Amazing Surface Barrier Transistor
By Edwin Bohr
availability of SB transistors now makes it important to learn something
about these new types.
Years ago, when junction transistors
were just beginning to nudge the one-megacycle limit of useful operation,
surface-barrier transistors were percolating away at 30 mc. and
higher. And, to top it all, they were doing this with absurdly small
values of collector voltage and current - small even by ordinary
transistor standards. Today, the surface-barrier transistor, in
its particular field of application, still remains without peer.
Yet, in the popular technical magazines, this surface-barrier
transistor has received hardly more than a casual mention. The reason
for this is simple. The SB transistor has not been an "available"
transistor. Surface-barrier transistors have been with us for a
long time, but only on an engineering sample basis; and their development
and potentialities largely have hidden in the laboratory and between
the pages of learned journals.
This situation has been given
a complete about face. Anyone with a few dollars in his pocket can
now buy a surface-barrier transistor. In fact, he can choose from
several types. Some units have cut-off frequencies as high as 60
Surface-barrier transistors are beginning to appear
in personal receivers and automobile radios. At least one piece
of ham gear has appeared using this fabulous transistor. Military
equipment, too, now employs the SB transistor, dispelling early
rumors that this unit was undesirably fragile and delicate. The
current availability of this component will undoubtedly kindle a
wildfire of interest in surface-barrier transistor applications
In this article we will hit the high spots
of the SBT, its theory, history of development, and present some
practical applications and circuits. Some of the things the SBT
can do really make a person's mouth hang open. As an enticement
to read further, we will tell you in advance that one of the circuits
is a scale-of-two counter containing only four components - two
SBT's and two resistors! If the SBT is unique, which it is, some
of its circuits are even more unique. The Name
Fig. 1. Jets of electrolyte stream from nozzles toward the
germanium wafer during the manufacture of the SB transistor.
By now, the curious reader has undoubtedly wondered how the name
"surface-barrier transistor" is derived.
Briefly, we can
hint at an answer to this question by indicating that the ordinary
diffused junction transistor contains two forms of semiconducting
material. In contrast, the SBT contains only one form. Available
diffused junction transistors are either p-n-p or n-p-n types. Available
SBT's are simply n-type transistors. The emitter and collector of
the SBT both are formed by plating to the surface of germanium,
forming a surface-barrier rectifying interface.
we are dropping the theory right here. An understanding of semiconductor
physics is, of course, anything but easy and necessitates a broad
knowledge in many direct and accessory fields. After all, to run,
one must first learn to crawl and then to walk. We, alas, can't
do it all in fifteen hundred words. However, we can still give the
reader plenty of good functional "walking" information. Don't worry,
we will be back to the theory in a few paragraphs.
The SBT is a development of the
Philco Corporation and, at present, they and Sprague Electric Co.
are the only manufacturers. Just as the transistor was an outgrowth
of research into the field of solids, the SET was the result of
further Philco research into changes in the properties of germanium
just beneath the crystal's surface.
Atoms of germanium behave
very differently at the surface of a crystal from the way they do
in the interior. The changed behavior extends from the surface into
the crystal for a depth of about one ten-thousandth of an inch,
forming a so-called surface barrier. Scientists found the SB effect
can be utilized to form a useful amplifying semiconductor device
if several special conditions can be met.
must be attached to the germanium in a way that will produce a minimum
distance between the collector and emitter. This distance between
collector and emitter must be the same order of thickness as the
surface barrier. Second, the germanium must be completely free from
contamination or physical strain.
These are problems of
the highest degree. Nevertheless, by the magic of modern technology,
they have been solved. In fact, the spacing between emitter and
collector in the SBT has actually been reduced to a few thousandths
of a millimeter and with tolerances of a millionth of an inch. This
small miracle is accomplished by a clever process called "electrolytic
machining." Electrolytic Machining
Fig. 2. (A) Oscillator and (B) Superregenerative circuit.
Fig. 3. (A) D.C. amp. (B) Bi-stable circuit.
Fig. 4. Wide-band video frequency amplifier.
To begin the manufacture of SBT's, blanks of single-crystal n-type
germanium are cut and etched to a thickness of 0.003 inch. The blank
is next placed between two tiny glass nozzles, mounted on a common
axis. Jets of electrolyte stream from the nozzles toward the germanium
wafer. An electric current passes through this stream of electrolyte,
removing the germanium under the point of impact, an action that
is the reverse of electroplating. Fig. 1 shows this arrangement
As the electrolytic machining proceeds, the emitter
and collector surface barriers begin to approach each other, the
current density reduces, thus slowing down the etching for vernier
control of the process. This reverse-plating, or etch process, has
now caused two pits to form in the germanium blank The remaining
thickness of germanium between the pits can be controlled to ± 5%.
Ninety to 120 seconds are required for this etch.
reversal of current through the electrolyte, the drilling process
is stopped and indium emitter and collector electrodes are plated
to the surfaces of the cavities. All of this is done without interrupting
the stream of electrolyte. Indium, incidentally, is the same metal
used to form the p-type germanium in p-n-p junction transistors.
In the finished transistor the collector is twice as large
in diameter as the emitter. Hairlike leads are attached to the indium
electrodes and the transistor is ready for hermetic sealing into
a small cylindrical case. Performance
Cut-off frequencies for all transistors are given in terms
of a grounded-base circuit. For grounded-emitter and grounded-collector
service, the high-frequency performance begins to roll off at a
frequency approximately equal to the grounded-base cut-off frequency
divided by the beta gain of the transistor.
rule, we see that a conventional diffused junction transistor, with
a 20 mc. cut-off and a beta of 60 performance-wise, begins to deteriorate
at one-third of a megacycle. In contrast, the SBT may have a cut-off
frequency of 60 mc. and a beta of 10. This means the SBT gain is
smooth up to six megacycles. Tests made with the SBT show that it
gives unprecedented performance as a superhet mixer. Too, it has
the largest bandwidth-gain product of any available transistor,
making it really practical for wide-band video and i.f. amplifier
To top it all, the SBT does this at collector
voltages and power levels remarkably lower than those of conventional
transistors. A 30 mc. SBT oscillator, for example, can easily operate
at a collector potential of 3 volts and a current of 0.5 milliamp!
A. portable receiver using the SBT's will operate from a small three-volt
battery. Using conventional transistors, about nine volts are usually
considered to be necessary. Characteristics
Table 1 provides the more important features on
available Philco units. Of these, the SB-100 was the first commercially
available SBT. This SB-100 and the L-5108 are generally the most
useful for high-frequency and amateur-band applications. The L-5116
will oscillate to 90 mc.
Three SBT's, the AO-1, L-5113-L, and L-5114-L, are types made available
for particular customer requirements. The AO-1 is an inexpensive
SBT and its user can probably expect widely varying characteristics.
Service technicians will find the L-5113-L and L-5114-L used in
battery sets. The L-5113-L is used for converter and second detector
service and the L-5114-L for i.f. applications.
and 2N129 are military-version SBT's. Undoubtedly, personnel in
the armed services will be seeing plenty of these transistors in
Another SBT, the 2N240, is available for computer
and high-speed switching circuits. This type has controlled saturation
characteristics, fitting it for numerous ultra-simple direct-coupled
"on-off" amplifier circuits. The meaning of "saturation characteristic"
will be explained later in the article.
Table 1. Types and characteristics of typical Philco SB transistors.
circuits are similar in most respects to those of the p-n-p diffused-junction
transistor. The electrode voltages and bias currents have the same
polarity. In the case of high-frequency operation there is really
no significant difference between schematics for SB and p-n-p transistors.
For computer applications, the differences are really quite startling.
Figs. 2A and 2B give surface-barrier circuits for operation
at 20 mc. and higher with suitable tuned circuits. For 30 mc. C
may be approximately 100 μμfd. and L 6 turns spaced to 1/2
inch on a 3/8-inch coil form. The 470-ohm resistors are insurance
against excessive collector current.
A direct-coupled amplifier
and bi-stable circuit using the 2N240 are shown in Figs. 3A and
3B. When you look at these circuits they appear to be printer's
errors or textbook-type simplified diagrams. But they aren't. These
are good workable circuits. Let's look at Fig. 3A and see how it
As you may remember, the 2N240 has a controlled
saturation characteristic, by this we mean the voltage from collector
to base, when the transistor is passing the maximum collector current
permitted by the collector resistor and the available collector
supply voltage. In other words, the voltage from collector to ground,
when the collector current has reached saturation, is called saturation
For the 2N240. the collector-to-emitter voltage
with a saturation current of 2 ma. is -0.07 volt and -0.1 volt for
a saturation current of 8 ma. Further characteristics of the 2N240
state that an input signal of -0.1 volt from base to emitter will
cause only -150 microamps of collector current.
Now if we
apply an input base current Ib1
of -0.3 ma. to V1
the collector voltage VC1
will drop to -0.07 volt which
is direct-coupled to V2
. This is not enough voltage to
conduct so that VC2
, the output voltage,
is practically equal to the supply voltage. However, if we decrease
the input current, VC1
will increase, driving V2
If we now connect the output lead to the
input, the direct-coupled bi-stable circuit of Fig. 3B results.
With the addition of proper steering and control circuits, this
type of counter is capable of operating at frequencies higher than
the best vacuum-tube counters. The power dissipation and space requirements
for this computer circuit are extremely small.
The SB transistor
is a hot-performing video amplifier. Using simple audio-amplifier-type
RC coupling, a two-stage SBT amplifier will have adequate response
out past 3 mc. Employing peaking coils, the circuit of Fig. 4 has
a 9 mc. bandwidth and 28 db gain. Removing the coils, the bandwidth
is still sufficient for good video response.
video amplifiers are non-microphonic. We have replaced industrial-TV
video preamps with three-stage SBT preamps, eliminating all but
trivial remaining microphonics in the vidicon.
radios, both portable and automobile, use the SBT, with circuits
almost identical to diffused-junction transistor sets.
there is a protective circuit to prevent burnout of the converter
or input r.f. stage caused by too-large signal from signal generators,
etc. This is necessary because the emitter and collector connection
wires inside the transistor are almost microscopic and the thin
base section is easily ruptured. Consequently, the SBT is faster
than the fastest fuse - and far more expensive.
gun-type soldering irons or conventional irons with isolation transformers
for bench work. Otherwise, possible leakage currents from the iron
and any other test instrument connected to the chassis may damage
the transistors. Conclusion
surface-barrier transistor, its performance, and fabrication, are
nothing short of a modern technological tour-de-force. Yet, it does
not stop here. Already surface-barrier transistors, using the diffusion
process (SBDT units) are able to operate at tremendously higher
frequencies than the present units. Some applications, in fact,
are spectacular enough to be classed as closely guarded military
Surface-barrier transistors, however, do not replace
diffused-junction transistors. They simply give the transistor circuit
engineer new inspiration and unprecedented performance in several
How far the SBT invades the entertainment
market depends, among other things, upon the number of SBT suppliers.
Today, Philco and Sprague are the only makers. Firms generally will
not use a transistor unless there are several sources acting as
alternate lifelines in the event of strikes, material shortages,
catastrophe, etc. This lack of suppliers until now has held back
transistorized power amplifiers for automobile set and it will have
the same effect on SBT radios.
Meanwhile, practical transistorization
has been pushed past the ten-meter band by SBT's. Next, the twelve
TV channels will fall before the transistor. Anyone want to service
this transistorized TV booster? Don't laugh, it isn't too far off.