The Amazing Surface Barrier Transistor
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According to the Transistor Museum 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
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 mc. 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 and circuitry. 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. Momentarily, 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. History 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. First, electrodes 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 clearly. 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. By instantaneous 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. Applying this 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 applications. 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. Types 2N128 and 2N129 are military-version SBT's. Undoubtedly, personnel in the armed services will be seeing plenty of these transistors in FM receivers. 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 Surface-barrier transistor 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 operates. 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 voltage. 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 make V2 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 into saturation. 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. Surface-barrier 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. Entertainment radios, both portable and automobile, use the SBT, with circuits almost identical to diffused-junction transistor sets. Sometimes 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. Philco recommends 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 The 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 secrets. Surface-barrier transistors, however, do not replace diffused-junction transistors. They simply give the transistor circuit engineer new inspiration and unprecedented performance in several special applications. 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.
Posted May 3, 2023 |
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