Application of the 4A germanium grown junction transistor in a special customerís set for persons with impaired hearing marks the first use of a transistor in telephone equipment installed in the home. This n-p-n transistor, which acts as a voice amplifier, requires so little battery power that it can be supplied from the "talking current" sent along the wire from the central office. The 4A, along with other grown junction transistors which have been developed for many purposes in the telephone system, has opened the door to a wide variety of possible improvements in telephone service. Several of these are already in the design stage. In the illustration above, the author (right) and D. F. Ciccolella are shown inspecting a rack of n-p-n grown transistors that are used in various laboratory tests.
The construction of an n-p-n grown junction transistor begins with the preparation of a single crystal of germanium which has the two p-n junctions grown into it. If in the growing process, a seed crystal is slowly withdrawn from a bath of molten germanium in such a way that more germanium solidifies onto the seed, which then grows into a rod-sized crystal. The molten germanium is n-type at the start. At the proper moment in the withdrawing process a small pellet of an acceptor (p-type) impurity is added, to be followed a few seconds later by a somewhat larger pellet of a donor (n-type) impurity. These impurities dissolve in the melt and control the type of the germanium subsequently grown. The resulting crystal then has a thin layer of p-type material sandwiched between terminal sections of n-type material. This structure is sketched in the upper left hand part of Figure 1. The p-type layer may be about a thousandth of an inch in thickness. The crystal is sliced, leaving the thin p-layer in the center of the slice, which is then cut into rectangular bars, square in cross section. Each bar is fabricated into a transistor, as illustrated, by soldering its ends to supporting and conducting leads comprising a stem assembly. The germanium bar is etched to reduce undesirable electrical effects caused by irregular surfaces, a very fine gold lead is then welded to the central p-layer, and finally the transistor is encased in a hermetically sealed can.
Some of the finer points of the fabrication techniques will be discussed later, in connection with the important effects they have on the transistorís characteristics, but it is useful now to discuss the principles of operation of this device. A preliminary discussion of the physics of p-n junctions has been given by M. Sparks in an earlier issue of the RECORD, and the following paragraphs will further develop some of the pertinent concepts.
A descriptive analogy is drawn in Figure 2 between a junction transistor and a waterfall. Both can be used as power amplifiers. Suppose the waterfall to be provided with a dam which can be moved up or down to control the flow of water. Raising or lowering the dam can be accomplished by the use of a small amount of mechanical power, while much more power is obtained from the wheel at the bottom of the fall. In the transistor, the left hand, or emitter, section acts as a reservoir for free electrons. The flow of these electrons over the left-hand junction is controlled by raising or lowering the electrical potential difference between the emitter and the central, or base, section of the transistor. This control requires the expenditure of only a small amount of electrical power, which is furnished by the signal to be amplified. These electrons then deliver much more power, in the form of an amplified replica of the input signal, as they flow over the electron potential fall at the right-hand collector junction. The extra power of this amplified signal comes from the battery in the collector circuit.
With certain reservations, the waterfall analogy can be pushed still further. In an actual waterfall, all the water flowing over the dam does not reach the water wheel, even in an ideal situation; some is lost by evaporation and as spray. In the transistor, losses occur in the current flow between emitter and collector because some of the electrons recombine with positive holes, which are plentiful in the base sandwich layer. By making the base layer very thin, one can keep this loss down to less than one percent of the current flowing over the emitter junction. The ratio of electron current collected to the total current entering the base layer is called the current transmission factor, alpha (a). As will be pointed out later in this article, this quantity is an important measure of transistor performance.
The analogy becomes fuzzy, however, if one tries to push it too far. In the waterfall, there is nothing that corresponds to the base layer of the transistor; water flowing over the dam at once enters the fall. In the transistor, a physical separation of the emitter and collector is necessary to provide for applying electrical voltages separately across the two junctions. Closer examination of the operation reveals also that the water losses occur in the fall, while in the transistor the electron current losses occur, not in the collector junction, but in the base layer.
The circuit performance characteristics of a transistor are directly related to its structure and fabrication. In the grown junction transistor it is possible to control the electrical properties of the emitter, base, and collector regions by proper additions of impurities to the molten germanium in the preparation of the crystal. The thickness of the base region is regulated by very precise temperature control and by careful timing of the impurity additions. The physical size and structure of the transistor are important design parameters also. The larger the transistor the greater the power it can dissipate without overheating. Since the electrical capacitance of a p-n junction is directly proportional to its area, however, increasing the size of the transistor increases its collector capacitance. But this, in turn, reduces its useful frequency range.
Germanium is a brittle material, about as hard as glass, so that when it is cut or shaped with a carborundum or diamond-faced wheel, the cut surface is so damaged as to be useless as a part of the transistor. The crystal perfection is disturbed for some distance below the surface, and this damaged layer is riddled with a network of tiny cracks and disoriented crystal debris. As was discussed in a previous article,* such a disturbed layer is a prolific generator of unwanted minority carriers. For example, a disturbed layer will generate positive carriers in negative-conductivity germanium. The liberation of such particles in the neighborhood of active junctions can so interfere with the processes of emission and collection of signal-carrying currents as to degrade the performance of the device seriously. All of this damaged material must therefore be removed from the germanium bar in the region near the p-n junctions before the transistor can be completed. This is done by an electrolytic etch, in which the germanium bar is made the positive electrode. About three thousandths of an inch are dissolved around the perimeter of the transistor bar by this etch, and this leaves the germanium surface clean and smooth, free of cracks or chips.
A delicate operation in fabricating the grown junction transistor is the attachment of the base lead to the p-region. The problem is to weld a gold wire, about two thousandths inch in diameter (considerably less than the diameter of an average human hair) to a particular region of the smooth germanium bar, when this region is itself thinner than the diameter of the wire. The end of the gold wire is pointed or flattened, and the wire is mounted in a micromanipulator (Figure 3) to allow accurate positioning of the contact. The point is then moved along the bar until it is known, by electrical test, that the point is in contact with the base region of the n-p-n element. The actual weld is made by passing a current of about two amperes through the gold wire to the germanium for a few milliseconds. This current heats the contact region sufficiently to form a little puddle of gold-germanium alloy; on cooling this bonds the wire to the germanium.
Even with the greatest care, this weld may be somewhat wider than the base layer. To avoid short circuiting the collector and emitter junctions at the weld, the gold wire is "doped" with a small percentage of a p-type impurity, such as gallium. This insures that the germanium-gold alloy region will be strongly p-type and will, in fact, act as part of the base region, even though it extends into what was n-type germanium before the bond was made. Figure 4 shows a section through a typical base bond. The etch used to bring out the emitter and collector p-n junctions has selectively attacked the material right at the bond, producing the illusion that the wire has become detached. The local enlargement of the base layer near the bond in consequence of the doping is easily seen. In particular, the right hand or collector junction has been locally "pushed over" so that it now extends slightly into what was previously the collector n-type portion of the structure.
During the fabrication process, tests are applied to insure that the electrical performance of the finished transistor will be as required. After the base lead is attached, the transistor is complete except for final mechanical protection. This is afforded by sealing the transistor inside a metal can. After a short aging period, the transistor is ready for final electrical tests and coding.
As mentioned earlier, the ratio of electron current collected to the total current entering the base layer (alpha) is one of the most important properties of the transistor. For junction transistors under normal operating conditions, alpha is slightly less than one, which means that there is a small loss in electron current that appears as a small current in the base lead. The ratio of collector current to this small base current is given by the expression a/1-a, a typical value of which may be about 50*. Now, if we connect the transistor into a circuit like that in Figure 5, where the input signal is introduced into the base lead, the ratio of collector output current to signal input current is given by the same expression. That is, in such a "common emitter" circuit the output current may be 50 times the input current. The closer alpha is to unity, the higher this figure will be. As has already been pointed out, one way to push the value of alpha closer to unity is to reduce the width of the base layer, thus reducing the time in which an electron could recombine with a hole as it crosses the base layer. Another way is to grow the base layer as free as possible from imperfections and impurities which might subsequently act as recombination catalyzers in the completed transistor.
Another important performance index is the way in which the current transmission factor alpha depends on the signal frequency. For any application it is desirable that alpha be independent of frequency over the frequency band of interest. The design theory of parallel-junction structures predicts that the frequency cut-off of alpha (the frequency at which alpha has decreased to 0.707 of its low-frequency value) is inversely proportional to the square of the base thickness. Thus, for high frequency operation the base must be very thin. In an n-p-n grown junction germanium transistor, a frequency cutoff of five megacycles per second requires that the base region of the transistor be less than one one-thousandth inch in thickness.
An example of the effect of the germanium material properties on performance of the transistor is found in the minority carrier lifetime. This quantity is a measure of the length of time an electron or hole can survive in material of the opposite conductivity type before it recombines and is hence lost as a carrier of current. Typical minority carrier lifetimes in transistor germanium are a few tens of microseconds. Lifetime is sensitively dependent upon the purity and perfection of the crystal and the cleanliness of its surfaces. Long lifetime is particularly desired in the base layer in order to reduce the transit loss of the electron current crossing from emitter to collector. It is desirable also in the emitter and collector regions of the transistor near the junctions because short lifetime material, like disturbed surface layers, is a copious generator of minority carriers which degrade junction performance.
Table I compares typical electrical parameter values for a production-code, grown junction n-p-n transistor with those of its development prototype. Observe that as the transistor passed from development to production, improvements were made in all the parameters cited. These improvements result from the close control of materials properties and fabrication processing which are made possible by the use of production methods. The 4C transistor is intended for voice-frequency amplifier service at low power levels. The M-1752 is a general purpose junction transistor for voice and carrier-frequency applications. Both of these transistors have rated power dissipations of 50 milliwatts, and they are made in a similar manner. Another transistor design using n-p-n grown crystal material is the high-frequency tetrode, which was the subject of an article previously published in the RECORD.**
Another n-p-n transistor, the A-1858, developed from the M-1752 prototype, is the amplifier device used in the P carrier system, now in experimental operation at Americus, Georgia. About forty of these transistors are associated with each channel, exclusive of those used in repeater equipment, serving in amplifiers, signal generators, and in control applications. The economy of operation of these devices is strikingly illustrated by the fact that an entire transistor terminal, using about 40 A-1858 transistors, is operated with a power input only one-fourth that required by the heater of one conventional telephone repeater tube.
KENNETH D. SMITH received the B.A. degree in Physics from Pomona College in 1928 and the M.A. degree in Physics from Dartmouth College in 1930. He then joined Bell Telephone Laboratories where for about ten years he assisted in the development of laboratory testing facilities at carrier and radio frequencies, and laboratory and field testing facilities for coaxial cable systems. During World War II, Mr. Smith worked on the design and development of proximity fuses, and the design of radar bombing equipment. Following the war, he engaged in development work on broadband microwave radio systems, and from 1951 to 1954 he was concerned with junction transistor development. At the present time, Mr. Smith is directing the work of a group responsible for developing large area silicon devices including the Bell Solar Battery.
Fig. 1 -Steps in the fabrication of a germanium grown junction transistor.
Fig. 2 - The waterfall (a) is analogous to the n-p-n junction transistor (b).
Fig. 3 - The correct position for bonding is determined by moving a gold wire over the surface of an n-p-n bar with the aid of a micromanipulator.
Fig. 4 - An n-p-n section of a transistor, with gold wire welded to central or base layer. Note broadening of p-layer at point of contact.
Fig. 5 - The "common emitter" circuit: signal introduced into base, lead results in high current gain.
Fig. 6 - R. L. Johnston bonding leads to n-p-n junction bar in micromanipulator. Oscilloscope at right tells when correct position for bonding is found.