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The intense interest in the transistor shown by electron-tube research and development engineers may be attributed to the fact that the transistor performs functions similar to those of triode-type electron tubes, although the mechanism of conduction is quite different.

The transistor is of particular interest to equipment designers, who see many circuit possibilities in its characteristics. It is very small in size, and the power requirements for its operation are extremely low. When suitable circuits are developed, space and power requirements for complex electronic equipment may be simplified to a large degree by the use of transistors.

Another promising feature is that the operating life of certain types of transistors shows indications of being very long (compared by tubes), thus minimizing replacement problems. The physical ruggedness of the transistor offers other obvious advantages. In addition, the transistor requires no "warm up" time but will operate instantaneously upon application of voltage to its electrodes.
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The limitations of the present developmental transistors, however, must not be overlooked. Transistor characteristics vary with ambient temperature changes, the noise is high compared with that of electron tubes, and the power output is relatively low.

Nevertheless, when the favorable characteristics of the transistor are weighed against its limitations, it appears that this device, even in its present developmental stage is destined for use in many applications. Further improvements in its characteristics undoubtedly will create new and expanding fields for its use.

At the same time, the principles of semi-conduction in solids may be expected to play an increasing part in the development of many new electronic devices, of which the present transistor is but the first.
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Two types of transistors,
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• the point-contact type and
• the junction type,

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will be discussed in this series. The point-contact transistor will be discussed first, and at greater length, because the development of this device has reached a more advanced stage and more is known about its performance with respect to frequency of operation, life, and uniformity of characteristics.

However, the junction transistor promises to be at least as important as the point-contact transistor in many applications.
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The heart of the transistor is the germanium crystal. Germanium is a semi-conductor, a metallic-like substance having conductivity greater than that of an insulator but less than that of a conductor. Its resistance, in contrast with that of metals, decreases as its temperature is raised. Other types of semi-conductors, such as silicon, lead sulphide, and selenium, have been used in transistor work, but, to the present time, germanium has proved the most successful.

Germanium is known mostly for its use in point-contact diode rectifiers which have been availabe commercially for several years. These devices have achieved widespread use in many present-day applications.
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The resistivity of the germanium, an important factor in transistor operation, is dependent upon the presence in the germanium of minute quantities of certain impurities (künstliche Verunreinigung). If no impurities are present in the germanium crystal, no transistor action takes nlace. If too many impurity atoms are present, however, the germanium becomes too conductive and transistor action is adversely affected.

The impurities which enhance the transistor operation should be present in the ratio of less than one atom to every 10,000,000 germanium atoms.

Because of their exceedingly low concentration, it is quite difficult to detect the quantity of impurities present in the germanium crystal. Some of these impurities are actually present in the germanium dioxide as it is delivered to the manufacturer.

It is desirable, however, to remove as many impurities as possible by purification techniques (Säuberungstechniken) so that controlled amounts of them may be added to obtain the desired values of resistivity.
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The initial process in the conversion of germanium dioxide to the final crystals for transistor use is the reduction of the dioxide to a germanium metallic powder. This process is performed in a hydrogen atmosphere at a temperature of approximately 650 degrees Centigrade.

The powder is then melted at a temperature of approximately 960 degrees Centigrade and is formed into ingots (Blöcke oder Barren). After the ingots are formed, they may be subjected to one or more stages of purification. In one type of purification process, the germanium ingot is placed in a furnace of an inertgas atmosphere, is melted (geschmolzen), and then is progressively cooled from one end to the other.

During this cooling process, impurities present in the germanium tend to concentrate at each end of the ingot. The inner portion of the ingot, therefore, has a higher purity than the ends where the impurities are concentrated. The low-purity ends of the ingot may be cut off and the process repeated if additional purification is needed.

The germanium ingot formed by these purification techniques is polycrystallinc. Greater uniformity is obtained in a further process in which a single crystal is formed from this polycrystalline ingot. In this process the polycrystalline germanium is placed in a graphite pot and melted.

A small single crystal of germanium is dipped into the surface of the melt, then withdrawn very slowly, pulling with it some molten germanium which solidifies on the crystal seed. The speed of withdrawal may be about l/4 inch per minute. The temperature of the germanium is controlled very closely during the crystal "growing," with a permissible variation of no more than ±1 degree Centigrade.
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The germanium crystal is composed of millions of germanium atoms, each consisting of a positively charged nucleus and a number of negatively charged electrons. All but four of the electrons are tightly bound to the nucleus and cannot enter into chemical reactions with electrons of other atoms.

The remaining four electrons, which are able to enter into chemical processes, are called valence elections. In the pure germanium crystal, however, the atoms are arranged in such a fashion that the valence electrons are fixed in place and contribute only slightly to electrical conductivity. The resistivity, which is the reciprocal of conductivity, of the pure germanium, therefore, is higher than that of germanium containing impurities.

As more impurity atoms of this type are added, the conductivity of the germanium increases and the resistivity decreases. Germanium having an excess of electrons due to the addition of such impurities is known as "n"-type germanium, that is, germanium having an excess of negative charges.

"N"-type impurities are also known as "donor" impurities because they donate electrons to the crystal conductivity. Typical donor impurities are arsenic, antimony, and phosphorus.

Conduction in germanium may also be increased by adding a second type of impurity such as aluminum, boron, or indium. Fig. 14 illustrates how these impurities, known as "p"-type impurities, create a deficiency of electrons. If impurities having three valence electrons per atom are added, each impurity atom (A) takes the place of a germanium atom and its three valence electrons form electron-pair bonds with electrons of neighboring germanium atoms.

In order to fit completely into the valence bond structure of the crystal, the impurity atom borrows an electron from an electron-pair bond from somewhere else in the crystal (B), thus leaving a net positive charge in the half-empty bond.

This positive charge is known as a "hole"; these holes contribute to the conductivity of the crystal in much the same manner as electrons because they also can move from atom to atom.

As more "p"-type impurities are added, more holes are formed and the conductivity of the crystal is increased. The main distinction between the two types of germanium is that the "n"-type has an excess of electrons while the "p"-type has an excess of holes.

Both "n" and "p" types are used in transistors: in certain types of transistors both exist in different parts of the same crystal. The "n"-type germanium is used predominantly in the present point-contact transistors.
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The third type of impurity includes those which do not have three or five valence electrons. These impurities, which are present in very small quantities, may not affect the conductivity of the germanium, but may disturb the crystal structure and adversely affect transistor properties.

The role which "p"-type and "n"-type impurities play in determining the resistivity of germanium may be appreciated by noting the change in resistivity which occurs with a change in the ratio of impurity atoms to germanium atoms.

The density of germanium atoms in pure germanium is approximately 4.5 x 10 hoch 22 atoms per cubic centimeter; there are approximately 3.7 x 10 hoch 19 germanium atoms in the average pellet used for a transistor. If 4.5 x l0 hoch 14 "n"-type impurity atoms are added to each cubic centimeter of pure germanium, or one impurity atom for every 100,000,000 germanium atoms, the resistivity of the germanium drops from 60 ohms per centimeter cube to approximately 3.8 ohm per centimeter cube, a value which is satisfactory for use in a point-contact transistor.

If 4.5 x 10 hoch 15 impurity atoms are added to the germanium, however, the resistivity drops to 0.38 ohms per centimeter cube, a value which is too low for transistor use.

This example illustrates how critical are the quantities of impurities which must be added. The problem is further complicated by the fact that "p"-type impurities may be present in the germanium ingot when the "n"-type impurities are introduced, and the holes and electrons furnished by the two types of impurities may cancel each other out.

If both type of impurities were present in equal amounts, the resistivity would be the same as if no impurities were present.
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Before the operation of the transistor in specific circuits is covered, it is important to discuss briefly the current rectification obtained when the metal point electrode makes contact with the germanium-crystal surface.

When this contact occurs, a nonlinear relationship exists between a voltage applied to and the current flowing through the point of contact. A so-called "barrier" (eine Sperre) to the flow of current will be present or absent depending upon the polarity of the voltage applied to the metal point.

For instance, if a metal point contacts the surface of an "n"-type germanium, the barrier will be absent and a large forward current will flow if the metal point is biased positively with respect to the crystal.

If the point is biased negatively with respect to the crystal, the barrier will be present and only a small reverse current will flow. If the germanium is a "p"-type, the forward current will flow when the point is biased negatively with respect to the crystal.

One explanation of this barrier is that it is a very thin layer at the surface of the crystal which acts as an insulating layer. If the germanium resistivity is too low, this insulating barrier at the surface does not exist because of the large number of current carriers present both in the interior and on the surface of the germanium, and poor rectification results.
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The current amplification factor is defined as the ratio of the change in collector current to a change in emitter current when the collector voltage is maintained constant.

A very significant characteristic of the transistor, however, is that this current amplification factor may actually be two or greater. Factors greater than unity are made possible by the electrical "forming" process.

One explanation of the results of this process is that a space charge of holes is formed around the collector point. It appears that this positive charge increases the electron flow from the metal collector to the germanium and account for the increased current amplification. *1)
*1) Shockley. W: "Electrons and Holes in Semiconductors." I). Van Nostrand, 1950.

The transistor amplifies not only input current, but also power. Because the emitter is biased in the forward direction, only a small impedance to the flow of current exists; therefore, the input impedance of the transistor is fairly low, on the order of 500 ohms. The collector, on the other hand, is biased in the reverse direction; it offers a higher impedance, therefore, to the flow of current.

The collector resistance comprises the greatest portion of the output impedance of the transistor. The load resistance, to provide a proper impedance match, must be fairly high, on the order of 10,000 to 20,000 ohms. With the input signal applied to the transistor at a low impedance and the output taken from a high impedance, power amplification results.

In the "p"-type transistor, electrons are emitted from a negatively biased emitter and are collected at a positively biased collector. In general, the "p"-type transistor has characteristics similar to the "n"-type unit, except that in operation all battery polarities are reversed.
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The frequency response of the point-contact transistor is limited by the "transit time" of the holes or electrons. Transit time is the time it takes the holes or electrons to travel from the emitter to the collector. The transit time in seconds may be calculated approximately through use of expressions.

Since an improved frequency response results from a small transit time, it can be seen from this expression that the response can be improved by using germanium of high resistivity and small contact spacings. The mobility of the holes or electrons is the velocity with which they move through the germanium when an electric field is applied.

In the case of "n"-type germanium, holes travel from the emitter to the collector; in the case of "p"-type germanium, electrons travel from the emitter to the collector. The mobility of electrons is greater than that of holes and, consequently, the frequency response of "p"-type germanium is slightly better than that of "n"-type germanium, provided that the contact spacings and resistivities are comparable.

The frequency response may be defined as the measure of the change in current amplification with increasing frequency. The current amplification factor of certain types of close-spaced point-contact transistors drops approximately 3db at 10 mc. A 3db drop in gain has been chosen to define the cut-off frequency.

This method of measuring frequency response, however, defines only one parameter as a function of frequency. If the power gain of the device is measured as a function of frequency in an amplifier with a high-impedance load, the response of the transistor deteriorates more rapidly. A transistor having a 3-decibel drop in the current amplification factor at 10 megacycles may have a cut-off of voltage or power gain at 4 megacycles or less.
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The amount of conduction of heat away from the contact area varies inversely with the ambient temperature. As the ambient temperature is increased, the temperature at the point of contact becomes too great and the collector resistance is reduced. Changes in other properties of the transistor, such as the emitter resistance, transfer resistance, and internal feedback resistance, may also occur.

The net result of these changes is a loss of power gain, changes in bias conditions, and possible permanent damage to the transistor. For best operation, germanium transistors should be operated at temperatures below 60 degrees centigrade.

The maximum dissipation ratings of the device should also be reduced as ambient temperatures are increased above normal room temperature. At ambient temperatures below 25 degrees centigrade the situation is less critical, and if the temperature is low enough higher dissipations may be used without loss of stability.

* Wallace. R. L. Jr. and Pietenpol. W. J.: "Some Circuit Properties and Applications of N-P-N Transistors." Proceedings of the I.R.E. Vol. 39, No. 7. pages 753-767, July, 1951.
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Other developmental germanium-crystal devices, known as "p-n" junction transistors, have somewhat different characteristics from those of the point-contact transistor. In comparison with currently produced point-contact types, the junction transistors have lower noise, higher power gain, greater efficiency of operation, and higher power-handling capabilities.
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When considering the possible circuit applications for the two types of transistors, one must be aware of the advantages and limitations of both types.

At the present time, the advantages of high gain, low noise, and greater stability of the simple junction transistor can be utilized at frequencies up to several megacycles in applications such as r.f. and i.f. amplifiers of standard broadcasting receivers.

In addition, power outputs greater than one watt appear to be possible in oscillator and amplifier applications in the audio frequency and low frequency ranges. Another feature of the junction transistor is its ability to amplify and oscillate with microwatt power inputs.

The frequency response of the point-contact transistors, on the other hand, is somewhat higher than that of junction types. As with junction types, point-contact types which are currently available, can be made to oscillate and amplify over the broadcast-frequency band. When used as an amplifier, point-contact transistors have a relatively flat response over the entire broadcast band and beyond.

Types now under development will operate at considerably higher frequencies. Feedback in these units has been reduced to values which make stable operation at radio frequencies practical. The point-contact transistor therefore, may also have considerable application in radio circuits and may be used in intermediate-frequency amplifiers, radio-frequency oscillators, and other circuits not associated with the high-power stages of r.f. systems.

Point-contact transistors have been developed which are capable of oscillating at frequencies well over 100 mc. Oscillations at frequencies higher than 200 mc. have been obtained - one developmental unit has oscillated at a frequency over 300 mc.
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One of the most important uses of the point-contact transistor probably will be in counter circuits. A number of recent publications describe some basic circuits which utilize the negative resistance properties of one or more transistors.

These circuits generate pulses of various waveforms, store information for varying periods of time, add, subtract, multiply, and divide. Up to the present time these functions, and many others, have been performed in electronic computers by large numbers of electron tubes for which the heater-power supplies alone have been considerable.

Use of the transistor would obviously alleviate this situation since no heater power is required. Furthermore little d.c. power is necessary for operation. The adverse characteristics of transistors with regard to frequency response, noise and power output are relatively unimportant factors in computer circuits. Computers which employ germanium devices would have the advantages of small size, ruggedness, and economy of operation and maintenance.

The progress in the field of germanium devices is not limited to the field of transsistors. While the point-contact germanium diode has already attained commercial acceptance, new types of diodes utilizing the "p-n" junction rectification characteristics are being developed. One diode power rectifier which utilizes a p-type or acceptor impurity metal diffused onto a pellet of germanium has already been described. *8)

Peak inverse voltages of 400 volts are permissible with these devices which have very low resistances in the forward direction and current-carying capabilities as high as 350 milliamperes. When the relative infancy of the germanium power rectifier is considered, it is difficult to estimate the ultimate importance of these devices.

Because of improved efficiency, however, they appear to be suitable both as a replacement for the selenium rectifier and as an advantageous substitute for certain types of rectifier tubes.
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*7) Eberhard. E.. Endrey, R. O.. and Moore.
R. P.: "Counter Circuits Using Transistors."
RCA Review, Vol. X, No. 4, page 459. December 1949.

*8) Saby. J. S.: "Recent Developments in Transistors and Related Devices" Tele-Tech, Vol, 10, No. 12. December, 1951.

Another germanium device of considerable significance is the phototransistor. *9) This photocell is a photo-conductive device and operates on the principle that light absorbed by germanium changes its conductivity. In the phototransistor, a point contact acts as the collector and draws a small amount of current. Light in the vicinity of the collector increases the conductivity of the germanium and the current through the collector.

The first transistor was announced only three and one-half years ago. Great strides have been made in learning the fundamental theory of operation of transistor devices, and much progress has been made in the knowledge of the control of transistor characteristics and manufactuting processes.

There appear to be a number of fields in which transistors will be used widely and to great advantage. Further improvements in their characteristics may be expected as research and development continue.

The author wishes to acknowledge the advice and contributions of Mr. E. W. Herold and Dr. J. Kurshan of the RCA Laboratories Division, Princeton, N. J. and of Mr. R. M. Cohen and Mr. H. Nelson of the RCA Tube Department. Harrison, N. J.