Meet the Transistor
January 1955 Popular Electronics
Shortly before Christmas, 1947, the experimental work of Bell Laboratories scientists John Bardeen, Walter Brattain, and William Shockley resulted in the world's first semiconductor transistor. With proper biasing, the germanium transistor demonstrated an ability to produce signal gain. The signal fed to the base resulted in a higher amplitude signal at the collector. Voila, the device which would ultimately replace the vacuum electron tube had been invented. The rest, as they say, is history. Aside from a few high power applications, the only new equipment produced that uses vacuum tubes are retro things like audio amplifiers and simple receivers. Of course, there is still a large cadre of vacuum tube users in the Amateur Radio real and vintage equipment restorers. If you have never watched a chassis full of tubes turn on and begin glowing, it is worth your while to find someone with an old radio - or even a TV - and take in the nostalgia.
January 1955 Popular Electronics
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Meet the Transistor
A tiny component that will revolutionize the entire electronics industry - can operate from dry cells.
Few electronic inventions have captured the public's interest as has the development of the transistor. This device, although requiring but 1/1000th the power and a small fraction of the space of a vacuum tube, may be used as an amplifier, detector, or oscillator and is thus capable of handling many of the jobs that vacuum tubes are normally employed to do. In addition, since the transistor has no filament to burn out and because it operates at comparatively low temperatures, it has a life expectancy from 10 to 20 times greater than the average tube.
For these reasons and because of its small size (see Fig. 1) it has virtually replaced tubes in the manufacture of hearing aids. It is also being used in military equipment, specialized communications gear, and in certain types of instruments. Eventually it will be used in portable receivers, home radio and TV sets, and auto receivers. Tiny transistorized "wristwatch" and pocket-sized transmitters and receivers have already been built experimentally and offer commercial possibilities.
Fig. 1. Miniature vacuum lube dwarfs the transistor which can replace it. Later models are even tinier.
Fig. 2. A cutaway view of a point-contact transistor. Note two "cat's whiskers".
Fig. 3. (A) Basic transistor amplifier. (B) Symbol for an "n-p-n" junction transistor.
Fig. 4. Diagram and photo of an "n-p-n" junction transistor. Note changes from Fig. 2.
Transistors are made possible by the electrical properties of a group of materials known as semi-conductors, consisting of substances which may act either as conductors or insulators, depending on their physical conditions. Germanium, silicon, and selenium are the most popular semi-conductors, with germanium being used almost exclusively in the manufacture of transistors.
In a normal conductor, such as copper or silver, current flows when an electrical voltage is applied to the material causing a movement of free electrons through the substance. In a semi conductor, the application of voltage alone may not be sufficient to initiate current flow - some other physical condition may be necessary such as the presence of light, heat, or of an additional electrical field. The current flow, when it does take place, may consist not only of the movement of free electrons but may also include the movement of electrical "holes" through the material.
A "hole" is formed when an individual molecule loses an electron. The molecule lacking an electron may pick up one from a nearby, electrically neutral molecule, thus leaving the second molecule with a hole and a net positive charge. In this way, the hole may travel through the substance, jumping from molecule to molecule and producing a current flow which acts just as if it consisted of movement of positively charged particles.
Although current flow through a particular substance may consist of a movement of both holes and electrons, if the current flow is made up primarily of a movement of holes, the material is called a "positive-carrier" or p-type semi-conductor. If the current flow is made up primarily of a movement of electrons, it is called a "negative-carrier" or n-type semi-conductor. A transistor is made up of a combination of these materials.
Types of Transistors
Transistors are usually divided into two basic types, depending on their method of construction, i.e., the point-contact and the junction types. A cutaway view of a point-contact type is shown in Fig. 2 while a junction type is illustrated in Fig. 4.
A point-contact transistor consists of a small cube of semi-conductor material with two fine wires or "cat's whiskers" contacting its surface. Electrical connections are made to the semi-conductor, called the "base", and to each of the two contact wires - one of which is called the "emitter", the other the "collector". If n-type semi-conductor material is used, it is called an n-base point-contact transistor. Small p-type areas are formed under the contact wires during manufacture. If p-type material is used in the base, the transistor is a p-base unit, and small n-type areas are formed under the tips of the contacts.
A junction transistor consists of a "sandwich" of two types of semi-conductor material, with the inner layer of different material from the two outer layers. The basic construction is shown in Fig. 4. If the inner layer is of p-type material, the unit is called an n-p-n junction transistor - if of n-type material, a p-n-p transistor results. The n-p-n type is shown in Fig. 4. Electrical connections are made to the two outer layers and to the inner layer of the "sandwich", with terminals identified as "emitter", "base", and "collector", just as in the case of the point-contact type.
How Transistors Work
Operation of the transistor may be understood by referring to a basic transistor amplifier circuit (see Fig. 3A). An n-p-n junction transistor is used.
In operation, the emitter-base circuit is "biased" by battery B1 in such a way that a low resistance is offered to the flow of current through the n-p emitter-base junction. The collector-base circuit, on the other hand, is biased with reverse polarity by battery B2 and offers a high resistance to the flow of current, in fact, current flow can only take place through the p-n base collector junction because of the excess of electrons produced by the current flow in the emitter-base circuit.
If a signal is applied to the input terminals (across R1
) the variations in the emitter-base current which result will cause a variation in the number of free electrons in the base, with resulting changes appearing as an amplified signal across R2
. In practice, the emitter and collector currents may be on the same order of magnitude, but a considerable signal power gain is obtained since the collector circuit represents a high impedance while the input (emitter-base) circuit represents a low impedance.
Operation of a p-n-p transistor amplifier is similar except that conduction in the base-collector circuit is principally by means of "holes" instead of electrons.
The schematic symbol used to identify an n-p-n type transistor in wiring diagrams is shown in Fig. 3B. The base is represented by a straight line, with the emitter and collector terminals identified by slanting lines to the base with the emitter further identified by an arrowhead pointing away from the base.
The p-n-p type transistor is wired directly opposite from the n-p-n type, that is, the collector voltage is negative while the emitter voltage is positive. The symbol for the p-n-p type is identical to that for the n-p-n except that the arrowhead points toward the base instead of away from it.
In this article we have discussed the basic principles of transistors and explained how they can be used in a simple amplifier. Subsequent issues will describe other amplifiers as well as detectors and oscillator circuits using transistors. END