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Part VIII — Transistor Principles

Just a little more than 20 years ago, a team of physicists at Bell Telephone Laboratories stuck some wires onto a solid crystal and came up with a device which acted like a cross between a transformer and a resis-or—and which in the two decades which followed has revolutionized all phases of electronics.

Despite the widespread application of the transistor, however, its operation and use has been almost ignored in the FCC examinations up until the latest versions. The new Advanced Class exams include a number of questions on semiconductors, and this month's chapter of our Advanced-Class study course will concentrate on this area.

Only three of the questions in the official FCC study list for the new exams deal directly with semiconductors. I hese, which we will go into this time, are;

32. Power dissipation in what part of a transistor warrants careful observation of power ratings?

40. Compare transistors and tubes. What are the advantages and disadvantages of each?

46. What is the vacuum tube counterpart of (1) a grounded base circuit; (2) grounded emitter circuit; (3) grounded collector circuit?

Lest you think that three questions is too small a number to spend an entire instalment on, read them again—and notice that two of the three require a rather comprehensive knowledge of transistor principles for adequate answers!

As we have done in ¡he past, let's re-phrase the official questions into another group which can he examined in a more orderly sequence.

The first question in any dealings with a transistor must be "How does a transistor work?" You don't; have to have a solid-state physicist s knowledge of the "how", but any use of the devices becomes much easier when you have some idea what s going on inside.

Almost all transistor specifications are given in terms of a "black-box analysis" which boils down to one of the thre® basic circuits listed in FCC question 46. That makes our second question for dissection become "What are the basic transistor circuits and how do they differ?"

An adequate comparison of transistors and tubes requires a knowledge of both the advantages of the transistor! in relation to the more familiar tube. Our third question thus is "What are the transistor's advantages?" and the fourth is "What are its disadvantages?"

Finally, the power-rating question (No. 32 in the FCC list) is only one of a number of possible similar questions dealing with critical points in the application of transistors. To he prepared for all these questions, let s find out "What are the critical factors in using transistors?"

For those among you who arc physicists, let's spell out in advance that this is a practical explanation of all these questions and as such, necessarily runs the risk of becoming oversimplified at some points. You aren't going to get very involved with "holes" or "minority carriers", and the matrix algebra so commonly encountered in any examination of transistor circuit approximations is going to be conspicuous by its absence. Many good books have already been written oil solid-state physics, and we've studied quite a few of tiem in preparation for this article. But this article aims to give sufficient understanding of what goes on to satisfy the exam requirements, and possibly to whet interest in pursuing the details later.

Okay? Let's get on with it.

How Does a Transistor Work? Back in those dear dead days before semiconductors, most of us learned that vacuum tubes amplifying by deflecting and/or rejecting elec-

1, i7i& swiicfti^g circuit shouts the basis of transistor action; details are explained in the text, trons in transit from cathode to plate, and that this action was brought about by the electrical charge on the tube's grid. The electrons were boiled oft' the cathode by the iieal; of the filament, and the tube had to have a vacuum because otherwise the grid couldn't accurately control the electron flow.

Al! of this is still true. So, how can a solid chunk of something very like sand do any amplifying when it has neither cathode, grid, nor plate, no vacuum in it, and no electric-charge effect worth talking about?

The answer is hinted at in the name of the device, which is a blend of "trans*' meaning "through" and 'resistor", The transistor is a special type of variable resistor, 111 which current injected (or withdrawn) from one terminal apparently goes "through" and affects current flow between the other two terminals, Incidentally, the familiar vacuum tube can be thought of in the same manlier, as a variable resistor whose resistance is varied by grid voltage—and then the opera Ling similarities between tubes and transistors become obvious. They operate in functionally the same way, except that transistors are operated by current while tubes operate by voltage.

Fig, 1 shows a simplified approximation of what a transistor docs in a circuit, Placing the switch in position A, with the voltage and resistances shown, would cause one amp of current to flow through Rl, with a resulting power dissipation of 10 watts, [f, when we throw the switch to position R we could keep that same one amp of current flowing through the circuit composed now of RI and R2 in series, we would increase the power dissipation by a hundred times. This would be a form of power amplification—and in effect, this is what a transistor does.

The trail si st or consists of two adjacent junctions, between different types of semiconductor material known as "n" and *p" type. You can think of it as a thin slice of ham between two thick slices of bread if you like-

In a single junction, current can flow much more easily in one direction than in the other. When negative polarity is applied to the "n" side of the junction and positive to the side, current flow is easy; when polarity is reversed, current flow is difficult and apparent resistance is high.

lie "easy" current flow is a combination of two processes known as "injection" and "collection". The material on one side of the junction "injects" electrical charge into the junction region, and that on the other side of the junction "collects" this charge.

To get from one side of the junction to the other, the charge must "diffuse' through the junction region, In a single junction the diffusion will be either aided or hindered by the types of material involved and the polarity of the charge. This is what makes the current flow easy in one direction— when both materials aid the flow—and difficult in the other—when both oppose.

Such a single junction is widely used, for many purposes, The familiar crystal diode is one example of such a junction. The silicon power rectifier is another.

When we place the second junction adjacent to the first to turn the diode into a transistor, a number of additional happenings come into the picture. The basic processes of injection, diffusion, and collection remain the same. To obtain transistor action, though, one of the two junctions must be biased in the forward or easy-current direction while the other must be biased in the reverse or high-resistance direction- The single slab of material in the middle, which corresponds to the ham in the sandwich and is common to both junctions, controls the action.

Not all of the charge injected into this middle region or "base" by the forward-biased junction from the "emitter" materia] is collected by the base. Like the stream of electrons in the vacuum tube which flow past the grid to the plate, some of the injected charge passes right on through the base into the second reverse-biased junction. However, this charge is of the proper polarity to tend to forward-bias— or at least reduce the amount of reverse bias on—this second junction,

And when the amount of reverse bias is reduced, the resistance across this second junction reduces right along with it.


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