Transistors are semiconductor devices used to amplify or switch electronic signals. They usually have three terminals for connection to an external circuit. Transistors are one of the basic building blocks of modern electronics. There are basically two types of transistors, Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET).
Bipolar transistors have two P-N junctions connected together. This is done in either of two ways: a P-type layer sandwiched between two N-type layers, or an N type layer between two P-type layers. Bipolar transistors, like diodes, can be made from various semiconductor substances. Silicon is probably the most common material used.
NPN versus PNP
A simplified drawing of an NPN transistor, and its schematic symbol, are shown in Fig. 22-1. The P-type, or center, layer is called the base. The thinner of the N-type semiconductors is the emitter, and the thicker is the collector. Sometimes these are labeled B, E, and C in schematic diagrams, although the transistor symbol alone is enough to tell you which is which.
A PNP bipolar transistor is just the opposite of an NPN device, having two P-type layers, one on either side of a thin, N-type layer (Fig. 22-2). The emitter layer is thinner, in most units, than the collector layer.
You can always tell whether a bipolar transistor in a diagram is NPN or PNP. With the NPN, the arrow points outward; with the PNP it points inward. The arrow is always at the emitter.
Generally, PNP and NPN transistors can do the same things in electronic circuits. The only difference is the polarities of the voltages, and the directions of the currents. In most applications, an NPN device can be replaced with a PNP device or vice versa, and the power-supply polarity reversed, and the circuit will still work as long as the new device has the appropriate specifications.
There are many different kinds of NPN or PNP bipolar transistors. Some are used for radio-frequency amplifiers and oscillators; others are intended for audio frequencies. Some can handle high power, and others cannot, being made for weak-signal work. Some bipolar transistors are manufactured for the purpose of switching, rather than signal processing. If you look through a catalog of semiconductor components, you'll find hundreds of different bipolar transistors, each with its own unique set of specifications.
Why, you might ask, need there be two different kinds of bipolar transistor (NPN and PNP), if they do exactly the same things? Sometimes engineers need to have both kinds in one circuit. Also, there are some subtle differences in behavior between the two types. These considerations are beyond the scope of this book. But you should know that the NPN/PNP duality is not just whimsy on the part of people who want to make things complicated.
NPN biasing
You can think of a bipolar transistor as two diodes in reverse series. You can't normally connect two diodes together this way and get a good transistor, but the analogy is good for modeling the behavior of bipolar transistors, so that their operation is easier to understand.
A dual-diode NPN transistor model is shown in Fig. 22-3. The base is formed by the connection of the two diode anodes. The emitter is one of the cathodes, and the collector is the other.
The normal method of biasing an NPN transistor is to have the emitter negative and the collector positive. This is shown by the connection of the battery in Fig. 22-3. Typical voltages for this battery (although it might be, and often is, a dc power supply) range from 3 V to about 50 V. Most often, 6 V, 9 V, or 12 V supplies are used.
The base is labeled "control" in the figure. This is because the flow of current through the transistor depends critically on the base bias voltage, EB, relative to the emitter-collector bias voltage, EC.
Zero bias
Suppose that the base isn't connected to anything, or is at the same potential as the emitter. This is zero base bias, sometimes simply called zero bias. How much current will flow through the transistor? What will the milliammeter (mA) show?
The answer is that there will be no current. The meter will register zero.
Recall the discussion of diode behavior from the previous chapter. No current flows through a P-N junction unless the forward bias is at least equal to the forward breakover voltage. (For silicon, this is about 0.6 V.) But here, the forward bias is zero. Therefore, the emitter-base current, often called simply base current and denoted IB, is zero, and the emitter-base junction does not conduct. This prevents any current from flowing between the emitter and collector, unless some signal is injected at the base to change the situation. This signal would have to be of positive polarity and would need to be at least equal to the forward breakover voltage of the junction.
Reverse bias
Now imagine that another battery is connected to the base at the point marked "control," so that EB is negative with respect to the emitter. What will happen? Will current flow through the transistor?
The answer is no. The addition of this new battery will cause the emitter-base (E-B) junction to be reverse-biased. It is assumed that this new battery is not of such a high voltage that avalanche breakdown takes place at the junction.
A signal might be injected to overcome the reverse-bias battery and the forward breakover voltage of the E-B junction, but such a signal would have to be of a high, positive voltage.
Forward bias
Now suppose that EB is made positive, starting at small voltages and gradually increasing.
If this forward bias is less than the forward breakover voltage, no current will flow. But as the base voltage EB reaches breakover, the E-B junction will start to conduct.
The base-collector (B-C) junction will remain reverse-biased as long as EB is less than the supply voltage (in this case 12 V). In practical transistor circuits, it is common for EB to be set at a fraction of the supply voltage.
Despite the reverse bias of the B-C junction, the emitter-collector current, called collector current and denoted IC, will flow once the E-B junction conducts. In a real transistor (Fig. 22-3B), the meter reading will jump when the forward breakover voltage of the E-B junction is reached. Then even a small rise in EB, attended by a rise in IB, will cause a big increase in IC. This is shown graphically in Fig. 22-4.
Saturation
If EB continues to rise, a point will eventually be reached where IC increases less rapidly. Ultimately, the IC vs. EB curve will level off. The transistor is then saturated or in saturation. It is conducting as much as it possibly can; it's "wide open."
This property of three-layer semiconductors, in which reverse-biased junctions can sometimes pass current, was first noticed in the late forties by the engineers Bardeen, Brattain, and Shockley at the Bell Laboratories. When they saw how current variations were magnified by a three-layer device of this kind, they knew they were on to something. They envisioned that the effect could be exploited to amplify weak signals, or to use small currents to switch much larger ones. They must have been excited, but they surely had no idea how much their discovery would affect the world.
PNP biasing
For a PNP transistor, the situation is just a "mirror image" of the case for an NPN device. The diodes are turned around the opposite way, the arrow points inward rather than outward in the transistor symbol, and all the polarities are reversed. The dual-diode PNP model, along with the actual bipolar transistor circuit, are shown in Fig. 22-5. In the discussion above, simply replace every occurrence of the word "positive" with the word "negative."
You need not be concerned with what actually goes on inside the semiconductor materials i NPN and PNP transistors. The important thing is the fact that either type of device can serve as a sort of "current valve." Small changes in the base voltage, EB, cause small changes in the base current, IB. This induces large fluctuations in the current IC through the transistor.
In the following discussion, and in most circuits that appear later in this book, you'll see NPN transistors used almost exclusively. This doesn't mean that NPN is better than PNP; in almost every case, you can replace each NPN transistor with a PNP, reverse the polarity, and get the same results. The motivation is to save space and avoid redundancy.
About the Author
Stan Gibilisco is one of McGraw-Hill's most prolific and popular authors, specializing in electronics and science topics. His clear, reader-friendly writing style makes his science books accessible to a wide audience, and his background in research makes him an ideal editor for professional references and course materials. He is the author of The Encyclopedia of Electronics; The McGraw-Hill Encyclopedia of Personal Computing; and several titles in the popular Demystified library of home-schooling and self-teaching books. His published works have won numerous awards. The Encyclopedia of Electronics was chosen a "Best Reference Book of the 1980s" by the American Library Association, which also named his McGraw-Hill Encyclopedia of Personal Computing a "Best Reference of 1996." Stan Gibilisco maintains a Web site at www.sciencewriter.net.
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