The transistor is a three-terminal device primarily used to amplify signals and control current within a circuit. [Figure 12-220]
The basic two-junction semiconductor must have one type of region sandwiched between two of the other type. The three regions in a transistor are the collector (C), which is moderately doped, the emitter (E), which is heavily doped, and the base (B), which is significantly less doped. The alternating layers of semiconductor material type provide the common commercial name for each type of transistor. The interface between the layers is called a junction. Selenium and germanium diodes previously discussed are examples of junction diodes. Note that the sandwiched layer or base is significantly thinner than the collector or the emitter. In general, this permits a “punching through” action for the carriers passing between the collector and emitter terminals.
The transistors are classified as either NPN or PNP according to the arrangement of their N and P-materials. The NPN transistor is formed by introducing a thin region of P-material between two regions of N-type material. The opposite is true for the PNP configuration.
The two basic types of transistors along with their circuit symbols are shown in Figure 12-221.
Note that the two symbols are different. The horizontal line represents the base, and two angular lines represent the emitter and collector. The angular line with the arrow on it is the emitter, while the line without is the collector. The direction of the arrow on the emitter determines whether or not the transistor is a PNP or an NPN type. If the arrow is pointing in, the transistor is a PNP. On the other hand, if the arrow is pointing out, then it is an NPN type.
As discussed in the section on diodes, the movement of the electrons and holes can be considered current. Electron current moves in one direction, while hole current travels in the opposite direction. In transistors, both electrons and holes act as carriers of current.
A forward biased PN junction is comparable to a lowresistance circuit element, because it passes a high current for a given voltage. On the other hand, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using Ohm’s Law formula for power (P = I2R) and assuming current is held constant through both junctions, it can be concluded that the power developed across the high resistance junction is greater than that developed across a low resistance junction. Therefore, if a crystal were to contain two PN junctions, one forward biased and the other reverse biased, and a low-power signal injected into the forward biased junction, a high-power signal could be produced at the reverse-biased junction.
To use the transistor as an amplifier, some sort of external bias voltage must modify each of the junctions. The first PN junction (emitter-base) is biased in the forward direction. This produces a low resistance. The second junction, which is the collector-base junction, is reverse biased to produce a high resistance. [Figure 12-222]
With the emitter-base junction biased in the forward direction, electrons leave the negative terminal of the battery and enter the N-material. These electrons pass easily through the emitter, cross over the junction, and combine with the hole in the P-material in the base. For each electron that fills a hole in the P-material, another electron leaves the P-material, which creates a new hole and enters the positive terminal of the battery.
The second PN junction, which is the base-collector junction, is reverse biased. This prevents the majority carriers from crossing the junction, thus creating a high-resistance circuit. It is worth noting that there still is a small current passing through the reversed PN junction in the form of minority carriers—that is, electrons in the P-material and holes in the N-material. The minority carriers play a significant part in the operation of the NPN transistor.
Figure 12-223 illustrates the basic interaction of the NPN junction. There are two batteries in the circuit used to bias the NPN transistor. Vbb is considered the base voltage supply, rated in this illustration at 1 volt, and the battery voltage Vcc, rated at 6 volts, is called the collector voltage supply.
Current within the external circuit is simply the movement of free electrons originating at the negative terminal of the battery and flowing to the N-material. [Figure 12-223]
As the electrons enter the N-material, they become the majority carrier and move through the N-material to the emitter-base PN junction. This emitter-base junction is forward biased at about 0.65 to 0.7 volts positive with respect to the emitter and presents no resistance to the flow of electrons from the emitter into the base, which is composed of P-material. As these electrons move into the base, they drop into available holes. For every electron that drops into a hole, another electron exits the base by way of the base lead and becomes the base current or Ib. Of course, when one electron leaves the base, a new hole is formed. From the standpoint of the collector, these electrons that drop into holes are lost and of no use. To reduce this loss of electrons, the transistor is designed so that the base is very thin in relation to the emitter and collector, and the base is lightly doped.
Most of the electrons that move into the base fall under the influence of the reverse bias of the collector. While collector-base junction is reverse biased with respect to the majority carriers, it behaves as if it is forward biased to the electrons or minority carriers in this case. The electrons are accelerated through the collector-base junction and into the collector. The collector is comprised of the N-type material; therefore, the electrons once again become the majority carrier. Moving easily through the collector, the electrons return to the positive terminal of the collector supply battery Vcc, which is shown in Figure 12-223 as Ic.
Because of the way this device operates to transfer current (and its internal resistances) from the original conduction path to another, its name is a combination of the words “transfer” and “resistor”—transistor.
PNP Transistor Operation
The PNP transistor generally works the same way as the NPN transistor. The primary difference is that the emitter, base, and collector materials are made of different material than the NPN. The majority and minority current carriers are the opposite in the PNP to that of the NPN. In the case of the PNP, the majority carriers are the holes instead of the electrons in the NPN transistor. To properly bias the PNP, the polarity of the bias network must be reversed.
Identification of Transistors
Figure 12-224 illustrates some of the more common transistor lead identifications. The methods of identifying leads vary due to a lack of a standard and require verification using manufacturer information to properly identify. However, a short description of the common methods is discussed below.
Figure 12-224D shows an oval-shaped transistor. The collector lead in this case is identified by the wide space between it and the lead for the base. The final lead at the far left is the emitter. In many cases, colored dots indicate the collector lead, and short leads relative to the other leads indicate the emitter. In a conventional power diode, as seen in Figure 12-224E, the collector lead is usually a part of the mounting bases, while the emitter and collector are leads or tines protruding from the mounting surface.