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Bipolar Junction Transistors
Chapter Summary

The bipolar junction transistor (BJT) is a three-terminal device. The terminals are called the emitter, base, and collector. As shown in Figure 6-1, the collector and emitter are made using the same type of semiconductor material (either n-type or p-type), and the base is the made up of the other.

Figure 6-1. Transistor construction.

The schematic symbols for the transistors in Figure 6-1 are shown in Figure 6-2. The arrow in the schematic symbol:

  • Helps identify the transistor terminals.
  • Always points toward the n-type material.
  • Indicates the direction of the emitter current.

Figure 6-2. Transistor schematic symbols.

The transistor terminal currents are shown in Figure 6.2 of the text. The emitter current () normally has the greatest value of the three, followed by the collector current (). Note that the current directions for the pnp transistor are opposite those for the npn transistor.

The values of and are determined primarily by the value of the base current (). Under normal circumstances, is varied, causing a similar change in and . For this reason, the transistor is referred to as a current-controlled device.

Under normal circumstances, the relationship between and is expressed as

where (the Greek letter beta) is the current gain of a transistor. The relationship among , , and is demonstrated throughout the chapter. The various voltages present in a typical transistor circuit are described in Table 6.1 and measured as shown in Figure 6.3.

Transistor Construction and Operation

The transistor has two pn-junctions, as shown in Figure 6-3. Normally, the component is biased using one of the three combinations shown in the figure.

  When a transistor is in cutoff, its collector-base and base-emitter junctions are both reverse biased. This biasing combination is illustrated in Figure 6.6. When biased in cutoff, the transistor allows only a small amount of leakage current to pass through the emitter and collector circuits. For all practical purposes, we can assume that when a transistor is biased in cutoff.

Figure 6-3. Junction biasing combinations.

When a transistor is zero biased, meaning that there are no external biasing potentials applied to the two junctions, depletion layers are present, and the transistor is effectively in cutoff. Zero biasing is illustrated in Figure 6.5.

The opposite of cutoff is saturation. When a transistor is in saturation, its collector-base and base-emitter junctions are both forward biased. In this case, and reach their maximum values, as determined by the supply voltage and resistance values in the collector and emitter circuits. This concept is illustrated in Figure 6.7.

When a transistor is operating in its active region, its base-emitter junction is forward biased and its collector-base junction is reverse biased. This biasing combination is illustrated in Figure 6.8. As shown in the figure, when a transistor is operating in its active region. The characteristics of the three transistor operating regions are summarized in Figure 6.9.

The transistor is a current-controlled device. As a result of its construction, a small change in results in a larger change in the other terminal currents. The larger change in and is a result of the transistor current gain, as demonstrated in Example 6.1.

According to Kirchhoff’s current law, the current(s) leaving a component must equal the current(s) entering the component. Therefore,

Since the value of is typically much greater than the value of , we normally assume that .

 The dc beta ( ) rating of a transistor is the ratio of to . The rating for a given transistor is provided on the component’s spec sheet. This rating is important because the most common transistor circuits use the base as an input terminal and the collector as an output terminal. Therefore, represents the ratio of dc output current to dc input current. When you know the values of and any single transistor terminal current, you can determine the values of the other two terminal currents. This principle is demonstrated in Examples 6.2 through 6.4.

The dc alpha ( ) rating of a transistor is the ratio of to . Since , the value of is always less than unity (1). Unlike , the rating for a given transistor is not typically provided on its spec sheet. However, its value can be calculated using the component’s rating, as follows:

The validity of this relationship is demonstrated in Example 6.5.

 The maximum allowable value of for a given transistor is typically listed on its spec sheet. As shown in Example 6.6, the maximum allowable value of base current can be found as

Another current rating commonly found on spec sheets is the maximum cutoff current. This is the maximum amount of leakage current that a transistor will allow when it is biased in cutoff, measured at specified values of and reverse . Typical voltage ratings include the following:

  • : The collector-base reverse breakdown voltage, measured with the emitter terminal open.
  • : The collector-emitter reverse breakdown voltage, measured with the base terminal open.
  • : The emitter-base reverse breakdown voltage, measured with the collector terminal open.

These voltage ratings are illustrated in Figure 6.20.

 The characteristic curve for a transistor illustrates the relationship among its values of , , and . The curve illustrates the saturation, active, and breakdown characteristics of the device, as shown in Figure 6.21. Normally, the operation of a transistor is represented using a composite of collector curves like the one in Figure 6.25. Note that the area of the curve below the line represents the cutoff region of operation.

 There are several other transistor operating curves that are typically of interest. Among them are the base curve and the beta curve. The base curve for a transistor is very similar to the forward operating curve of a diode. The transistor beta curve illustrates the relationship among beta, , and operating temperature. As the beta curve indicates:

  • Beta increases as increases to some specified value (which is listed on the device spec sheet). Once this value of is exceeded, further increases in collector current result in a decrease in beta.
  • Beta increases as operating temperature increases.

Typical base and beta curves for a transistor can be seen in Figures 6.26 and 6.27.

 Transistor spec sheets list a variety of maximum ratings, thermal characteristics and electrical characteristics. The maximum ratings portion of the spec sheet lists the parameters that cannot be exceeded without risking damage to the component.

 The thermal characteristics portion of the spec sheet provides the thermal resistance () values for the component. Thermal resistance is the opposition to the flow of heat, measured in degrees Celsius per Watt (). The value of thermal resistance indicates the rise in temperature that occurs per Watt of power dissipated. The ideal component would have a rating of , indicating that it could dissipate any amount of power without experiencing a rise in temperature.

The electrical characteristics portion of the spec sheet is typically divided into rating groups, such as off characteristics and on characteristics. These ratings indicate the guaranteed operating characteristics of the component, measured under specified conditions. A sample transistor spec sheet can be seen in Figure 6.28.

 A transistor can be tested using an ohmmeter, as illustrated in Figure 6.30. Each transistor junction is tested as shown in the figure. A problem is indicated if either junction has:

  • Low forward and reverse resistance
  • High forward and reverse resistance

(Remember: A pn-junction should have low forward resistance and high reverse resistance.) Once the two junctions are tested, the transistor resistance is measured from collector to emitter. This resistance should be extremely high in both directions. If the transistor fails any of these tests, it is faulty and must be replaced. Note that transistors are more commonly tested using a transistor checker. Each transistor checker has its own instructions for testing the various types of transistors.

 The principles discussed in terms of npn transistors apply to pnp transistors as well. The difference between the two lies in the voltage polarities and current directions, as shown in Figure 6.33.

 The supply voltages used to bias transistors are normally derived from the system dc power supply, as shown in Figure 6.33.

 There are a variety of npn and pnp transistors, each with its unique strengths. Among them are the following:

  • Integrated transistors contain more than one transistor in integrated form. These components save space and reduce production costs. (See Figure 6.34 of the text.)
  • High-voltage transistors have extremely high reverse breakdown ratings. These components are designed for use in systems that contain high biasing voltages. (See Figure 6.35 of the text.)
  • High-current transistors have extremely high maximum collector current ratings. These components are designed for use in high-current applications. (See Figure 6.36 of the text.)
  • High-power transistors have extremely high power dissipation ratings. They are designed for use in circuits where the relatively high voltages and currents result in high power dissipation requirements. (See Figure 6.37 of the text.)
  • Surface-mount transistors are contained in packages that are much smaller and lighter than their standard IC counterparts. These components are used in low-power applications where size is an important consideration. (See Figure 6.38 of the text.)

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