As is the’ case in a reverse-biased diode, the current through the collector-base junction of it transistor may increase suddenly if the reverse-biasing voltage across it is made sufficiently large. This increase in current is typically caused by the avalanching mechanism already described in connection with diode breakdown. However! in a transistor it can also be the result of a phenomenon called punch through. Punch through occurs when the reverse bias widens the collector-base depletion region to the extent that it meets the base-emitter depletion region. This joining of the two regions effectively shorts the collector to the emitter and C? ises
a substantial current flow. Remember that the depletion region extends farther into the lightly doped side of a junction, and that the base is more lib!ltly doped than the collector. Furthermore, the base is made very thin, so the two junctions are already relatively close to each other. Punch through can be a limiting design factor in determining the doping level and base width of a transistor. Figure 4-14 shows
Although the base-emitter junction is not normally reverse biased, there are practical applications in which it is periodically subjected to reverse bias. Of course, it too can break down, and its reverse breakdown voltage is usually much less than that of the collector-base junction. Base-emitter breakdown is often destructive, so designers must he aware of the manufacturer’s specified maximum reverse baseemitter voltage. “,
As a final note on transistor operation, we should mention that some transistors can he (and occasionally are) operated in what is called an inverted mode. In this mode, the emitter is used as the collector and vice versa. normerly. the emitter is the most heavily doped of the three regions, so unless a trausistor is specifically designed lor inverted operation, it will not perform well in that mode. The (.\’in the inverted mode, designated ai, is generally smaller than the (.\’that can he realized in conventional operation.
COMMON-EMITIER CHARACTERISTICS
The next transistor bias arrangement we will study is called the conunon-cmittcr (C’E) conliguration. It is illustrated in Figure 4-15. Note that the external voltage source V/ill is used to forward bias the base-emitterjunction and the external source Vcr is used to reverse bias the collector-base junction. The magnitude of Vcc must be greater than Villi to ensure that the collector-base junction remains reverse biased. since, as can be seen in the figure, VCII = Vn – Villi. (Write Kirchhoff’s voltage law around the loop from the collector, through Vu’, through Villi. and back to the collcctor.) The emitter terminal is, of course, the ground, or common, terminal in this configuration.
Figure 4-16 shows that the input voltage in the CE configuration is the basee mitter voltage (VII’ for an NPN and V”II for a PNP), and the output voltage is the collector-emitter voltage (Vn. for an NPN and V/:c for a PNP). The input current is 1″ and the output current is lc- The common-emitter configuration is the most useful and most widely used transistor configuration and we will study it in considerable detail. In the process we will learn some new facts about transistor behavior.
ICEO and Beta
Before investigating the input and output characteristics of the CE configuration. we will derive a new relationship between lc and /(‘/10. Although this derivation does not depend in any way on the bias arrangement used, it will provide us with some new parameters that are useful for predicting leakage in the CE configuration and for relating CE input and output currents. Equation 4-4 states thatUsing equation 4-6, we can obtain an expression for reverse “leakage” current in the CE configuration. Figure 4-17 shows NPN and PNP transistors in which the base-emitter circuits are left open while the reverse-biasing voltage sources remain connected. As can be seen in Figure 4-17, the only current that can flow when the base is left open is reverse current across the collector-base junction. This current flows from the collector, through the base region, and into the emitter. It is designated Icw-Collector-to-Emitter current with the base Open. (Note, once again)
that this “reverse” current is in the same direction as normal collector current through the transistor.) Since I/J must equal 0 when the base is open, we can substitute 1/1 = 0 in equation 4-6 to obtain
Beta is always greater than I and for typical transistors ranges from around 20 to several hundred. When a is close to I, a small/increase in a causes a large increase in the vahe of {3. For example, if a = 0.99, then (3 = 0.99/(1 – 0.99) = 99. If a is increased hy 0.005 to 0.995, then (3 = 0.995/(1 – 0.995) = 199. Because a small change in a causes a large change in {3, small manufacturing variations in transistors that are supposed to be of the same type cause them to have a wide range of {3 values. It is not unusual for transistors of the same type to have betas that vary from 50 to 200.
In terms of {3,equation 4-6 becomesc
or
Although lceo is much greater than ICBo, it is generally quite small in comparison to {31/J’ especially in silicon transistors, and it can be neglected in many practical circuits. Neglecting lcso in equation 4-10, we obtain the approximation lc “‘”{3IH• This approximation is widely used in transistor circuit analysis, and we will often
Common-Emitter Input Characteristics
Since the input to a transistor in the CE configuration is across the base-to-emitter junction (see Figure 4- t 6), the CE input characteristics resemble a family of forward- biased diode curves. A typical set of CE input characteristics for an NI>N transistor is shown in Figure 4-18. Note that /1/ increases as Vu: decreases for a
fixed value of VI/f:. A large value of Vo: results in a arge reverse ias of the collector-base junction, which widens the depletion region and makes the base smaller. When the base is smaller, there are fewer recombinations of injected minority carriers and there is a corresponding reduction in base current. In contrast to the CB input characteristics, note in Figure 4-18 that the input current is plotted in units of microamperes. /1/ is, of course, much smaller than either h or I, (1/1 ~ /(/(3). The CE input characteristics are often called the base chnractcristics