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3.5.3: Transistor I-V Characteristics

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    89963
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    Let's now take a look at some current-voltage relationships for the bipolar transistor. In the absence of any voltage or current on the emitter-base junction, if we were to make a plot of \(I_{C}\) as a function of \(V_{\text{CB}}\), it would look something like Figure \(\PageIndex{1}\). Check back with the voltage convention in the figures on the structure and forward active biasing of a bipolar transistor to make sure you agree with what I drew. All we've got here is a PN junction or diode. It just happens to be biased in a reverse direction, so it conducts when \(V_{\text{CB}}\) is negative and not when \(V_{\text{CB}}\) is positive. Thus, all we need to do is draw a diode curve, but upside down!

    Graph of I_C vs V_CB. The graph approaches 0 for all I_C greater than a certain negative number, and decreases exponentially for all values of I_C more negative than that number.
    Figure \(\PageIndex{1}\): I-V graph for the collector-base terminals of the bipolar transistor

    What happens if we now also have some bias applied to the emitter-base junction? As we saw, so long as the base-collector junction is reverse biased, almost all of the collector current consists of electrons which have been injected into the base by the emitter, diffuse across the base region, and then fall down the base-collector junction. The rate at which electrons fall down the junction does not depend on how large a drop there is (e.g. how big \(V_{\text{CB}}\) is). The only thing that matters, so far as the collector current is concerned, is how fast electrons are being injected into the base region, which is, of course, determined by the emitter current \(I_{E}\). Thus for several different values of emitter current, \(I_{E_{1}}\), \(I_{E_{2}}\), and \(I_{E_{3}}\), we might see something like Figure \(\PageIndex{2}\).

    Three versions of the curve from Figure 1 above on the same axis, each shifted upwards by some amount. The highest curve approaches alpha times I_E3 at large voltages, the middle curve approaches alpha times I_E2, and the lowest curve approaches alpha times I_E1.
    Figure \(\PageIndex{2}\): Common base characteristics of the bipolar transistor

    In the first quadrant, which is in the "forward active bias mode," the output from the collector terminal looks more or less like a current source; that is, \(I_{C}\) is a constant, regardless of what \(V_{\text{CB}}\) is. Note however, that we must use a controlled source, in this case, a current-controlled current source, since \(I_{C}\) depends on what \(I_{E}\) happens to be. Obviously, looking in the (forward biased) emitter-base terminal, we see the usual p-n junction. Thus, if we were interested in building a "model" of this device, we might come up with something like Figure \(\PageIndex{3}\). Note that the base terminal is common to both inputs. Since we would actually like to think of the transistor as a two-port device (with an input and an output) the model for the transistor is often drawn as shown in Figure \(\PageIndex{4}\).

    A current source is connected to the right, anode side of a diode. The terminal to the right side of the current source has an applied voltage of V_CB and an incoming current of I_C. The terminal to the left of the diode has an applied voltage of V_EB and an outgoing current of I_E. In between the current source and diode is an incoming current I_B. The current source points to the left and has a value of alpha times I_E.
    Figure \(\PageIndex{3}\): Model for the common base transistor
    The C terminal at the top right has an incoming current of I_C, which flows down through a current source providing current of alpha times I_E, also in the downwards direction. This connects to a horizontal wire, which is connected to a vertical wire that rises to connect to the anode of a diode. The cathode of the diode leads to terminal E, which has an outgoing current of I_E. The two ends of the horizontal wire segment are each labeled as terminal B.
    Figure \(\PageIndex{4}\): Re-drawn common base transistor

    The only drawback with what we have so far is that except in some specialized high-frequency circuits, the bipolar transistor is very rarely used in the common base configuration. Most of the time, you will see it in either the common emitter configuration (Figure \(\PageIndex{5}\)), or the common collector configuration. The common emitter is probably the way the transistor is most often used.

    A npn bipolar transistor is oriented with the collector at the top, base in the middle, and emitter at the bottom. A current I_C flows into the collector, and a current I_E flows out of the emitter. The wire leading out of the emitter splits into two: one wire connects to the negative end of a V_CE voltage source, with the I_C current entering the collector flowing out of the positive end of this voltage source. The other wire connects to a current source I_B, with the current of I_B that it provides flowing into the base.
    Figure \(\PageIndex{5}\): Configuration for the common emitter circuit

    Note that we have a current source driving the base, and we have applied just one battery all the way from the collector to the emitter. The battery now has to do two things: a) It has to provide reverse bias for the base-collector junction and b) it has to provide forward bias for the base emitter junction. For this reason, the \(I_{C}\) as a function of \(V_{\text{CE}}\) curves look a little different now. It is now necessary for \(V_{\text{CE}}\) to become slightly positive in order to get the transistor into its active mode. The other difference, of course, is that the collector current is now shown as being \(\beta I_{B}\), the base current, instead of \(\alpha I_{E}\), the emitter current.

    Graph with axes of I_C vs V_CE, showing four curves that each originate at the origin and rise sharply for a short horizontal distance before leveling out. The highest curve is labeled with beta I_B4, the next highest with beta I_B3, the next with beta I_B2, and the lowest with beta I_B1.
    Figure \(\PageIndex{6}\): Common emitter characteristic curves for the transistor
     

    This page titled 3.5.3: Transistor I-V Characteristics is shared under a CC BY-NC-SA 1.0 license and was authored, remixed, and/or curated by Bill Wilson via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.