2.2 Transistor i-v characteristics

Let's now take a look at some current voltage relationships for the bipolar transistor. In the absence of any voltage orcurrent on the emitter-base junction, if we were to make a plot of $I_C$ as a function of $V_{\mathrm{CB}}$ it would look something like . 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 bebiased in a reverse direction, so it conducts when $V_{\mathrm{CB}}$ is negative and not when $V_{\mathrm{CB}}$ is positive. Thus, all we need to do is draw a diode curve, but upside down!

What happens if we now also have some bias applied to the emitter-base junction? As we saw, so long as thebase-collector junction is reverse biased, almost all of the collector current consists of electrons which have been injectedinto the base by the emitter, diffuse across the base region, and then fall down the base-collector junction. The rate atwhich electrons fall down the junction does not depend on how large a drop there is (e.g. how big $V_{\mathrm{CB}}$ is). The only thing that matters, in so far as the collector current is concerned, is how fast electrons are being injectedinto 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 $I_{E_1}$ , we might see something like . In the first quadrant, which is in the "forward active biasmode," the output from the collector terminal looks more or less like a current source; that is $I_C$ is a constant, regardless of what $V_{\mathrm{CB}}$ 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, ifwe were interested in building a "model" of this device, we might come up with something like . Note that the base terminal is common to both inputs. Since we wouldactually like to think of the transistor as a two-port device (with an input and an output) the model for the transistor isoften drawn as shown in .

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

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