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only the latter in this chapter Figure 94 illustrates the approximate construction, symbols, and nomenclature for the two types of BJTs
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Collector C p Base n p+ Emitter E Circuit symbols E Emitter B B Base C n p n+ E Circuit symbols E VS IE E = Electron flow = Hole flow B B + B IB p n+ Collector C C n C
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Figure 94 Bipolar junction transistors
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The operation of the npn BJT may be explained by considering the transistor as consisting of two back-to-back pn junctions The base-emitter (BE) junction acts very much like a diode when it is forward-biased; thus, one can picture the corresponding ow of hole and electron currents from base to emitter when the collector is open and the BE junction is forward-biased, as depicted in Figure 95 Note that the electron current has been shown larger than the hole current, because of the heavier doping of the n side of the junction Some of the electron-hole pairs in the base will recombine; the remaining charge carriers will give rise to a net ow of current from base to emitter It is also important to observe that the base is much narrower than the emitter section of the transistor Imagine, now, reverse-biasing the base-collector (BC) junction In this case, an interesting phenomenon takes place: the electrons emitted by the emitter with the BE junction forward-biased reach the very narrow base region, and after a few are lost to recombination in the base, most of these electrons are collected by the collector Figure 96 illustrates how the reverse bias across the BC junction is in such a direction as to sweep the electrons from the emitter into the collector This phenomenon can take place because the base region is kept particularly narrow Since the base is narrow, there is a high probability that the electrons will have gathered enough momentum from the electric eld to cross the reverse-biased collector-base junction and make it into the collector The result is that there is a net ow of current from collector to emitter (opposite in direction to the ow of electrons), in addition to the hole current from base to emitter The electron current owing into the collector through the base is substantially larger than that which ows into the base from the external circuit One can see from Figure 96 that if KCL is to be satis ed, we must have IE = IB + IC (91)
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The BE junction acts very much as an ordinary diode when the collector is open In this case, IB = IE
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Figure 95 Current ow in an npn BJT
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C + V2 I B B + V1 E = Electron flow = Hole flow When the BC junction is reversebiased, the electrons from the emitter region are swept across the base into the collector IE Emitter IC Collector
Base
The most important property of the bipolar transistor is that the small base current controls the amount of the much larger collector current IC = IB (92)
Figure 96 Flow of emitter electrons into the collector in an npn BJT
9
Transistor Fundamentals
The operation of the BJT is defined in terms of two currents and two voltages: iB , iC , vCE, and vBE + vCB B + vBE E KCL: iE = iB + iC KVL: vCE = vCB + vBE iE iB vCE C iC +
Figure 97 De nition of BJT voltages and currents
where is a current ampli cation factor dependent on the physical properties of the transistor Typical values of range from 20 to 200 The operation of a pnp transistor is completely analogous to that of the npn device, with the roles of the charge carriers (and therefore the signs of the currents) reversed The symbol for a pnp transistor was shown in Figure 94 The exact operation of bipolar transistors can be explained by resorting to a detailed physical analysis of the npn or pnp structure of these devices The reader interested in such a discussion of transistors is referred to any one of a number of excellent books on semiconductor electronics The aim of this book, on the other hand, is to provide an introduction to the basic principles of transistor operation by means of simple linear circuit models based on the device i-v characteristic Although it is certainly useful for the non-electrical engineer to understand the basic principles of operation of electronic devices, it is unlikely that most readers will engage in the design of high-performance electronic circuits or will need a detailed understanding of the operation of each device The present chapter will therefore serve as a compendium of the basic ideas, enabling an engineer to read and understand electronic circuit diagrams and to specify the requirements of electronic instrumentation systems The focus of this section will be on the analysis of the i-v characteristic of the npn BJT, based on the circuit notation de ned in Figure 97 The device i-v characteristics will be presented qualitatively, without deriving the underlying equations, and will be utilized in constructing circuit models for the device The number of independent variables required to uniquely de ne the operation of the transistor may be determined by applying KVL and KCL to the circuit of Figure 97 Two voltages and two currents are suf cient to specify the operation of the device Note that, since the BJT is a three-terminal device, it will not be suf cient to deal with a single i-v characteristic; two such characteristics are required to explain the operation of this device One of these characteristics relates the base current, iB , to the base-emitter voltage, vBE ; the other relates the collector current, iC , to the collector-emitter voltage, vCE The latter characteristic actually consists of a family of curves To determine these i-v characteristics, consider the i-v curves of Figures 98 and 99, using the circuit notation of Figure 97 In Figure 98, the collector is open and the BE junction is shown to be very similar to a diode The ideal current source, IBB , injects a base current, which causes the junction to be forward-biased By varying IBB , one can obtain the open-collector BE junction i-v curve shown in the gure If a voltage source were now to be connected to the collector circuit, the voltage vCE and, therefore, the collector current, iC , could be varied, in addition to the base current, iB The resulting circuit is depicted in Figure 99(a) By varying both the base current and the collector-emitter voltage, one could then generate a plot of the device collector characteristic This is also shown in Figure 99(b) Note that this gure depicts not just a single iC -vCE curve, but an entire family, since for each value of the base current, iB , an iC -vCE curve can be generated Four regions are identi ed in the collector characteristic: 1 The cutoff region, where both junctions are reverse-biased, the base current is very small, and essentially no collector current ows 2 The active linear region, in which the transistor can act as a linear ampli er, where the BE junction is forward-biased and the CB junction is reverse-biased
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