半导体物理学半导体 (57).pdf

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1、The Bipolar TransistorPhysical structureSimplified Transistor Current RelationThe minority carrier concentrations are again shown in Figure for an npn bipolar transistor biased in the forward-active mode.Ideally,the minority carrier electron concentration in the base is a linear function of distance

2、,which implies no recombination.The electrons diffuse across the base and are swept into the collector by the electric fi eld in the BC space charge region.Minority carrier distributions and basic currents in a forward-biased npn bipolar transistor.Simplified Transistor Current RelationAssuming the

3、ideal linear electron distribution in the base,the collector current can be written as a diffusion current given by:where ABEis the cross-sectional area of the BE junction,nBOis the thermalequilibrium electron concentration in the base,and Vt is the thermal voltage.The diffusion of electrons is in t

4、he+x direction so that the conventional current is in the-x direction.Considering magnitudes only,Equation(13.1)can be written as:Collector Current(13.1)Simplified Transistor Current RelationBasic working principle of bipolar transistor:The collector current is controlled by the baseemitter voltage,

5、that is,the current at one terminal of the device is controlled by the voltage applied to the other two terminals of the device.Simplified Transistor Current RelationOne component of emitter current,iE1,shown in Figure 12.6 is due to the fl ow of electrons injected from the emitter into the base.Thi

6、s current,then,is equal to the collector current given by:iE1=iC(13.1)Since the baseemitter junction is forward biased,majority carrier holes in the base are injected across the BE junction into the emitter.These injected holes produce a pnjunction current iE2as indicated in Figure 12.6.This current

7、 is only a BE junction current so this component of emitter current is not part of the collector current.Since iE2is a forward-biased pn junction current,we can write(considering magnitude only):Emitter Current where IS2involves the minority carrier hole parameters in the emitter.Simplified Transist

8、or Current Relation The total emitter current is the sum of the two components:Simplified Transistor Current Relationcommon-base current gain：Since all current components in Equation(13.4)are functions of exp(vBE/Vt),the ratio of collector current to emitter current is a constant.We can writewhere i

9、s called the common-base current gain.By considering Equation(13.4),we see that iCiEor 1.Since iE2 is not part of the basic transistor action,we would like this component of current to be as small as possible.We would then like the common-base current gain to be as close to unity as possible.constan

10、t current source Simplified Transistor Current RelationConstant current source:Note that the emitter current is an exponential function of the baseemitter voltage and the collector current iC=iE.To a first approximation,the collector current is independent of the basecollector voltage as long as the

11、 BC junction is reverse biased.We can sketch the common base transistor characteristics.The bipolar transistor acts like a constant current source.Common base circuitIdeal bipolar transistor common-base currentvoltage characteristics.constant current source Simplified Transistor Current RelationThe

12、component of emitter current iE2 is a BE junction current so that this current is also a component of base current shown as iBa.This component of base current is proportional to exp(vBE/Vt).There is also a second component of base current.We have considered the ideal case in which there is no recomb

13、ination of minority carrier electrons with majority carrier holes in the base.However,in reality,there will be some recombination.Since majority carrier holes in the base are disappearing,they must be resupplied by a fl ow of positive charge into the base terminal.This flow of charge is indicated as

14、 a current iBbin Figure 13.6.iBb.Base CurrentSimplified Transistor Current Relationwhere is called the common-emitter current gain.Normally,the base current will be relatively small so that,in general,the common-emitter current gain is much larger than unity(on the order of 100 or larger).The ratio

15、of collector current to base current：since both currents are directly proportional to exp(vBE/Vt).We can then writeThe number of holes per unit time recombining in the base is directly related to the number of minority carrier electrons in the base.Therefore,the current iBbis also proportional to exp(vBE/Vt).The total base current is the sum of iBaand iBaand is proportional to exp(vBE/Vt).Simplified Transistor Current Relation