《电子器件BJ》PPT课件.pptx

Chapter 7.5: Generalized Biasing,Presented by: Lei Wang,7.5 Generalized biasing,Normal biasing(normal active mode): the emitter junction is forward-biased and the collector is reverse biased.,2,7.5 Generalized biasing,3,The collector saturation current is negligible. The active part of the base and the two junctions are of uniform cross-sectional area A; current flow in the base is essentially one-dimensional from emitter to collector. The emitter junction is forward biased; the collector junction is reverse biased.,7.5 Generalized biasing,The emitter and collector junctions are both forward biased; what will happen？ Moreover, how about the cases when the emitter junction is reverse biased and the collector is forward biased or the emitter and collector junctions are both reverse biased?,4,7.5 Generalized biasing,5,Generalized biasing? E-M equations(J.J. Ebers &J.L.Moll 1954),J.J. Ebers (1921-1957) J.J.Ebers Award was established in 1971 by IEEE Electron Device Society For outstanding technical contributions to electron devices. IEEE International Electron Device Meeting, USA,7.5.1 The coupled-diode model,6,The emitter and collector junctions are both forward biased; what will happen？,7.5.1 The coupled-diode model,7,7.5.1 The coupled-diode model,8,+,IEN and ICN stand for the normal mode components IEI and ICI stand for the inverted mode components,7.5.1 The coupled-diode model,9,+,7.5.1 The coupled-diode model,10,For the symmetrical transistor,For Normal Mode,7.5.1 The coupled-diode model,11,IE-ICI, IC-IEI, PE PC, PC PE(=0),For inverted mode,7.5.1 The coupled-diode model,12,The emitter and collector junctions are both forward biased,We can see from these equations that a linear superposition of the normal and inverted components does give the result we derived previously for the symmetrical transistor.,7.5.1 The coupled-diode model,13,(7-18) becomes:,7.5.1 The coupled-diode model,14,For the asymmetrical transistor,For comparison,Where, IES is the emitter saturation current with the collector junction short circuited.,7.5.1 The coupled-diode model,15,For the asymmetrical transistor,For comparison,Where, ICS is the collector saturation current with the emitter junction short circuited.,7.5.1 The coupled-diode model,16,Ebers-Moll equations,An interesting feature of the Ebers-Moll equations is that IE and IC are described by terms resembling diode relations (IEN and ICI), plus terms which provide coupling between the properties of the emitter and collector (IEI and ICN).,7.5.1 The coupled-diode model,17,7.5.1 The coupled-diode model,18,ICO is the magnitude of the collector saturation current with the emitter junction open(IE=0). IEO is the magnitude of the emitter saturation current with the collector junction open (IC=0).,7.5.1 The coupled-diode model,19,7.5.1 The coupled-diode model,20,What will be the base current ?,7.5.2 Charge Control Analysis,21,The emitter injection efficiency =1; The collector saturation current=0 t : the transit time for a average excess hole from emitter to collector p :the average hole lifetime Lp: the hole diffusion length,7.5.2 Charge Control Analysis,Because the base width Wb Lp, t p In steady state there are excess electrons and holes in the base. The charge in the electron distribution Qn is replaced every p seconds. iB=Qn/ p The charge in the hole distribution Qp is collected every t seconds. iC=Qp/ t For space charge neutrality, Qn=Qp,22,7.5.2 Charge Control Analysis,23,7.5.2 Charge Control Analysis,24,+,7.5.2 Charge Control Analysis,The effects of time dependence of stored charge can be included in above equations by adding the rate of change of stored charge to each of the injection currents IEN and ICI.,25,7.5.2 Charge Control Analysis,26,Time dependence of stored charge,x,x,x+x,Diffusion law: the number of charges through the cross-section per unit area per unit time.,The net increase in hole charge within the total volume of differential element per unit time due to diffusion.,The net increase in hole particle per unit volume per unit time,At point x, the net decrease in hole particle per unit volume per unit time due to recombination.,At point x, the net increase in hole particle per unit volume per unit time.,At point x, the change of hole concentration with time. the Continuity equation(连续性方程）(diffusion equation),7.5.2 Charge Control Analysis,27,x,x,x+x,7.6 Characteristics of BJT,28,(1) input characteristics,Common-base, IEVEB,7.6 Characteristics of BJT,29,Common-emitter, IBVEB,7.6 Characteristics of BJT,30,(2) Output Characteristics,Common-base, ICVCB,VCB0, i.e. the collector junction is positive biased.At some point,Ic=0,saturation,VEB=0 Cut-off,7.6 Characteristics of BJT,31,VCB-kT/q,Common-emitter, ICVCE,VCB=0, saturation,IB=0, Cut off,7.6 Characteristics of BJT,Example Assume that a p-n-p transistor is doped such that the emitter doping is ten times that in the base, the minority carrier mobility in the emitter is one-half that in the base, and the base width is one-tenth the minority carrier diffusion length. The carrier lifetimes are equal. Calculate and for this transistor.,32,7.6 Characteristics of BJT,Solution: We have known , pp/nn=10, np/pn=1/2, Wb/Lpn=1/10,33,7.7 Other important effects,7.7.1 Drift in the Base Region,34,One important result of a graded base region is that a built-in electric field exists from emitter to collector .,Within the base region, the net doping concentration decreases along a profile,This electric field will add a drift components to the transport of holes across the base.,7.7.1 Drift in the Base Region,At equilibrium, in the base region,35,7.7.1 Drift in the Base Region,Since this field aids the transport of holes across the base region from emitter to collector, the transit time is reduced below that of a comparable uniform base transistor.,36,This shortening of transit time can be very important in high frequency devices.,7.7.2 Base Narrowing,37,Base Width Modulation， Early Effect（基区宽度调制效应，Early 效应）,The decrease in Wb causes to increase.,When VCB is increased far enough, it is possible to decrease Wb to the extent that the collector depletion region fills the entire base- punch through. At this point, The transistor action is lost.,7.7.2 Base Narrowing,As a result, the collector current Ic increases with collector voltage rather than staying constant as predicted from the simple treatment. The slope introduced by Early effect is almost linear with Ic.,38,7.7.3 Avalanche Breakdown,39,BVCBO ：the breakdown voltage when IE=0 in the common-base case,BVCEO ：the breakdown voltage for IB=0 in the common-emitter case,BVCEOBVCBO,There is a strong Influence of carrier multiplication over a fairly broad range of collector voltage.,7.7.3 Avalanche Breakdown,40,For common-base case, IE=0, IC=MICO,For common-emitter case, IB=0, IC=IE=MICO/(1-MN),BVCEOBVCBO,MN1,The breakdown voltage is defined as the inverse biased when the reverse current reaches the given value .,7.7.3 Avalanche Breakdown,Physically, in the common-emitter case (for IB=0)when an ionizing collision occurs in the collector junction depletion region, a secondary hole and electron are created. The primary and secondary holes and electrons are swept into the collector and the base respectively. The supply of electrons in the base is increased, the hole injection at the emitter must increase to maintain space charge neutrality. An increased holes injection from the emitter causes an increased multiplication current at the collector Junction; this in turn increases the rate at which secondary electrons are swept into the base, calling for more hole injection.,41,7.7.4 Injection Level; Thermal Effect,42,Ideally， we have assumed and are independent of carrier injection level. Actually?,The emitter injection efficiency,The current transfer ratio,The amplification factor,The Base transport factor iC=BiEP,Injection Level,7.7.4 Injection Level; Thermal Effect,43,For very low injection, the recombination in the junctions must be considered. This is particularly important in the emitter junction, where any recombination tends to degrade the emitter injection efficiency . Therefore, ,should decrease for low values of IC, causing the curves of the collector characteristics to be spaced more closely for low currents than for higher currents