Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01Bipolar Transistors (Briefly)A) Motivation and Diode CarriersB) Bipolar Transistor Structure and Carrier FlowC) Bipolar Transistor I vs. VD) Bipolar Transistor CircuitsReading: Schwarz and Oldham, pp. 499-509, 594-596Lecture 21Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01pn Junction Carrier FlowForward bias 0.7 V (positive on the p-side):This is the direction of easy current flow. + charges flow to meet up with – charges. Essentially unlimited conduction.n pp- -----+ +++ + +AC-+ND= 1020/cm3NA= 1018/cm3100 electrons => Forward direction1 hole <= Reverse directionAfter the carriers reach the other side of the junction they circle around a carrier of the opposite type and eventually recombine.Carriers types flowing are proportional to doping so see many more electrons than holes crossing the junction.Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor Structurennp- -----++BEC---+-+VBEVCERaise collector voltage with respect to emitter so it is electrically asymmetrical and electrons will prefer collector over emitter.Be sure that the base thinner than the electron diffusion length.ND= 1020/cm3NA= 1018/cm3ND= 1016/cm3Create a npn structure with decreasing doping levels.Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor Carrier ProcessAdd an extra hole in base (by positive charge flowing into base lead).nnp- -----++BEC---+-+VBEVCE+-This attracts an electron from the emitter (emitter current). The electron and hole circle about each other. But if the electron gets too close to the collector the additional voltage on the collector sweeps it into the collector (current).In a good npn transistor 149 electrons go into the collector for every one that recombines with a hole in the base. To accomplish this the base must be much narrower than the electron diffusion length.Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor Carrier Flownnp- -----++BEC---+-+VBEVCEND= 1020/cm3NA= 1018/cm3ND= 1016/cm3Gain mechanism: A hole in base attracts 150 electrons of which 149 go to collector and one recombines with the hole.holeselectronsExtra loss mechanism: Like in a diode there is a small reverse flow (injection) of holes from the base into the emitter such as10 out of every 100 holes.Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor Gain100 holes into base.nnp- -----++BEC---+-+VBEVCEND= 1020/cm3NA= 1018/cm3ND= 1016/cm3holeselectronsβ = (IC/IB) = (13,410)/(10+90) = 13410 lost to emitter leaving 90 to attract electrons.Each of the 90 attracts 149 or 13,410 electrons that reach collector. α = (IC/IE) = (13,410)/(13,4010+10+90) = 0.9926Note that since IE= IC+ IB, α and β are related by β = α/(1-α)Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor CircuitBEnnp- -----++---+VINWe need a model for the base and model for IOUTversus VOUTcollector side of the bipolar transistor.VCCVOUTRCRBECB-+VINVOUTVCCIOUTRCRBCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor: Base Circuit ModelThe base-emitter is modeled as a large signal diode.BEnnp- -----++---+VINVCCVOUTRCRBDiodeBE-+VINRBIB+-0.7VLarge Signal Diode ModelCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01DIODE I-V CHARACTERISTICS AND MODELSImproved “Large-Signal Diode” Model:If we choose not to ignore the small forward-bias voltage drop of a diode, it is a very good approximation to regard the voltage drop in forward bias as a constant, about 0.7V. the “Large signal model” results.Reverse bias0Vany ,0I <≅Forward bias0Iany ,0.7V>≅0100200300400-5 -3 -1 1forward bias (V)Current (microam+−VIThe Large-Signal Diode Model- 0.7+VI0.7Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor: Base Circuit AnalysisThe base circuit analysis is the same as that for a diode.BE-+VINRBIB+-0.7VLarge Signal Diode ModelVBE< 0.7V => IB=0CutoffOnVBE= 0.7VIB= (VIN – 0.7V)/RBIB= (2.5V – 0.7V)/10kΩ = 180 µΑExample: VIN= 2.5V, RB= 10kΩCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor: Collector Circuit ModelThe collector current is programmed by the base current to be β times the base current. However, the current stops growing larger whenever the collector base voltage drops to 0.2V.ECB-+VINVOUTVCCIOUTRCRBIC= βIB when VCE> 0.2VEVOUTVCCIOUTRCRBCCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor: Collector Circuit AnalysisThe collector current is programmed by the base current to be β times the base current. However, the current stops growing larger whenever the collector base voltage drops to 0.2V.EVOUTVCCIOUTRCRBIC= βIB when VCE> 0.2VCCutoff: VBE< 0.7V IB= IC= 0Forward Active: VBE= 0.7V and VCE> VCE-SAT=0.2V IC= βIBIB= (VIN – 0.7V)/RBIB= (VIN – 0.7V)/RBSaturated: VBE= 0.7V and VCE= VCE-SAT=0.2V IC-SAT= (VCC– 0.2V)/RCCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 21: 11/19/01 A.R. NeureutherVersion Date 11/25/01npn Bipolar Transistor: ICvs. VCEGraphThe ICversus VCEgraph for the first order model is a set of horizontal lines with heights β times IB. Loadline techniques can easily be
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