1EE105 - Fall 2005Microelectronic Devices and CircuitsLecture 25Bipolar Junction Transistor2AnnouncementsHomework 11 due after ThanksgivingNo lab this weekLab 9 next weekReading: Chapter 7 (7.1-3.)23Midterm 2Hi: 50Lo: 12Average: 38Median 39.54Midterm 2051015202550-45-5040-4535-4030-3525-3020-2515-2010-15<10Midterm 2Midterm 135Lecture MaterialLast lectureMultistage amplifiersCascode amplifier design exampleThis lectureFinish cascode amplifierBipolar junction transistor13Ideal BJT StructureNPN or PNP sandwich (Two back-to-back diodes)How does current flow? Base is very thin.A good BJT satisfies the followingBase (P)Collector (N)Emitter (N)CIBIEI−BEV+−CEV+−Base (N)Emitter (P)Collector (P)EIBICI−EBV+−ECV+−CEII≈−CBII>>BEqVkTCSIIe≈414Actual BJT Cross SectionVertical npn sandwich (pnp is usually a lateral structure)n+ buried layout is a low resistance contact to collectorBase width determined by vertical distance between emitter diffusion and base diffusion 15BJT LayoutEmitter area most important layout parameter516BJT Schematic SymbolCollector current is control by base current linearly Collector current is an exponential function of the base-emitter voltageBIEI−BEV+−CEV+−BVCVEVBEqVkTCSIIe≈CBIIβ=17BJT Collector CharacteristicGround emitterFix VCEDrive base with fixed current IBMeasure the collector current618Collector Characteristics (IB)Forward ActiveRegion (Very High Output Resistance)Saturation Region (Low Output Resistance)Reverse Active(Bad Transistor)BreakdownLinear Increase19Base-Emitter Voltage ControlExponential IncreaseForward ActiveRegion (High Output Resistance)Reverse Active(Bad Transistor)Saturation Region (Low Output Resistance)~0.3VBreakdown720Transistor ActionBase-emitter junction is forward biased and collector-base junction is reverse biasedElectrons “emitted” into base much more than holes since the doping of emitter is much higherMagic: Most electrons cross the base junction and are swept into collectorWhy? Base width much smaller than diffusion length. Base-collector junction pulls electrons into collectorBase (p)Emitter (n+)Collector (n)0BEV+>−0CBV+>−ehehhrecombination21Diffusion CurrentsMinority carriers in base form a uniform diffusion current. Since emitter doping is higher, this current swamps out the current portion due to the minority carriers injected from base822BJT CurrentsCFEIIα=−Collector current is nearly identical to the (magnitude)of the emitter current … defineKCL: ECBIII−=+DC Current Gain:()CFEFBCIIIIαα=−= +1FCBFBFIIIαβα==−.999Fα=.9999991.001FFFαβα===−23Origin of αFBase-emitter junction: some reverse injection of holes into the emitter Æ base current isn’t zero E BCTypical:Some electrons lost due to recombination.99Fα≈100Fβ≈924Collector CurrentDiffusion of electrons across base results in0BEqVpnpBdiffkTnnBdn qD nJqD edx W⎛⎞==⎜⎟⎝⎠BEqVkTCSIIe=0npB ESBqD n AIW⎛⎞=⎜⎟⎝⎠25Base CurrentDiffusion of holes across emitter results in01BEqVpnEdiffnEkTppEqD pdpJqD edx W⎛⎞⎛⎞=− = −⎜⎟⎜⎟⎝⎠⎝⎠01BEqVpnE EkTBEqD p AIeW⎛⎞⎛⎞=−⎜⎟⎜⎟⎝⎠⎝⎠1026Current Gain00npBoEpBBCnEFpnEoEBpnEBEqD n AnWIDWqD p AIDpWWβ⎛⎞⎜⎟⎛⎞⎛⎞⎛⎞⎝⎠== =⎜⎟⎜⎟⎜⎟⎜⎟⎛⎞⎝⎠⎝⎠⎝⎠⎜⎟⎝⎠20,,20,,ipBAB DEinE A BDEnnNNnpNN⎛⎞==⎜⎟⎝⎠Minimize base widthMaximize doping in emitter27Ebers-Moll EquationsDerivation: Write emitter and collector currents in termsof internal currents at two junctions()()//11BE th BC thVV VVEES RCSIIe Ieα=− − + −()()//11BE th BC thVV VVCFES CSIIe Ieα=−−−FES RCSIIαα=1128Ebers-Moll Equivalent CircuitBuilding blocks: diodes and I-controlled I sources29Forward Active RegionB-C junction is not forward-biased Æ IRis very smallTypical Values:0.7BEV=0.2CEV >1230Simplified Ebers-MollForward-Active Case:Saturation: both diodes are forward-biases Æ batteries0.7BEV =CFBIIβ=BIBCECI0.7BEV
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