Lecture 14: Bipolar Junction TransistorsLecture OutlineDiode Small Signal ModelDiode CapacitanceCharge StorageIdeal BJT StructureActual BJT Cross SectionBJT LayoutBJT Schematic SymbolBJT Collector CharacteristicCollector Characteristics (IB)Base-Emitter Voltage ControlTransistor ActionDiffusion CurrentsBJT CurrentsOrigin of αFCollector CurrentBase CurrentCurrent GainEbers-Moll EquationsEbers-Moll Equivalent CircuitForward Active RegionSimplified Ebers-MollSlide 24Transconductance gmTransconductance (cont)Comparison with MOSFETBJT Base CurrentsSmall Signal Current GainInput Resistance rπOutput Resistance roGraphical Interpretation of roBJT Small-Signal ModelBJT CapacitancesComplete Small-Signal ModelDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14Lecture 14: Bipolar Junction TransistorsProf. NiknejadDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadLecture OutlineDiode Small Signal ModelDiode Charge Storage (6.4.4)Diode CircuitsThe BJT (7.1)BJT Physics (7.2)BJT Ebers-Moll Equations (7.3)BJT Small-Signal ModelDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadDiode Small Signal ModelThe I-V relation of a diode can be linearized ( )1d d d dq V v qV qvkT kT kTD D S SI i I e I e e+� �+ = - �� �� �1dD D DqvI i IkT� �+ � + +� �� �L2 312! 3!xx xe x= + + + +LDD d d dqIi v g vkT� =Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadDiode Capacitance We have already seen that a reverse biased diode acts like a capacitor since the depletion region grows and shrinks in response to the applied field. the capacitance in forward bias is given byBut another charge storage mechanism comes into play in forward biasMinority carriers injected into p and n regions “stay” in each region for a whileOn average additional charge is stored in diode01.4Sj jdepC A CXe= �Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadCharge StorageIncreasing forward bias increases minority charge density By charge neutrality, the source voltage must supply equal and opposite chargeA detailed analysis yields: p side n side -Wp Wn xn -xp ( )0d dq V vkTnp e+0np0pn( )0d dq V vkTpn e+12dd TqICkTt=Time to cross junction(or minority carrier lifetime)Extra chargeStored in diode12d d TC g t=Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadIdeal 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+-C EI I�-C BI I>>BEqVkTC SI I e�Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadActual 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 diffusionDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBJT LayoutEmitter area most important layout parameterMulti-finger device also possible for reduced base resistanceDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBJT Schematic SymbolCollector current is control by base current linearly Collector is controlled by base-emitter voltage exponentiallyBIEI-BEV+-CEV+-BVCVEVBEqVkTC SI I e�C BI Ib=Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBJT Collector CharacteristicGround emitterFix VCEDrive base with fixed current IBMeasure the collector currentDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadCollector Characteristics (IB)Forward ActiveRegion (Very High Output Resistance)Saturation Region (Low Output Resistance)Reverse Active(Crappy Transistor)BreakdownLinear IncreaseDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBase-Emitter Voltage ControlExponential IncreaseForward ActiveRegion (High Output Resistance)Reverse Active(Crappy Transistor)Saturation Region (Low Output Resistance)~0.3VBreakdownDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadTransistor 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+>-ehehhrecombinationDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadDiffusion 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 baseDepartment of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBJT CurrentsC F EI Ia=-Collector current is nearly identical to the (magnitude)of the emitter current … defineKirchhoff: E C BI I I- = +DC Current Gain:( )C F E F B CI I I Ia a=- = +1FC B F BFI I Iaba= =-.999Fa =.9999991 .001FFFaba= = =-Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadOrigin of αFBase-emitter junction: some reverse injection of holes into the emitter base current isn’t zero E B C Typical:Some electrons lost due to recombination.99Fa �100Fb �Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadCollector CurrentDiffusion of electrons across base results in0BEqVp n pBdiffkTn nBdn qD nJ qD edx W� �= =� �� �BEqVkTC SI I e=0n pB ESBqD n AIW� �=� �� �Department of EECS University of California, BerkeleyEECS 105 Fall 2003, Lecture 14 Prof. A. NiknejadBase CurrentDiffusion of holes across emitter results in01BEqVp nEdiffnEkTp pEqD pdpJ qD edx W� �� �=-
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