EECS 105 Fall 2003 Lecture 14 Lecture 14 Bipolar Junction Transistors Prof Niknejad Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Lecture Outline Department of EECS Diode Small Signal Model Diode Charge Storage 6 4 4 Diode Circuits The BJT 7 1 BJT Physics 7 2 BJT Ebers Moll Equations 7 3 BJT Small Signal Model University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Diode Small Signal Model The I V relation of a diode can be linearized qVd qvd d vd q VkT I D iD I S e 1 I S e kT e kT 2 3 x x e x 1 x L 2 3 qvd I D iD I D 1 L kT qI iD D vd g d vd kT Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Diode 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 by eS C j A 1 4C j 0 X dep But another charge storage mechanism comes into play in forward bias Minority carriers injected into p and n regions stay in each region for a while On average additional charge is stored in diode Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Charge Storage pn 0 e p side n p 0e q Vd vd kT n side q Vd vd kT Extra charge Stored in diode pn 0 np0 Wp xp xn Wn Increasing forward bias increases minority charge density By charge neutrality the source voltage must supply equal and opposite charge 1 qI d tT A detailed analysis yields Cd 2 kT 1 Time to cross junction C d g dt T or minority carrier lifetime 2 Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Ideal BJT Structure IC Collector N IB Base P VBE Emitter N VCE Emitter P Base N IE VEB IB Collector P IE VEC IC NPN or PNP sandwich Two back to back diodes How does current flow Base is very thin A good BJT satisfies the following I C I E I C I B Department of EECS I C I S e qVBE kT University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Actual BJT Cross Section Vertical npn sandwich pnp is usually a lateral structure n buried layout is a low resistance contact to collector Base width determined by vertical distance between emitter diffusion and base diffusion Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad BJT Layout Emitter area most important layout parameter Multi finger device also possible for reduced base resistance Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad BJT Schematic Symbol I C b I B IB VBE I C I S e VCE IE qVBE kT VC VB VE Collector current is control by base current linearly Collector is controlled by base emitter voltage exponentially Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad BJT Collector Characteristic Ground emitter Fix VCE Drive base with fixed current IB Measure the collector current Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Collector Characteristics IB Saturation Region Low Output Resistance Breakdown Linear Increase Reverse Active Crappy Transistor Forward Active Region Very High Output Resistance Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Base Emitter Voltage Control Saturation Region Low Output Resistance 0 3V Breakdown Exponential Increase Reverse Active Crappy Transistor Forward Active Region High Output Resistance Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Transistor Action recombination Collector n VCB 0 VBE 0 e Base p h h h e Emitter n Base emitter junction is forward biased and collector base junction is reverse biased Electrons emitted into base much more than holes since the doping of emitter is much higher Magic Most electrons cross the base junction and are swept into collector Why Base width much smaller than diffusion length Base collector junction pulls electrons into collector Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Diffusion Currents Minority 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 base Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad BJT Currents Collector current is nearly identical to the magnitude of the emitter current define I C a F I E Kirchhoff a F 999 I E I C I B DC Current Gain I C a F I E a F I B I C aF IC I B bF I B 1 a F Department of EECS aF 999 bF 999 1 a F 001 University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Origin of F Base emitter junction some reverse injection of holes into the emitter base current isn t zero Some electrons lost due to recombination E Typical Department of EECS a F 99 B C bF 100 University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Collector Current Diffusion of electrons across base results in J ndiff qVBE qD n n pB 0 kT qDn e dx WB dn p qDn n pB 0 AE I S WB I C I S e Department of EECS qVBE kT University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Base Current Diffusion of holes across emitter results in J pdiff qVBE qD p dpnE p nE 0 kT qD p e 1 dx WE qD p pnE 0 AE I B WE Department of EECS qVkTBE e 1 University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Current Gain qDn n pBo AE I C WB Dn bF I B qD p pnEo AE D p W E n pB 0 WE p WB nE 0 Minimize base width ni2 n pB 0 N A B N D E 2 N A B pnE 0 ni N D E Maximize doping in emitter Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Ebers Moll Equations Exp 6 measure E M parameters Derivation Write emitter and collector currents in terms of internal currents at two junctions I E I ES eVBE Vth 1 a R I CS eVBC Vth 1 I C a F I ES eVBE Vth 1 I CS eVBC Vth 1 a F I ES a R I CS Department of EECS University of California Berkeley EECS 105 Fall 2003 Lecture 14 Prof A Niknejad Ebers Moll Equivalent Circuit Building blocks diodes and …
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