Lecture 18 PNP Bipolar Junction Transistors (BJTs)

Lecture 18PNP Bipolar Junction Transistors (BJTs)In this lecture you will learn: The operation of bipolar junction transistors Forward and reverse active operations, saturation, cutoff Ebers-Moll modelECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityNPN Bipolar Junction TransistorVBE-- NdCVCB -CBVBE -EECE 315 – Spring 2007 – Farhan Rana – Cornell University1

PNP Bipolar Junction TransistorVBE-- VCB -NaCCBVBE -EECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityA Silicon PNP BJTMetal base contactInsulator(SiO2)N (contact)N (base)Metal emitter contactP (emitter)Metal collector contactInsulator(SiO2)Insulator(SiO2)P (collector)P (contact layer)P (substrate)ECE 315 – Spring 2007 – Farhan Rana – Cornell University2

PNP BJT: Basic OperationVBE 0-- VCB 0EmitterBaseCollectorP-dopedN-dopedP-dopedN aENdBN aC xn 0 x pWEWBWCSuppose:The base-emitter junction is forward biasedVBE 0The base-collector junction is zero biasedVCB 0This biasing scheme will putthe device in the “forwardactive” operation (to bediscussed fully later)ECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Basic OperationVBE 0- electronsN aEWEVCB 0- swept holesholesNdBN aC x p 0 xnWBWCE-fieldConsider the action in the base first (VBE 0 and VCB 0) The holes diffuse from the emitter, cross the depletion region, and enter the base In the base, the holes are the minority carriers In the base, the holes diffuse towards the collector As soon as the holes reach the base-collector depletion region they are immediatelyswept away into the collector by the strong electric fields in the depletion regionECE 315 – Spring 2007 – Farhan Rana – Cornell University3

PNP BJT: Electron-Hole PopulationsVBE 0-VCB 0- electronsN aEWEConsider the base first:swept holesholesNdB x p 0 xnWBN aCWCE-fieldIn the base, the hole population can be written as:p x pno p' x Equilibrium hole densitypno Excess hole densityni2NdBIn the base, the excess electron population satisfies the differential equation: p' x 2 x 2 p' x L2p q VBE e KT 1 2 qVBCnp' x n WB i e KT 1 0 NdB Boundaryconditions 0p' x n ni2NdBECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Electron-Hole PopulationsVBE 0-VCB 0- p' x electronsN aEWEswept holesholesNdBN aC x p 0 xnWBWCE-field Ignore carrier recombination (i.e. assume Lp ) 2 p' x x 2Boundaryconditions 0Solution is: x x n ni2p' x p' x p 1 WB NdB q VBE e KT 1 2 qVBCni KT 1 0ep' x n WB NdB p' x n ni2NdB qVBE e KT 1 1 x x n WB ECE 315 – Spring 2007 – Farhan Rana – Cornell University4

PNP BJT: Electron-Hole PopulationsVBE 0-VCB 0- p' x electronsN aEholesNdB x p 0 xnWEConsider the emitter now:WBswept holesN aCWCE-fieldIn the emitter, the electron population can be written as:n x n po n' x Equilibrium electron densityn po Excess electron densityni2N aEIn the emitter, the excess electron population satisfies the differential equation: n' x 2 x 2 n' x L2n q VBE e KT 1 n' x p WE 0Boundaryconditions 0n' x p ni2N aEECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Electron-Hole PopulationsVBE 0-VCB 0- p' x electronsn' x N aEWEholesNdBN aC x p 0 xnWBWC Ignore carrier recombination (i.e. assume Ln ) 2 n' x x 2swept holesBoundaryconditions 0E-field q VBE e KT 1 n' x p WE 0n' x p ni2N aESolution is:x x p ni2 n' x n' x p 1 WE N aE qVBE e KT 1 1 x x p WE ECE 315 – Spring 2007 – Farhan Rana – Cornell University5

Area APNP BJT: Electron and Hole Current DensitiesVBE 0-- N aEVCB 0N aCNdBJ p x Jn x x p 0 xnWEIn the base: The hole current is:J p x q D pWCDp p x qni2 xNdBWB qVBE e KT 1 In the emitter: The electron current is:J n x q Dn n x Dn qni2 xN aEWE qVBE e KT 1 ECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityArea APNP BJT: Terminal CurrentsVBE 0-- N aENdBJn x VCB 0N aCJ p x IEWE x p 0 xnWCEmitter current: The current flowing out of the emitter is the sum of the total electron and total holecurrents in the emitter: qVBE D p Dn e KT 1 I E qni2 A N aE WE NdBWB ECE 315 – Spring 2007 – Farhan Rana – Cornell University6

VBE 0PNP BJT: Terminal Currents-- Area AVCB 0IBN aEN aCNdBJn x J p x IEIC x p 0 xnWEWCE-fieldCollector Current: The current going into the collector is due to the holes that got swept from the Basethrough the Base-Collector depletion region by the electric-fields: qV D p KTBE e 1 IC qni2 A NdBWB Base Current: The current going into the Base is due to the electrons that got injected from the baseinto the emitter: DnI B qni2 A N aE WE e qVBEKT 1 ECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityVBE 0PNP BJT: Terminal Currents-- Area AVCB 0IBN aEN aCNdBJn x J p x IEWEIC x p 0 xn qVBE D p Dn e KT 1 I E qni2 A N aE WE NdBWB qVBE D KTp e 1 IC qni2 A NWdBB qVBE Dn KT 1 I B qni2 A e NW aE E WCVCB 0IC -CBIBVBE 0 -EIEIE IB ICECE 315 – Spring 2007 – Farhan Rana – Cornell University7

PNP BJT: Circuit Level ParametersCurrent gain F :Current gain of the BJT in the forward activeoperation is defined as the ratio of the collectorand base currents: FD p N aE WEI C I B NdBWB Dn VCB 0 -IC FIE FIBBIC F I BVBE 0Typical values of F are between 20-200 and:N aE NdB N aC F :CIBE -IEIn the forward active operation F is defined asthe ratio of the collector and emitter currents:I E I B ICDp F IC IENdBWBDpDn N aE WE NdBWBTransistor relation: F and F are related: F IC F I E F1 FECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Ebers-Moll Model for Forward Active OperationVBE 0Suppose:VCB 0VCVCICIB F IFVBCVBICIBBIESEIFVEVEIEIE qVBE D p Dn e KT 1 I F qni2 A N aE WE NdBWB qVBE I ES e KT 1 The circuit level simplified model with an ideal diode and a current-controlledcurrent source models the PNP transistor in the forward active operationECE 315 – Spring 2007 – Farhan Rana – Cornell University8

PNP BJT: Forward and Reverse Active OperationsVCBVBE 0IC -CVCB 0BVCB 0VBEE -IB -VBEIEForward active operationEIEReverse active operation F ICIB R F ICIE R F CBVBE 0IBIC -VCB F R 1 FIED p N aCWC I B NdBWB DnIEIC R1 RIn a well designed transistor: F RECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Ebers-Moll Model for Reverse Active OperationVCB 0Suppose:ICVBE 0VCVCICICSCVBIRVBIBB R IRIBEVEVEIEIEIR D p Dn e N aCWC NdBWB qni2 A qVCBKT 1 qVCB ICS e KT 1 The circuit level simplified model with an ideal diode and a current-controlledcurrent source models the PNP transistor in the reverse active operationECE 315 – Spring 2007 – Farhan Rana – Cornell University9

PNP BJT: Ebers-Moll Model and Terminal CurrentsTerminal currents:VCICVCIC F IFCVBBICSIRVBIBIBEIESVE R IRIFVEIEIEI R ICS qVCB e KT qVBC e KT 1 ICS qVEB qVBE I F I ES e KT 1 I ES e KT 1 And 1 IB 1 F IF 1 R IRIC F IF IRI E IF R I RECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: Different Regimes of OperationICVCReversedbiased F IFICSVCForwardbiased F IFIRVBICSIES R IRIFForwardbiasedVEIEVBE 0VCB 0Forward ActiveICSIRVBIBIESForwardbiased F IFIRVBIBForwardbiasedICICVC R IRIFIBIESReversedbiasedVE R IRIFVEIEIEVCB 0VCB 0VBE 0VBE 0SaturationReverse ActiveECE 315 – Spring 2007 – Farhan Rana – Cornell University10

PNP BJT: Regimes of Operation - IIn forward active operation:ICBVCEIBI INVBE 0IB 0CSince: -VCB 0VCE VCB VBE In forward active operation: VCE VBE qV D p KTBE eIC qni2 A 1 F I B NdBWB IndependentEIEof VCEICForward active:Base-emitter junction forward biasedBase-collector junction reversed biasedVBEVCEI B 0 VBE 0 VCB 0Saturation:Base-emitter junction forward biasedSaturation Base-collector junction forward biasedIB 0I B 0 VBE 0 VCB 0VCE VBEForward activeIB 0 & IC F IBVCE VBEECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityCarrier Densities in Different Regimes of OperationForward active:VBE 0-n' x electronsN aEWESaturation:VCB 0- p' x electronsholesNdB x p 0 xnWBholesN aCWCN aE NdB N aCVBE 0-n' x N aEWEVCB 0- electronsholes xn 0 x pp' x NdBelectronsholesWBn' x N aCWCThe forward biased base-collector junction reduces the magnitude of thecollector current!ECE 315 – Spring 2007 – Farhan Rana – Cornell University11

PNP BJT: Regimes of Operation - IIICForward active:Base-emitter junction forward biasedBase-collector junction reversed biasedCBVCEIB -I B 0 VBE 0 VCB 0EI INSaturation:Base-emitter junction forward biasedBase-collector junction forward biasedIEICIB 0 (cutoff)I B 0 VBE 0 VCB 0VCECutoff:Base current zeroCurves forincreasing IB VCE VBEForward activeIB 0 & IC F IBVCE VBEIB 0SaturationIB 0VCE VBEReverse active:Base-emitter junction reverse biasedBase-collector junction forward biasedI B 0 VBE 0 VCB 0ECE 315 – Spring 2007 – Farhan Rana – Cornell UniversityPNP BJT: A Simple Amplifier CircuitCurrent gain (in forward active regime):VDDEIINIC FIBIEBIBLoad line equation:CICVCE IC R VDDVCR IC VDD VCERIC VDDIB 0 (cutoff)Curves forincreasing IB Lesson: Don’t let the basecollector junction becomeforward biasedVCE VCE VBEForward activeIB 0 & IC F IBVCE VBEVDDRSaturationIB 0VCE VBEECE 315 – Spring 2007 – Farhan Rana – Cornell University12

A Silicon PNP BJTMetal base contactInsulator(SiO2)N (contact)N (base)Metal emitter contactP (emitter)Metal collector contactInsulator(SiO2)Insulator(SiO2)P (collector)P (contact layer)P (substrate)ECE 315 – Spring 2007 – Farhan Rana – Cornell University13

PNP BJT: Ebers-Moll Model for Reverse Active Operation IC IE IB IC IE IB. 10 ECE 315 –Spring 2007 –Farhan Rana –Cornell University PNP BJT: Ebers-Moll Model a