Bipolar Junction Transistors: Basics

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ECE 663Transistor/switch/amplifier – a 3 terminal deviceDamLaserHeartAxonal conductionMOSFETBJT

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All of these share a feature with… Output current can toggle between large and small (Switching  Digital logic; create 0s and 1s) Small change in ‘valve’ (3rd terminal) creates Large change in output between 1st and 2nd terminal (Amplification  Analog applications; Turn 0.5  50)

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Example: BJT common emitter characteristicsGain = 300

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http://www.computerhistory.org/semiconductor/timeline.html#1940s

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Aim of this chapter How can we get ‘Gain’? What is the structure of the device to get gain? What is the equation for gain? How can we use this equation to maximize gain? How can we model this device as a circuit element? What are its AC characteristics and speed?

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Recall p-n junction

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So if we combine these by fusing their terminals…PNWVappl > 0-+Holes from P region (“Emitter”) of 1st PN junction driven by FB of 1st PN junction into central N region (“Base”) Driven by RB of 2nd PN junction from Base into P region of 2nd junction (“Collector”) 1st region FB, 2nd RB If we want to worry about holes alone, need P+ on 1st region For holes to be removed by collector, base region must be thin

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Bipolar Junction Transistors: BasicsIE = IB + IC ………(KCL) VEC = VEB + VBC ……… (KVL)

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ECE 663BJT configurationsGAIN CONFIG

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ECE 663Bipolar Junction Transistors: Basics

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ECE 663Bipolar Junction Transistors: Basics

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ECE 663BJT Fabrication

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ECE 663PNP BJT Electrostatics

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ECE 663PNP BJT Electrostatics

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ECE 663 NPN Transistor Band Diagram: Equilibrium

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ECE 663PNP Transistor Active Bias ModeFew recombine in the baseVEB > 0 VCB > 0

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ECE 663P+NPForward Active minority carrier distribution

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ECE 663PNP Physical Currents

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ECE 663PNP transistor amplifier actionClearly this works in common emitter configuration

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ECE 663Emitter Injection Efficiency - PNPECIEpICpIEnICnIBIEICCan we make the emitter see holes alone?

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ECE 663Base Transport FactorECIEpICpIEnICnIBIEICCan all injected holes make it to the collector?

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ECE 663Common Base DC current gain - PNP Common Base – Active Bias mode:IC = aDCIE + ICB0ICp = aTIEp = aTgIEIC = aTgIE + ICnaDC = aTg

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ECE 663Common Emitter DC current gain - PNP Common Emitter – Active Bias mode:IE = bDCIB + ICE0bDC = aDC /(1-aDC)IC = aDCIE + ICB0 = aDC(IC + IB) + ICB0IC = aDCIB + ICB0 1-aDCGAIN !!

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ECE 663Common Emitter DC current gain - PNPThin base will make aT  1 Highly doped P region will make g  1

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ECE 663PNP BJT Common Emitter Characteristic

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Last Updated: 8th March 2018

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