MIT6_012S09_lec18

MIT6_012S09_lec18 - Lecture 18 The Bipolar Junction...

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Lecture 18 The Bipolar Junction Transistor (II) Regions of Operation Outline • Regions of operation • Large-signal equivalent circuit model • Output characteristics Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.3, 7.4 & 7.5 6.012 Spring 2009 Lecture 18 1
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1. BJT: Regions of Operation V BE - forward saturation V BC active + B V CE± V BC V BE reverse - cut-off - Forward active : device has high voltage gain and high β ; Reverse active : poor β ; not useful ; Cut-off : negligible current: nearly an open circuit; Saturation : device is flooded with minority carriers; takes time to get out of saturation 6.012 Spring 2009 Lecture 18 2
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Forward-Active Region: V BE > 0, V BC <0 n-Emitter p-Base n-Collector I E <0 I B >0 I C >0 V BE > 0 V BC < 0± Minority Carrier profiles ( not to scale ): n pB p nE p nC n pBo p nEo p nCo 0 W B - X BE W B +X BC -W E -X BE W B +X BC +W C x ± emitter base collector 6.012 Spring 2009 Lecture 18 3
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Forward-Active Region: V BE > 0, V BC < 0 Emitter injects electrons into base , collector extracts (collects) electrons from base : I C = I S e V BE V th [ ] ; I S = qA E n pBo D n W B Base injects holes into emitter , holes recombine at emitter contact: I B = I S β F e V BE V th [ ] 1 ; I S F = qA E p nEo D p W E Emitter current: I E = − I C I B = − I S e V BE V th [ ] I S F e V BE V th [ ] 1 • State-of-the-art IC BJT’s today: I S ≈ 0.1 - 1 fA β F ≈ 50 - 300. β F hard to control tightly: circuit design techniques required to be insensitive to variations in β F .
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MIT6_012S09_lec18 - Lecture 18 The Bipolar Junction...

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