Lecture 17 The Bipolar Junction Transistor (II) Regimes of Operation Outline Regimes of operation Large-signal equivalent circuit model Output characteristics Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.3, 7.4 & 7.5 Announcement: Quiz #2: November 15, 7:30-9:30 PM at Walker. Calculator Required. Open book. Quiz Review: Monday, November 13, 7:00PM, 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 1
Summary of Key Concepts Forward-active regime: most useful, device has gain and isolation. For bias calculations: Saturation Regime: device flooded with minority carriers. Not useful. For bias calculations: Cut-off Regime: device open. Useful. For bias calculations: 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 2
1. BJT: Regimes of Operation Forward active: device has good isolation and high gain; most useful regime; Saturation: device has no isolation and is flooded with minority carriers; takes time to get out of saturation avoid this regime Reverse: poor gain; not useful; Cut-off: negligible current: nearly an open circuit; useful. 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 3
Forward-Active Regime: V BE > 0, V BC <0 Minority Carrier profiles (not to scale): 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 4
Forward-Active Regime: V BE > 0, V BC <0 Emitter injects electrons into base, collector extracts (collects) electrons from base: I C = I S exp qv BE kt ; I S = n pbo D n W B Base injects holes into emitter, holes recombine at emitter contact: I B = I S β F Emitter current: exp qv BE kt 1 ; I E = I C I B = I S exp qv BE kt I S β F = p neo D p W E I S β F exp qv BE kt 1 State-of-the-art IC BJT s today: I C 0.1-1 ma and β F 50-300. β F hard to control tightly: circuit design techniques required to be insensitive to variations in β F. β F = I C I B = n pbo D n W B p neo D p W E = N de D n W E N ab D p W B 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 5
Reverse-Active Regime: V BE < 0, V BC >0 Minority Carrier Profiles (not to scale): 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 6
Reverse-Active Regime: V BE < 0, V BC >0 Collector injects electrons into base, emitter extracts (collects) electrons from base: I E = I S exp qv BC kt ; I S = n pbo D n W B Base injects holes into collector, holes recombine at collector contact and buried layer: I B = I S β R Collector current: exp qv BC kt 1 ; I S β R = p nco D p W C I C = I E I B = I S exp qv BC kt I S β R exp qv BC kt 1 Typically, β R 0.1-5 << β F. β R = I E I B = n pbo D n W B p nco D = p W C N dc D n W C N ab D p W B 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 7
Cut-Off Regime: V BE < 0, V BC <0 Minority Carrier Profiles (not to scale): 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 8
Cut-Off Regime: V BE < 0, V BC <0 Base extracts holes from emitter: I B1 = I S β F = I E Base extracts holes from collector: I B2 = I S β R = I C These are tiny leakage currents ( 10-12 A). 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 9
Saturation Regime: V BE > 0, V BC >0 Minority Carrier profiles (not to scale): 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 10
Saturation Regime: V BE > 0, V BC >0 Saturation is superposition of forward active + reverse active: I C = I S exp qv BE kt exp qv BC kt I S exp qv BC kt 1 β R I B = I S β F exp qv BE kt 1 + I S β R exp qv BC kt 1 I E = I S exp qv BE kt exp qv BC kt I S β F exp qv BE kt 1 I C and I E can have either sign, depending on relative magnitudes of V BE and V BC and β F and β R. In saturation, collector and base are flooded with excess minority carriers it takes lots of time to get transistor out of saturation. 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 11
2. Large-signal equivalent circuit model System of equations that describes BJT operation: I C = I S exp qv BE kt exp qv BC kt I S β R exp qv BC kt 1 I B = I S β F exp qv BE kt 1 + I S β R exp qv BC kt 1 I E = I S exp qv BE kt exp qv BC kt I S β F exp qv BE kt 1 Equivalent-circuit model representation (non-linear hybrid-π model) [particular rendition of Ebers-Moll model in text]: Three parameters in this model: I S, β F, and β R. 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 12
Simplification of equivalent circuit model: Forward-active regime: V BE > 0, V BC < 0 For today s technology: V BE,on 0.7 V. I B depends on outside circuit. Reverse-active regime: V BE < 0, V BC > 0 For today s technology: V BC,on 0.6 V 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 13
Simplification of equivalent circuit model: Saturation regime: V BE > 0, V BC > 0 For today s technology: V CE,sat 0.1 V. I C and I B depend on outside circuit. Cut-off regime: V BE < 0, V BC < 0 Only negligible leakage currents. 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 14
3. Output Characteristics Common-base output characteristics: Common-emitter output characteristics: 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 15
Common-base output characteristics 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 16
Common-Emitter Output Characteristics 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 17
What did we learn today? Summary of Key Concepts Forward-active regime: most useful, device has gain and isolation. For bias calculations: Saturation Regime: device flooded with minority carriers. Not useful. For bias calculations: Cut-off Regime: device open. Useful. For bias calculations: 6.012 Electronic Devices and Circuits Fall 2000 Lecture 17 18