Lecture 16. The Bipolar Junction Transistor (I) Forward Active Regime. Outline. The Bipolar Junction Transistor (BJT): structure and basic operation

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1 Lecture 16 The Bipolar Junction Transistor (I) Forward Active Regime Outline The Bipolar Junction Transistor (BJT): structure and basic operation I-V characteristics in forward active regime Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.1, Electronic Devices and Circuits Fall 2000 Lecture 16 1

2 Summary of Key Concepts npn BJT in forward active regime: Emitter injects electrons into Base, Collector collects electrons from Base I C controlled by V BE, independent of V BC (transistor effect) I C exp qv BE kt Base: injects holes into Emitter I B I B I C Electronic Devices and Circuits Fall 2000 Lecture 16 2

3 1. BJT: structure and basic operation Uniqueness of BJT: high-current drivability per input capacitance fast excellent analog and front-end communications applications Electronic Devices and Circuits Fall 2000 Lecture 16 3

4 Simplified one-dimensional model of intrinsic device: BJT=two neighbouring pn junctions back-to-back Close enough for minority carriers to interact can diffuse quickly through the base Far apart enough for depletion regions not to interact prevent punchthrough Electronic Devices and Circuits Fall 2000 Lecture 16 4

5 Basic Operation: forward-active regime Transistor Effect : electrons injected from the Emitter to the Base, extracted by the Collector Electronic Devices and Circuits Fall 2000 Lecture 16 5

6 Basic Operation: forward-active regime Carrier profiles in thermal equilibrium: Carrier profiles in forward-active regime: Electronic Devices and Circuits Fall 2000 Lecture 16 6

7 Basic Operation: forward-active regime Dominant current paths in forward active regime: I C : electron injection from Emitter to Base and collection by Collector I B : hole injection from Base to Emitter I E : I E = -(I C +I B ) Key dependencies (choose one): I C on V BE : I C on V BC : I B on V BE : I B on V BC : exp qv BE kt, 1, no dep., other V BE exp qv BC kt, 1, no dep., other V BC exp qv BE kt, 1, no dep., other V BE exp qv BC kt, 1, no dep., other V BC I C on I B : exp onential, quadratic, no dep., other Electronic Devices and Circuits Fall 2000 Lecture 16 7

8 Basic Operation: forward-active regime V BE controls I C ( transistor effect ) I C independent of V BC ( isolation ) Price to pay for control: I B (base current) Comparison with MOSFET: Feature MOSFET BJT in saturation in FAR Controlling terminal Gate Base Common terminal Source Emitter Controlled terminal Drain Collector Functional dependence of controlled current DC current in controlling terminal Quadratic Exponential 0 Exponential Figure of Merit for BJT: Common-emitter current gain: Want it as large as possible Common-base current gain: Want it close to 1 β F = I C I B α F = I C I E Electronic Devices and Circuits Fall 2000 Lecture 16 8

9 2. I-V characteristics in forward-active regime Collector current: focus on electron diffusion in base Boundary conditions: n pb (0) = n pbo exp qv BE kt, n pb( W B )=0 Electron profile: n pb (x) = n pb (0) 1 x W B Electronic Devices and Circuits Fall 2000 Lecture 16 9

10 Electron current density: J nb = qd n dn pb dx = qd n n pb (0) W B Collector current scales with area of base-emitter junction A E : Collector terminal current: or D I C = J nb A E = qa n E n pbo exp qv BE W B kt I C = I S exp qv BE kt I S transistor saturation current Electronic Devices and Circuits Fall 2000 Lecture 16 10

11 Base current: focus on hole injection and recombination in emitter Boundary conditions: p ne ( x BE ) = p neo exp qv BE kt ; p ne( W E x BE ) = p neo Hole profile: p ne (x)=[ p ne ( x BE ) p neo ] 1+ x + x BE + pneo W E Electronic Devices and Circuits Fall 2000 Lecture 16 11

12 Hole current density: J pe = qd p dp ne dx = qd p p ne ( x BE ) p neo W E Base current scales with area of base-emitter junction A E : Base terminal current: D p I B = J pe A E = qa E p neo exp qv BE W E kt Electronic Devices and Circuits Fall 2000 Lecture 16 12

13 Forward Active Region: Current gain 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 To maximize β F : N de >> N ab W E >> W B want npn, rather than pnp design because D n > D p Electronic Devices and Circuits Fall 2000 Lecture 16 13

14 Plot of I C and I B Electronic Devices and Circuits Fall 2000 Lecture 16 14

15 Current Gain Electronic Devices and Circuits Fall 2000 Lecture 16 15

16 What did we learn today? Summary of Key Concepts npn BJT in forward active regime: Emitter injects electrons into Base, Collector collects electrons from Base IC controlled by V BE, independent of V BC (transistor effect) I C exp qv BE kt Base: injects holes into Emitter I B I B I C Electronic Devices and Circuits Fall 2000 Lecture 16 16