Lecture 18 The Bipolar Junction Transistor (II) Regimes of Operation

Transcription

1 Lecture 18 The ipolar Junction Transistor (II) Regimes of Operation Outline Regimes of operation Large-signal equivalent circuit model Output characteristics Reading Assignment: Howe and Sodini; hapter 7, Sections 7.3, 7.4 & 7.5 Announcement: Quiz #2: April 25, 7:30-9:30 PM at Walker. alculator Required. Open book Spring 2007 Lecture 18 1

2 1. JT: Regions of Operation V + + V - + V forward active saturation V V - - cut-off reverse Forward active: device has high voltage gain and high β; Reverse active: poor β; not useful; ut-off: negligible current: nearly an open circuit; Saturation: device is flooded with minority carriers; takes time to get out of saturation Spring 2007 Lecture 18 2

3 Forward-Active Regime: V > 0, V <0 n-mitter p-ase n-ollector I <0 I >0 I >0 V > 0 V < 0 Minority arrier profiles (not to scale): emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W Spring 2007 Lecture 18 3

4 Forward-Active Regime: V > 0, V < 0 mitter injects electrons into base, collector extracts (collects) electrons from base: I = I S e ] ; IS = qa n po D n V W ase injects holes into emitter, holes recombine at emitter contact: I = I V] S Vth e 1 β F ; I S = qa p no D p β F W mitter current: I = I I = I S e ] I S V β F e V ] 1 State-of-the-art I JT s today: I S fa β F β F hard to control tightly: circuit design techniques required to be insensitive to variations in β F. β F = I I = n po D n W p no D p W = N d D n W N a D p W Spring 2007 Lecture 18 4

5 Reverse-Active Regime: V < 0, V > 0 n-mitter p-ase n-ollector I >0 I <0 I >0 V < 0 V > 0 Minority arrier Profiles (not to scale): emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W Spring 2007 Lecture 18 5

6 Reverse-Active Regime: V < 0, V > 0 ollector injects electrons into base, emitter extracts (collects) electrons from base: I = I S e ] ; IS = qa n po D n V W ase injects holes into collector, holes recombine at collector contact and buried layer: I = I ( V) S Vth e 1 β R ; I S = qa p no D p β R W ollector current: I = I I = I S e Typically, β R << β F. ] I S V β R e V ] 1 β R = I I = n po D n W p no D p W = N dd n W N a D p W Spring 2007 Lecture 18 6

7 ut-off Regime: V < 0, V < 0 n-mitter p-ase n-ollector I >0 I >0 I <0 V < 0 V < 0 Minority arrier Profiles (not to scale): emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W Spring 2007 Lecture 18 7

8 ut-off Regime: V < 0, V < 0 ase extracts holes from emitter: I 1 = I S β F = I ase extracts holes from collector: I 2 = I S β R = I These are tiny leakage currents ( A) Spring 2007 Lecture 18 8

9 Saturation Regime: V > 0, V > 0 n-mitter p-ase n-ollector I I I <0 V > 0 V > 0 Minority arrier profiles (not to scale): emitter base collector p n n p pn pno n po p no x -W -X -X 0 W W +X W +X +W Spring 2007 Lecture 18 9

10 Saturation Regime: V > 0, V > 0 Saturation is superposition of forward active + reverse active: V ] V ] V I = I S e th e I V ] S V e th 1 β R I = I S β F e V] Vth 1 + I S β R e V] Vth 1 I = I S e ] V] e V I S β F e V] 1 I and I can have either sign, depending on relative magnitudes of V and V and β F and β R Spring 2007 Lecture 18 10

11 Saturation - The Flux Picture oth junctions are injecting and collecting. lectrons injected from emitter into base are collected by the collector as in Forward Active case. lectrons injected from collector into the base are collected by the emitter as in Reverse Active case. Holes injected into emitter recombine at ohmic contact as in Forward Active case. Holes injected into collector recombine with electrons in the n + buried layer ;;; majority hole flux from base contact hole diffusion flux n + polysilicon majority electrons electron diffusion ;;;;;; ;;;;;; n + emitter p-type base ;;;;;;;;;;;; ;;;;;;;;;; ;;;;;;;;; ;;;;;;;; ;;;;;;;; ;;;;;;;;;;; ;;;;;;; ;;;;;;;;; ;;;;; n-type collector minority hole diffusion flux majority electron flux from collector contact supplying injection into base ;;;;;;; ;;; ;; ;;;;; ;;;;;; ;;;;;; ;;;;; ;;;;;; ;;;;; ;;;; n + buried layer ;;; ;;; majority electron flux from collector contact to recombine with hole diffusion flux ; majority electron flux to collector contact ; Spring 2007 Lecture 18 11

12 2. Large-signal equivalent circuit model System of equations that describes JT operation: V ] V ] V I = I S e th e I V ] S V e th 1 β R I = I V] S Vth e 1 β F + I V] S Vth e 1 β R V] V] V I = I S e th V e th I V] S V e th 1 β F quivalent-circuit model representation (non-linear hybrid-π model) particular rendition of bers-moll model in text]: - I V I S /β R + + I I R I F β F I F - β R I R =I S exp(qv /kt) - exp(qv /kt)] I S /β F V - I Three parameters in this model: I S, β F, and β R Spring 2007 Lecture 18 12

13 Simplification of equivalent circuit model: Forward-active regime: V > 0, V < 0 V,on I S e ] V For today s technology: V,on 0.7 V. I depends on outside circuit. Reverse-active regime: V < 0, V > 0 V,on I S e ] V For today s technology: V,on 0.6 V Spring 2007 Lecture 18 13

14 Simplification of equivalent circuit model: Saturation regime: V > 0, V > 0 + V,on V,on V,sat V,on V,on - For today s technology: V,sat 0.1 V. I and I depend on outside circuit. ut-off regime: V < 0, V < 0 Only negligible leakage currents Spring 2007 Lecture 18 14

15 3. Output haracteristics ommon-emitter output characteristics: I I I =0 0 0 V =V +V V,sat Spring 2007 Lecture 18 15

16 ommon-mitter Output haracteristics Spring 2007 Lecture 18 16

17 What did we learn today? Summary of Key oncepts Forward-active regime: For bias calculations: V,on I S e ] V Saturation Regime: For bias calculations: + V,on V,sat V,on - ut-off Regime: For bias calculations: Spring 2007 Lecture 18 17