MIT6_012S09_lec18

Reading Assignment:
Howe and Sodini; Chapter 7, Sections 7.3, 7.4 & 7.5
6.012 Spring 2009
Lecture 18
1
1. BJT: Regions of Operation
VBE VBC + B +
VBE E
VCE
IB>0 VBE < 0 VBC > 0
Minority Carrier Profiles (not to scale):
emitter base collector
pnE
npB
pnC
pnCo npBo
x
pnEo -WE-XBE
-XBE 0
WB WB+XBC WB+XBC+WC
6.012 Spring 2009
Lecture 18
5
Reverse­Active Region: VBE < 0, VBC > 0
• Collector injects electrons into base, emitter extracts (collects) electrons from base: VBC qAC n pBo Dn Vth IE = IS e ; IS = WB
B
[ ] Ie
VBC Vth
S
E
E
For today’s technology: VBC,on ≈ 0.6 V
6.012 Spring 2009 Lecture 18 13
Simplification of equivalent circuit model:
[ ]
• Base injects holes into collector, holes recombine at collector contact and buried layer:
IS e IB = βR
( )− 1 ;
VBC Vth

βR
VBC Vth
IS
=
qAC pnCo D p WC
6.012 Spring 2009
Lecture 18
8
Saturation Region: VBE > 0, VBC > 0
n-Emitter IE p-Base n-Collector IC
IB <0 IB >0 VBE > 0 VBC > 0
Minority Carrier profiles (not to scale):
• State-of-the-art IC BJT’s today: IS ≈ 0.1 - 1 fA • βF ≈ 50 - 300. • βF hard to control tightly: ⇒ circuit design techniques required to be insensitive to variations in βF.
6.012 Spring 2009
Lecture 18
10
Saturation - The Flux Picture
Both junctions are injecting and collecting. Electrons injected from emitter into base are collected by the collector as in Forward Active case. Electrons 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
C C
B
B VBE,on
[ ] Ie
VBE Vth
S
E
E
For today’s technology: VBE,on ≈ 0.7 V. IB depends on outside circuit. • Reverse­active region: VBE < 0, VBC > 0
C C
VBC,on B
VBC Vth
• Collector current:
IS [ ] [ ] I C = − I E − I B = − I Se − e −1
βR

• Typically, βR ≈ 0.1 - 5 << βF .
Dn IE WB N dC DnWC βR = = = D p N aB D pWB IB pnCo • WC n pBo •
VBC Vth
[ ]
− 1
VBE Vth
IE = − IS e
I [ ]− e[ ] [ ] − S e − 1 βF
• IC and IE can have either sign, depending on relative magnitudes of VBE and VBC and βF and βR.
Minority Carrier Profiles (not to scale):
emitter base collector
pnE
npB
pnC
pnCo npBo
x
pnEo -WE-XBE
-XBE 0
WB WB+XBC WB+XBC+WC
6.012 Spring 2009
Lecture 18
7
Cut­Off Region: VBE < 0, VBC < 0
Dn I WB N dE DnWE βF = C = = D IB N aB D pWB pnEo • p WE n pBo •
6.012 Spring 2009 Lecture 18 4
Reverse­Active Region: VBE < 0, VBC > 0
n-Emitter IE>0 p-Base n-Collector IC<0
emitter base collectorpEnpBpnC
pnCo npBo
x
pnEo -WE-XBE
-XBE 0
WB WB+XBC WB+XBC+WC
6.012 Spring 2009
Lecture 18
9
Saturation Region: VBE > 0, VBC > 0
Saturation is superposition of forward active + reverse active:
[ ] [ ]
[ ]
[ ]
[ ]
[ ] [ ]
[ ]
Equivalent-circuit model representation (non­linear hybrid­π model) [particular rendition of Ebers-Moll C model in text]: IC VBC I S / βR B + + IB IR IF I S / βF VBE E IE βFIF - βRIR =IS[exp(qVBE/kT) - exp(qVBC/kT)]
Three parameters in this model: IS, βF, and βR.
6.012 Spring 2009 Lecture 18 12
Simplification of equivalent circuit model:
• Forward­active region: VBE > 0, VBC < 0
– ⇒ takes time to get out of saturation
6.012 Spring 2009
Lecture 18
2
Forward­Active Region: VBE > 0, VBC <0
n-Emitter p-Base n-Collector IC>0
IE<0
IB>0 VBE > 0 VBC < 0
VBC cut-off reverse C
+
forward� active
saturation
• Forward active: device has high voltage gain and high β; • Reverse active: poor β; not useful; • Cut­off: negligible current: nearly an open circuit; • Saturation: device is flooded with minority carriers;
• Emitter injects electrons into base, collector extracts (collects) electrons from base:
[ ]; I C = I Se
VBE Vth
IS =
qAE npBo Dn WB
• Base injects holes into emitter, holes recombine at emitter contact:
• Base extracts holes from emitter: