trnasistor chapter 5th 2013_09_13_15_57_13
DESCRIPTION
TRANSCRIPT
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2
The Bipolar Junction Transistor (BJT)
n p n
baseemitter collector
p n p
baseemitter collectornpn (Discrete) Transistor Fabrication (e.g. BC107, 108, 109)
200m n+ n-typewafer
epitaxialn-type layern10m
SiO2
p n
emitter basebase
collector
3
n p n
baseemitter collector
200mn+
n 10mp n
emitter basebase
collector
• base is deliberately made thin, ~1
• BJT’s should be connected as labelled, otherwise gains and breakdown voltages will be drastically reduced
4
Energy bands for an npn transistor under zero applied bias
emitter base collector
n p n
depletion regions
ConductionBand
ValenceBandE
lect
ron
En
erg
y
EF
Fig. 112
5
Energy bands for an npn transistor under normal biasing conditions
ConductionBand
ValenceBand
Ele
ctro
n E
ner
gy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons
6
n-type emitter p-type base n-type collector
base-emitterjunction
collector-basejunction
VBE VCB+ +
IE IC
IB
electrons
holes
|IE|
(1-)|IE|
- emitter efficiency
(lightly doped)
α|IE|
α – common-base current gain
ho
les
ICBO
electrons
holes
IC = αIE + ICBO
BJT CARRIER FLOWSFig. 114
7
10
8
6
4
2
0
IE / mA
0 0.2 0.4 0.6 0.8 VBE / V
VCB = 0VVCB = 25V
)(
26
mAIeI
kT
dI
dVrd
For diode:
For BJT:
CEe eI
kT
eI
kT
dI
dVr
re – dynamic emitter resistance
CE
e IIr
2626
For = 1, T = 300 K and IE, IC in mA:
Fig. 116: Input Characteristic – CB Configuration
Increasing VCB
rd – dynamic resistance
8
10
8
6
4
2
0
IE / mA
0 0.2 0.4 0.6 0.8 VBE / V
VCB = 0VVCB = 25V
IE ≈ IC
b
e c
VBE
IE
IB
IC
VCB
INPUT OUTPUT
IC(VBE)TRANSFER
CHARACTERISTIC
Transconductance, gm, is slope of transfer characteristic, hence:
em rg
1
9
LECTURE 18
BJT CHARACTERISTICS
Common base
Common emitter
Common collector
The Early effect
10
IC / mA
VCB / V-1 0 1 2 3 4 5 6 7 8
1
0.75
0.5
0.25
IE = 1.0 mA
IE = 0.75 mA
IE = 0.50 mA
IE = 0.25 mA
IE = 0
Breakdownregion
ICBOSat
ura
tio
n r
egio
n
Active regionIC ≈ αIE, α ≈ 1
Cutoff region
Fig. 117: Output Characteristics – CB Configuration
b
e c
VBE
IE
IB
IC
VCBIC(VCB)
11
bc
VBE
IB
IE
IC
VCE
INPUT
OUTPUT
COMMON EMITTER CONFIGURATION
IB(VBE) IC(VCE)
e
Fig. 115 (b)
INPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS
n
p
n
b
c
e
12
10
8
6
4
2
0
IB / A
0 0.2 0.4 0.6 0.8 VBE / V
VCE = 20VVCE = 5V
Fig. 118: Input Characteristic – CE Configuration
Increasing VCE
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IC / mA
VCE / V 0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdownregion
ICEO
Sat
ura
tio
n r
egio
n
Active region
Cutoff region
Fig. 119: Output Characteristics – CE Configuration
bc
VBE
IB
IE
IC
VCE
e IC(VCE)
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be
VCB
IB
IC
IE
VCE
INPUT
OUTPUT
COMMON COLLECTOR CONFIGURATION
IB(VCB) IE(VCE)
c
Fig. 115 (c)
INPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS
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IE / mA
VCE / V 0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdownregion
ICEO
Sat
ura
tio
n r
egio
n
Active region
Cutoff region
Fig. 121: Output Characteristics – CC Configuration
CC CE
IE(VCE) IC(VCE)
IE ≈ IC since α ≈ 1
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IB / A
VCB / V 0 5 10 15 20
80
60
40
20
Fig. 120: Input Characteristics – CC Configuration b
e
VCB
IB
IC
IE
VCE
c
Transistor on VBE ≈ 0.7V
0.7V
VCB ≈ VCE – 0.7V
4.3
VCE = 5V VCE = 10V VCE = 15V
As VCB VCE, VBE 0, transistor turns off
IB vs. VCB for different values of VCE
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THE EARLY EFFECT, OR BASE-WIDTH MODULATION
baseemitter collector
n p n
e-b junction
c-b junction
depletion regions
VBE VCB+ +IB
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+–- +- +- +- +
VVB
p n
Effect of bias on width of the depletion region
Fig. 55Reverse bias (p-type -ve w.r.t. n-type)
Potential
Distance
VB+V
VB
-20
-4
-6-8
-10
-12
-14
- +- +- +- +
Depletionregionwidens
19
+ –- +- +- +- +
VVB
p n
Effect of bias on width of the depletion region
Fig. 55Forward bias (p-type +ve w.r.t. n-type)
Potential
DistanceVB-VVB
0.10
0.2
0.30.4
0.5
0.6
0.7
Depletionregion
narrows
- +
- +
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VCB=6VVCB=5VVCB=4VVCB=3VVCB=2V
emitter collector
n p n
e-b junction
VBE + +IB
VCB=1V
THE EARLY EFFECT, OR BASE-WIDTH MODULATION
depletion regions
c-b junction
VCB
base
effectivewidth of base
21
If the effective width of the base decreases: –
1. There will be less recombination in the base, so α (and hence β) will increase.
β = α / (1- α)
nn--type emitter ptype emitter p--type base ntype base n--type collectortype collector
basebase--emitteremitterjunctionjunction
collectorcollector--basebasejunctionjunction
VVBEBE VVCBCB++ ++
IIEE IICC
IIBB
electronselectrons
holesholes
|I|IEE||
(1(1-- )|I)|IEE||
(lightly doped)(lightly doped)
αα|I|IEE||
ho
les
ho
les
IICBOCBO
electronselectrons
holesholes
IICC = = ααIIEE + I+ ICBOCBO
nn--type emitter ptype emitter p--type base ntype base n--type collectortype collector
basebase--emitteremitterjunctionjunction
collectorcollector--basebasejunctionjunction
VVBEBE VVCBCB++ ++
IIEE IICC
IIBB
electronselectrons
holesholes
|I|IEE||
(1(1-- )|I)|IEE||
(lightly doped)(lightly doped)
αα|I|IEE||
ho
les
ho
les
IICBOCBO
electronselectrons
holesholes
IICC = = ααIIEE + I+ ICBOCBO
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If the effective width of the base decreases: –
2. The minority carrier concentration gradient (Δn/Δx) will increase:
so |IE| will increase.
|IE| Δn/Δx (Δx is the basewidth)
Δx
|IE|Electronconcentration Δn
emitter base collectorn p n
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emitter collector
n p n
e-b junction
VBE + +IB
c-b junction
VCB
base
If the effective width of the base decreases: –
3. The c-b depletion region may extend all the way over to the e-b junction – PUNCH-THROUGH
24
Energy bands for an npn transistor under normal biasing conditions
ConductionBand
ValenceBand
Ele
ctro
n E
ner
gy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons
25
Energy bands for an npn transistor under normal biasing conditions
ConductionBand
ValenceBand
Ele
ctro
n E
ner
gy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons
26
Energy bands for an npn transistor under normal biasing conditions
ConductionBand
ValenceBand
Ele
ctro
n E
ner
gy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons
27
Energy bands for an npn transistor under normal biasing conditions
ConductionBand
ValenceBand
Ele
ctro
n E
ner
gy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons
28
IC / mA
VCB / V-1 0 1 2 3 4 5 6 7 8
1
0.75
0.5
0.25
IE = 1.0 mA
IE = 0.75 mA
IE = 0.50 mA
IE = 0.25 mA
IE = 0 ICBOSat
ura
tio
n r
egio
n
Active regionIC ≈ αIE
Cutoff region
b
e c
VBE
IE
IB
IC
VCBIC(VCB)
Early effect implies α and |IE| increases as VCB increases, hence IC
(≈ αIE )increases
Common-base output characteristics
Breakdownregion
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IC / mA
VCE / V 0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdownregion
ICEO
Sat
ura
tio
n r
egio
n
Active region
Cutoff region
bc
VBE
IB
IE
IC
VCE
e
As VCE increases, VCB increases. Early effect implies α, and hence β, increases as VCB increases. IC ≈ βIB, hence IC increases
Common-emitter output characteristics
IC ≈ βIB
30
10
8
6
4
2
0
IE / mA
0 0.2 0.4 0.6 0.8 VBE / V
VCB = 0VVCB = 25V
Increasing VCB
Common-base input characteristics
Early effect implies |IE| increases as VCB increases.
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10
8
6
4
2
0
IB / A
0 0.2 0.4 0.6 0.8 VBE / V
VCE = 20VVCE = 5V
Increasing VCE
Common-emitter input characteristics
Early effect implies less recombination in the base as VCB/VCE increases hence IB decreases
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IC = ICBO for IE = 0 IC ≈ IE in flat regions IC 0 as VCB goes negative IC increases at large values of VCB due to
breakdown at the reverse-biased c-b junctionIICC / mA/ mA
VVCBCB / V/ V--1 0 1 2 3 4 5 1 0 1 2 3 4 5 6 7 86 7 8
11
0.750.75
0.50.5
0.250.25
IIEE = 1.0 mA= 1.0 mA
IIEE = 0.75 mA= 0.75 mA
IIEE = 0.50 mA= 0.50 mA
IIEE = 0.25 mA= 0.25 mA
IIEE = 0 = 0
BreakdownBreakdownregionregion
IICBOCBOSat
ura
tio
n r
egio
nS
atu
rati
on
reg
ion
Active regionActive regionIICC ≈≈ ααIIEE, , αα ≈≈ 11
Cutoff regionCutoff region
IICC / mA/ mA
VVCBCB / V/ V--1 0 1 2 3 4 5 1 0 1 2 3 4 5 6 7 86 7 8
11
0.750.75
0.50.5
0.250.25
IIEE = 1.0 mA= 1.0 mA
IIEE = 0.75 mA= 0.75 mA
IIEE = 0.50 mA= 0.50 mA
IIEE = 0.25 mA= 0.25 mA
IIEE = 0 = 0
BreakdownBreakdownregionregion
IICBOCBOSat
ura
tio
n r
egio
nS
atu
rati
on
reg
ion
Active regionActive regionIICC ≈≈ ααIIEE, , αα ≈≈ 11
Cutoff regionCutoff region
• CB output characteristics are plots of IC vs. VCB:Summary
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Common-emitter configuration
• Input characteristics (IB vs. VBE) resemble that for a forward-biased diode but depend on output voltage VCE due to Early effect
1010
88
66
44
22
00
IIBB / / AA
0 0.2 0.4 0.6 0.80 0.2 0.4 0.6 0.8 VVBEBE / V/ V
VVCE CE = 20V= 20VVVCECE = 5V= 5V
Increasing VIncreasing VCECE
1010
88
66
44
22
00
IIBB / / AA
0 0.2 0.4 0.6 0.80 0.2 0.4 0.6 0.8 VVBEBE / V/ V
VVCE CE = 20V= 20VVVCECE = 5V= 5V
Increasing VIncreasing VCECE
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• Output characteristics are plots of IC vs. VCE for different values of the input current IB:
IC = ICEO for IB = 0 IC >> IB in flat regions due to current amplification IC increases at large values of VCE due to
breakdown at the reverse-biased c-b junctionIICC / mA/ mA
VVCECE / V/ V0 5 10 0 5 10 15 2015 20
44
33
22
11
IIBB = 40 = 40 AA
IIBB = 30 = 30 AA
IIBB = 20 = 20 AA
IIBB = 10= 10AA
IIBB = 0= 0
BreakdownBreakdownregionregion
IICEOCEO
Sat
ura
tio
n r
egio
nS
atu
rati
on
reg
ion
Active regionActive region
Cutoff regionCutoff region
IICC / mA/ mA
VVCECE / V/ V0 5 10 0 5 10 15 2015 20
44
33
22
11
IIBB = 40 = 40 AA
IIBB = 30 = 30 AA
IIBB = 20 = 20 AA
IIBB = 10= 10AA
IIBB = 0= 0
BreakdownBreakdownregionregion
IICEOCEO
Sat
ura
tio
n r
egio
nS
atu
rati
on
reg
ion
Active regionActive region
Cutoff regionCutoff region
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Common-collector configuration
• Input characteristics (IB vs. VCB)
VCB = VCE – VBE hence:
If transistor is “on” VCB is fixed at VCE – 0.7VAs VCB VCE device turns off, IB 0
IIBB / / AA
VVCBCB / V/ V0 5 10 0 5 10 15 2015 20
8080
6060
4040
2020
4.34.3
VVCECE = 5V= 5V VVCECE = 10V= 10V VVCECE = 15V= 15VIIBB / / AA
VVCBCB / V/ V0 5 10 0 5 10 15 2015 20
8080
6060
4040
2020
4.34.3
VVCECE = 5V= 5V VVCECE = 10V= 10V VVCECE = 15V= 15V
36
• Output characteristics are plots of IE vs. VCE for different values of IB
Since IE ≈ IC, CC output characteristics are essentially the same as those for CE
VVCBCB=6V=6V
emitter coemitter collectorllector
n p nn p n
ee--b b junctionjunction
VVBEBE ++ ++IIBB
depletion regionsdepletion regions
cc--b b junctionjunction
VVCBCB
basebase
effectiveeffectivewidth of basewidth of base
VVCBCB=6V=6V
emitter coemitter collectorllector
n p nn p n
ee--b b junctionjunction
VVBEBE ++ ++IIBB
depletion regionsdepletion regions
cc--b b junctionjunction
VVCBCB
basebase
effectiveeffectivewidth of basewidth of base
THE EARLY EFFECT, OR BASE-WIDTH MODULATION