1. 1

2. 2
The Bipolar Junction Transistor (BJT)
n p n
base
emitter collector
p n p
base
emitter collectornpn (Discrete) Transistor Fabrication (e.g. BC107, 108, 109)
200m n+ n-type
wafer
epitaxial
n-type
layern10m
SiO2
p n
emitter basebase
collector

3. 3
n p n
base
emitter collector
200mn+
n 10mp 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. 4
Energy bands for an npn transistor
under zero applied bias
emitter base collector
n p n
depletion regions
Conduction
Band
Valence
Band
ElectronEnergy
EF
Fig. 112

5. 5
Energy bands for an npn transistor
under normal biasing conditions
Conduction
Band
Valence
Band
ElectronEnergy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons

6. 6
n-type emitter p-type base n-type collector
base-emitter
junction
collector-base
junction
VBE VCB+ +
IE IC
IB
electrons
holes
|IE|
(1-)|IE|
 - emitter efficiency
(lightly doped)
α|IE|
α – common-base current gain
holes
ICBO
electrons
holes
IC = αIE + ICBO
BJT CARRIER FLOWSFig. 114

7. 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
dV
rd 

For diode:
For BJT:
CE
e
eI
kT
eI
kT
dI
dV
r


re – dynamic emitter resistance
CE
e
II
r
2626

For  = 1, T = 300 K and IE,
IC in mA:
Fig. 116: Input
Characteristic –
CB Configuration
Increasing VCB
rd – dynamic resistance

8. 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:
e
m
r
g
1


9. 9
LECTURE 18
BJT CHARACTERISTICS
 Common base
 Common emitter
 Common collector
 The Early effect

10. 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
Breakdown
region
ICBO
Saturationregion
Active region
IC ≈ αIE, α ≈ 1
Cutoff region
Fig. 117: Output
Characteristics –
CB Configuration
b
e c
VBE
IE
IB
IC
VCB
IC(VCB)

11. 11
b
c
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. 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

13. 13
IC / mA
VCE / V0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdown
region
ICEO
Saturationregion
Active region
Cutoff region
Fig. 119: Output
Characteristics –
CE Configuration
b
c
VBE
IB
IE
IC
VCE
e
IC(VCE)

14. 14
b
e
VCB
IB
IC
IE
VCE
INPUT
OUTPUT
COMMON COLLECTOR CONFIGURATION
IB(VCB) IE(VCE)
c
Fig. 115 (c)
INPUT
CHARACTERISTICS
OUTPUT
CHARACTERISTICS

15. 15
IE / mA
VCE / V0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdown
region
ICEO
Saturationregion
Active region
Cutoff region
Fig. 121: Output
Characteristics –
CC Configuration
CC CE
IE(VCE)  IC(VCE)
IE ≈ IC since α ≈ 1

16. 16
IB / A
VCB / V0 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

17. 17
THE EARLY EFFECT, OR BASE-WIDTH MODULATION
baseemitter collector
n p n
e-b
junction
c-b
junction
depletion regions
VBE VCB+ +
IB

18. 18
+–
- +
- +
- +
- +
V
VB
p n
Effect of bias on width of the depletion region
Fig. 55
Reverse bias
(p-type -ve w.r.t. n-type)
Potential
Distance
VB+V
VB
-2
0
-4
-6
-8
-10
-12
-14
- +
- +
- +
- +
Depletion
region
widens

19. 19
+ –
- +
- +
- +
- +
V
VB
p n
Effect of bias on width of the depletion region
Fig. 55
Forward bias
(p-type +ve w.r.t. n-type)
Potential
Distance
VB-VVB
0.1
0
0.2
0.3
0.4
0.5
0.6
0.7
Depletion
region
narrows
- +
- +

20. 20
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
effective
width of base

21. 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--emitteremitter
junctionjunction
collectorcollector--basebase
junctionjunction
VVBEBE VVCBCB++ ++
IIEE IICC
IIBB
electronselectrons
holesholes
|I|IEE||
(1(1--)|I)|IEE||
(lightly doped)(lightly doped)
αα|I|IEE||
holesholes
IICBOCBO
electronselectrons
holesholes
IICC == ααIIEE + I+ ICBOCBO
nn--type emitter ptype emitter p--type base ntype base n--type collectortype collector
basebase--emitteremitter
junctionjunction
collectorcollector--basebase
junctionjunction
VVBEBE VVCBCB++ ++
IIEE IICC
IIBB
electronselectrons
holesholes
|I|IEE||
(1(1--)|I)|IEE||
(lightly doped)(lightly doped)
αα|I|IEE||
holesholes
IICBOCBO
electronselectrons
holesholes
IICC == ααIIEE + I+ ICBOCBO

22. 22
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|Electron
concentration Δn
emitter base collector
n p n

23. 23
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. 24
Energy bands for an npn transistor
under normal biasing conditions
Conduction
Band
Valence
Band
ElectronEnergy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons

25. 25
Energy bands for an npn transistor
under normal biasing conditions
Conduction
Band
Valence
Band
ElectronEnergy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons

26. 26
Energy bands for an npn transistor
under normal biasing conditions
Conduction
Band
Valence
Band
ElectronEnergy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons

27. 27
Energy bands for an npn transistor
under normal biasing conditions
Conduction
Band
Valence
Band
ElectronEnergy
Fig. 113
emitter base collector
VBE VCB+ +
n p n
electrons

28. 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 = 0ICBO
Saturationregion
Active region
IC ≈ αIE
Cutoff region
b
e c
VBE
IE
IB
IC
VCB
IC(VCB)
Early effect implies α
and |IE| increases as
VCB increases, hence
IC (≈ αIE )increases
Common-base output characteristics
Breakdown
region

29. 29
IC / mA
VCE / V0 5 10 15 20
4
3
2
1
IB = 40 A
IB = 30 A
IB = 20 A
IB = 10A
IB = 0
Breakdown
region
ICEO
Saturationregion
Active region
Cutoff region
b
c
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. 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.

31. 31
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

32. 32
 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 junction
IICC / mA/ mA
VVCBCB / V/ V--1 0 1 2 3 4 51 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
BreakdownBreakdown
regionregion
IICBOCBO
SaturationregionSaturationregion
Active regionActive region
IICC ≈≈ ααIIEE,, αα ≈≈ 11
Cutoff regionCutoff region
IICC / mA/ mA
VVCBCB / V/ V--1 0 1 2 3 4 51 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
BreakdownBreakdown
regionregion
IICBOCBO
SaturationregionSaturationregion
Active regionActive region
IICC ≈≈ ααIIEE,, αα ≈≈ 11
Cutoff regionCutoff region
• CB output characteristics are plots of IC vs. VCB:
Summary

33. 33
 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
VVCECE = 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
VVCECE = 20V= 20VVVCECE = 5V= 5V
Increasing VIncreasing VCECE

34. 34
• 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 junction
IICC / mA/ mA
VVCECE / V/ V0 5 100 5 10 15 2015 20
44
33
22
11
IIBB = 40= 40 AA
IIBB = 30= 30 AA
IIBB = 20= 20 AA
IIBB = 10= 10AA
IIBB = 0= 0
BreakdownBreakdown
regionregion
IICEOCEO
SaturationregionSaturationregion
Active regionActive region
Cutoff regionCutoff region
IICC / mA/ mA
VVCECE / V/ V0 5 100 5 10 15 2015 20
44
33
22
11
IIBB = 40= 40 AA
IIBB = 30= 30 AA
IIBB = 20= 20 AA
IIBB = 10= 10AA
IIBB = 0= 0
BreakdownBreakdown
regionregion
IICEOCEO
SaturationregionSaturationregion
Active regionActive region
Cutoff regionCutoff region

35. 35
 Common-collector configuration
• Input characteristics (IB vs. VCB)
VCB = VCE – VBE hence:
If transistor is “on” VCB is fixed at VCE – 0.7V
As VCB  VCE device turns off, IB  0
IIBB // AA
VVCBCB / V/ V0 5 100 5 10 15 2015 20
8080
6060
4040
2020
4.34.3
VVCECE = 5V= 5V VVCECE = 10V= 10V VVCECE = 15V= 15V
IIBB // AA
VVCBCB / V/ V0 5 100 5 10 15 2015 20
8080
6060
4040
2020
4.34.3
VVCECE = 5V= 5V VVCECE = 10V= 10V VVCECE = 15V= 15V

36. 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--bb
junctionjunction
VVBEBE ++ ++
IIBB
depletion regionsdepletion regions
cc--bb
junctionjunction
VVCBCB
basebase
effectiveeffective
width of basewidth of base
VVCBCB=6V=6V
emitter coemitter collectorllector
n p nn p n
ee--bb
junctionjunction
VVBEBE ++ ++
IIBB
depletion regionsdepletion regions
cc--bb
junctionjunction
VVCBCB
basebase
effectiveeffective
width of basewidth of base
 THE EARLY EFFECT, OR BASE-WIDTH MODULATION