trnasistor chapter 5th 2013_09_13_15_57_13

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


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


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

13

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)

14

be

VCB

IB

IC

IE

VCE

INPUT

OUTPUT

COMMON COLLECTOR CONFIGURATION

IB(VCB) IE(VCE)

c

Fig. 115 (c)

INPUT CHARACTERISTICS

OUTPUT CHARACTERISTICS

15

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

16

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

17

THE EARLY EFFECT, OR BASE-WIDTH MODULATION


n p n

e-b junction

c-b junction

depletion regions

VBE VCB+ +IB

18

+–- +- +- +- +

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

- +

- +

20

VCB=6VVCB=5VVCB=4VVCB=3VVCB=2V

emitter collector

n p n

e-b junction

VBE + +IB

VCB=1V


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

22


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

23

emitter collector

n p n

e-b junction

VBE + +IB

c-b junction

VCB

base


3. The c-b depletion region may extend all the way over to the e-b junction – PUNCH-THROUGH

24


ConductionBand

ValenceBand

Ele

ctro

n E

ner

gy

Fig. 113


VBE VCB+ +

n p n

electrons

25


ConductionBand

ValenceBand

Ele

ctro

n E

ner

gy

Fig. 113


VBE VCB+ +

n p n

electrons

26


ConductionBand

ValenceBand

Ele

ctro

n E

ner

gy

Fig. 113


VBE VCB+ +

n p n

electrons

27


ConductionBand

ValenceBand

Ele

ctro

n E

ner

gy

Fig. 113


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

29

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.

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

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


IICBOCBOSat

ura

tio

n r

egio

nS

atu

rati

on

reg

ion

Active regionActive regionIICC ≈≈ ααIIEE, , αα ≈≈ 11


• CB output characteristics are plots of IC vs. VCB:Summary

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

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

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 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


IICEOCEO

Sat

ura

tio

n r

egio

nS

atu

rati

on

reg

ion

Active regionActive 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


IICEOCEO

Sat

ura

tio

n r

egio

nS

atu

rati

on

reg

ion

Active regionActive region


35

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


trnasistor chapter 5th 2013_09_13_15_57_13

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