bipolar_junction_transistor-Unit-2_EDCppt

BJT

1. Bipolar Junction Transistor
UNIT-II

2. Syllabus
•Introduction to Bipolar Junction Transistor
•BJT Operation
•BJT Configurations
•BJT Biasing

3. Reference Books
1.“Integrated Electronics” by Millman & Halkias
2.“Electronic Devices and Circuit Theory” by
Boylestad & Nashelsky

4. Introduction
•Solid state transistor was invented by a team of scientists at
Bell laboratories during 1947-48
•It brought an end to vacuum tube era
•Advantages of solid state transistor over vacuum devices:
–Smaller size, light weight
–No heating elements required
–Lower power consumption and operating voltages
–Low price

5. Introduction
Figure showing relative sizes of
transistor, IC and LED
Figure showing different transistor packages

6. Introduction
•Bipolar Junction Transistor (BJT) is a sandwich consisting of
three layers of two different types of semiconductor
•Two kinds of BJT sandwiches are: NPN and PNP

7. Introduction
•The three layers of BJT are called Emitter, Base and Collector
•Base is very thin compared to the other two layers
•Base is lightly doped. Emitter is heavily doped. Collector is
moderately doped
•NPN –Emitter and Collector are made of N-type
semiconductors; Base is P-type
•PNP –Emitter and Collector are P-type, Base is N-type
•Both types (NPN and PNP) are extensively used, either
separately or in the same circuit

8. Introduction
•Transistor symbols:
Note: Arrow direction from P to N (like diode)

9. Introduction
•BJT has two junctions –Emitter-Base (EB) Junction and
Collector-Base (CB) Junction
•Analogous to two diodes connected back-to-back:
–EB diode and CB diode
•The device is called “bipolar junction transistor” because
current is due to motion of two types of charge carriers –free
electrons & holes

10. Transistor Operation
•Operation of NPN transistor is discussed here; operation of
PNP is similar with roles of free electrons and holes
interchanged
•For normal operation (amplifier application)
–EB junction should be forward biased
–CB junction should be reverse biased
•Depletion width at EB junction is narrow (forward biased)
•Depletion width at CB junction is wide (reverse biased)

11. Transistor Operation

12. Transistor Operation
Un-biased transistor showing barriers at the junctions

13. Transistor Operation
C-B junction is reverse biased –increased barrier height

14. Transistor Operation
E-B junction is forward biased –aids charge flow

15. Transistor Operation
Electron-hole combination –leads to small base current

16. Transistor Operation
•When EB junction is forward biased, free electrons from
emitter region drift towards base region
•Some free electrons combine with holes in the base to form
small base current
•Inside the base region (p-type), free electrons are minority
carriers. So most of the free electrons are swept away into the
collector region due to reverse biased CB junction

17. Transistor Operation
•Three currents can be identified in BJT
1.Emitter current
•This is due to flow of free electrons from emitter to base
•Results in current from base to emitter
2.Base current
•This is due to combination of free electrons and holes in the base
region
•Small in magnitude (usually in micro amperes)
3.Collector current
•Has two current components:
•One is due to injected free electrons flowing from base to collector
•Another is due to thermally generated minority carriers

18. Transistor Operation
•Note the current directions in NPN and PNP transistors
•For both varieties: ---(1)
C
E
B
I
C
I
E
I
B
NPN
C
E
B
I
C
I
E
I
B
PNPBCE III 

19. Transistor Operation
•As noted earlier, collector current has two components:
–One due to injected charge carriers from emitter
–Another due to thermally generated minority carriers
•Both results in current in the same direction. Hence
---(2)
α
dcis the fraction of charge carriers emitted from emitter, that
enter into the collector region
I
CBOis the reverse saturation current in CB diode
---(3)CBOEdcC III  E
CBOC
dc
I
II


20. Transistor Operation
•As approximation, we can neglect I
CBOcompared to I
Eand I
C
•Hence approximate equations are:
•Like the reverse saturation current of ordinary diode, I
CBOalso
doubles for every 10
o
C rise in temperature.
•So I
CBOcannot be neglected at higher temperatures
•The parameter α
dcis called common-base dc current gain
•Value of α
dcis around 0.99EdcC II E
C
dc
I
I


21. Transistor Operation
•We have
•Substituting for I
E, we getCBOEdcC III   
CBOBCdcC IIII  CBOBdcCdc III  )1( )1()1(
dc
CBO
B
dc
dc
C
I
II





 CEOBdcC III 
•Where and)1(
dc
dc
dc




  
CBOdc
dc
CBO
CEO I
I
I 1
)1(


 

---(4)

22. Transistor Operation
•Equations (2) and (4) are two alternate forms of BJT current
equation
•Since value of α
dcis around 0.99, I
CEO>> I
CBO
•However, I
CEOis still very small compared to I
C
•Hence approximation of (4) gives: or
•Parameter β
dcis called common emitter dc current gain
•Values of α
dcand β
dcvary from transistor to transistor. Both
α
dcand β
dcare sensitive to temperature changesBdcC II B
C
dc
I
I


23. Problems
1.A BJT has alpha (dc) 0.998 and collector-to-base reverse sat
current 1μA. If emitter current is 5mA, calculate the
collector and base currents.
(Ans: 4.99 mA, 10 μA)
2. An npntransistor has collector current 4mA and base current
10 μA. Calculate the alpha and beta values of the transistor,
neglecting the reverse sat current I
CBO
(Ans: 0.9975, 400)
3.In a transistor, 99% of the carriers injected into the base cross
over to the collector region. If collector current is 4mA and
collector leakage current is 6 μA, Calculate emitter and base
currents
(Ans: 4.034 mA, 34 μA)

24. Transistor Configurations
•BJT has three terminals
•For two-port applications, one of the BJT terminals needs to
be made common between input and output
•Accordingly three configurations exist:
–Common Base (CB) configuration
–Common Emitter (CE) configuration
–Common Collector (CC) configuration
•(The last one is not discussed in this course)
Input Output
2-port
device

25. Transistor Configurations
•Common Base configuration
(Resistors are not shown here
for simplicity)
•Base is common between input and output
–Input voltage: V
EB Input current: I
E
–Output voltage: V
CBOutput current: I
C

26. Transistor Configurations
•CB Input characteristics
–A plot of I
Eversus V
EB
for various values of V
CB
–It is similar to forward
biased diode
characteristics
–As V
CBis increased, I
E
increases only slightly
–Note that second letter in
the suffix is B(for base)

27. Transistor ConfigurationsconstVwith
I
V
r
CB
E
EB
i


 constIwith
V
V
A
E
EB
CB
V



•Input resistance r
i
•Voltage amplification factor A
V
•Both can be determined from the CB input characteristics

28. Transistor Configurations21
21
EE
EBEB
i
II
VV
r


 21
21
EBEB
CBCB
V
VV
VV
A




29. Transistor Configurations
•CB Output characteristics
–A plot of I
Cversus V
CBfor various values of I
E
–Three regions are identified: Active, Cutoff, Saturation
–Active region:
•E-B junction forward biased
•C-B junction reverse biased
•I
Cis positive, V
CBis positive
•I
Cincreases with I
E
•For given I
E, I
Cis almost constant; increases only
slightly with increase in V
CB. This is due to base-width
modulation (Early effect)

30. Transistor Configurations
CB Output characteristics

31. Transistor Configurations
•Base Width modulation
–As the reverse bias voltage V
CBis increased, the depletion
region width at the C-B junction increases. Part of this
depletion region lies in the base layer. So, effective base
width decreases. Hence number of electron-hole
combination at the base decreases. So base current reduces
and collector current increases.
–Note that I
E= I
C+ I
B

32. Transistor Configurations
•When I
E= 0, I
C= I
CBO
–I
CBOis collector to base current with emitter open
–Below this line we have cut-off region
–Here both junctions are reverse biased
•Region to the left of y-axis (V
CBnegative) is saturation region
–Here both junctions are forward biased
–I
Cdecreases exponentially, and eventually changes
direction

33. Transistor Configurations
•Both can be measured from output characteristicsconstIwith
I
V
r
E
C
CB
O



•Output resistance r
o
•Current amplification factor A
Iorα
acconstVwith
I
I
CB
E
C
ac




34. Transistor Configurations
•Common Emitter configuration
(Resistors are omitted for simplicity)
•Emitter is common between input and output
–Input voltage: V
BE Input current: I
B
–Output voltage: V
CE Output current: I
C

35. Transistor Configurations
CE input characteristics
•Plot of I
Bversus V
BEfor
various values of V
CE
•Similar to diode
characteristics
•As V
CEis increased, I
B
decreases only slightly
•This is due to base-width
modulation
•Note that second suffix is E
(for emitter)

36. Transistor Configurations
•CE output characteristics
–A plot of I
Cversus V
CEfor various values of I
B
–Three regions identified: Active, Cut-off, Saturation
–Active region:
•Linear region in the output characteristics
•E-B junction forward biased
•C-B junction reverse biased
•I
Cincreases with I
B
•For given I
B, I
Cincreases slightly with increase in V
CE;
this is due to base-width modulation (Early effect)

37. Transistor Configurations
CE output characteristics

38. Transistor Configurations
•Note that V
CE= V
CB+ V
BE
•So if V
CEis increased, effectively V
CBalso increases
•For saturation to take place, C-B junction should be forward
biased.
•This happens when V
CEis approximately 0.3 V (or less) for Si
•Note that when V
CE= 0.3V, and V
BE= 0.6 V, V
CB= –0.3V (a
forward bias of 0.3 V)
•So region to the left of the vertical line V
CE=V
CE(sat)=0.3V (for
silicon) is considered as saturation region
•Region below I
B=0 line (or I
C=I
CEO) is cut-off region

39. Transistor Configurations
•I
CEOis much larger than I
CBObecause of the relation:dc
CBO
CEO
I
I


1
•Note that value of α
dcis around 0.99
•The values of α
dc& α
ac, and β
dc& β
acare almost the same.
Hence the subscripts can be omitted for simplicity

40. Transistor Configurations
•Input resistance r
i
•Voltage gain A
V
•Output resistance r
o
•Current gain A
Iorβ
acconstVwith
I
V
r
CE
B
BE
i 


 constIwith
V
V
A
B
BE
CE
V 


 constIwith
I
V
r
B
C
CE
o 


 constVwith
I
I
CE
B
C
ac 



•All these parameters can be determined from CE characteristics

41. Transistor Configurations
•Experimental
setup for
determining CE
characteristics:

42. Tutorials
1.A Ge transistor with β= 100 has collector-to-base leakage
current of 5 µA. If the transistor is connected in common-
emitter operation, find the collector current for base current
(a) 0 (b) 40 µA.
Sol:Given that I
CBO= 5µA, and β= 100
When I
B= 0, I
C= I
CEO= (β+1)I
CBO= 505 µA
When I
B= 40 µA, I
C= βI
B+ I
CEO
= (100 ×40 ×10
–6
) + (505 ×10
–6
)
= 4.505 mA

43. Tutorials
2.A Ge Transistor has collector current of 51 mA when the base
current is 0.4 mA. If β= 125, then what is its collector cutoff
current I
CEO?
(Ans: 1 mA)
3.In a transistor circuit, when the base current is increased from
0.32 mA to 0.48 mA, the emitter current increases from 15
mA to 20 mA. Find α
acand β
acvalues.
(Ans: 0.968, 30.25)
4.A transistor withα= 0.98 and I
CBO= 5 µA has I
B= 100 µA.
Find I
Cand I
E.
(Ans: 5.15 mA, 5.25 mA)

44. Transistor Biasing
•Oneofthemostcommonapplicationsoftransistorisin
amplifiers.
•E-Bjunctionshouldbeforwardbiased;C-Bjunction
shouldbereversebiased(activeregion)
•Forfaithfulamplificationwerequirethattransistorbe
operatedinactiveregionthroughoutthedurationofinput
signal.
•Toensurethis,properdcvoltagesshouldbeapplied.
ThisiscalledBiasing.

45. Operating point
Whennoinputsignalisappliedtotransistorcircuit,and
onlydcvoltagesaresupplied,currentsI
C,I
Bandvoltage
V
CEwillhavecertainvalues.
Ifthesevaluesareplottedoverthetransistoroutput
characteristics,thepointwegetiscalled‘Operatingpoint’.
Itisalsocalled‘Quiescentpoint’orjustQ-point.

46. •Inabovefigure,currentsI
BQ,I
CQandvoltageV
CEQare
plottedaspointQ.Inpractice,wehavetochooseQ-point
accordingtoourrequirement.Ifwewanttooperateinthe
middleofactiveregion,wemaychooseQasQ-point.

47. •Ifwewanttooperatenearsaturation,wemaychoose
Q’asQ-point.
•Ifwewanttooperatenearcutoff,wemaychooseQ’’as
Q-point.
•Notethatifnobiasingisused,Q-pointwillbeinthe
originofgraph.
•So,biasingisusedtofixtheQ-pointaccordingtoour
need.

48. Type of Biasing in Transistor
Fixed bias
Voltage divider bias

49. Fixed Bias
•BaseresistorR
BisconnectedtoV
cc(InsteadofV
BB).
NegativeterminalofV
ccisnotshown.Itisassumedto
beatground.
•ApplyingKVLtotheinput,B
BECC
B
R
VV
I



50. V
ccis constant, V
BEis almost constant (0.7V for silicon). So
by selecting proper R
B, we can fix I
Bas required.
Applying KVL to output side we get:
•I
Cis related to I
Bby β
•So, V
CEcan be fixed by selecting proper R
C.

51. Load Line
We have,
This is an equation of straight line with V
CEand I
Cas two
variables. This straight line is called load line. Now, output
characteristic is also a function of same two variables.
IfR
BandR
CareheldconstantandV
CCisvaried,thenload
lineshifts,maintainingsameslope.

53. •IfR
BandR
CareheldconstantandV
CCisvaried,then
loadlineshifts,maintainingsameslope.Fromthese
graphsweinferthat:
•witheverythingelseheldconstant,ifR
Bisincreased,
transistorgoestowardscutoff.
•ifR
Bisdecreased,transistorgoestowardssaturation.

54. •Witheverythingelseheldconstant,ifR
Cisincreased,
transistorgoestowardssaturation.
•ifR
Cisdecreased,transistorgoestowardsactive
region.
•Witheverythingelseheldconstant,ifV
CCisincreased,
transistorgoestowardsactiveregion.
•ifV
CCisdecreased,transistorgoestowardssaturation.

55. Advantagesoffixedbias
•Simpletoanalyzeanddesign
•Usesveryfewcircuitcomponents
Disadvantagesoffixedbias
•Q-pointisnotstable.i.e.,iftemperaturevaries,βwill
vary,henceI
Cwillvary.
•Iftransistorisreplacedbyanothertransistorhaving
differentβthenQ-pointwillshift.

56. Voltage divider bias or Self bias
•UsestworesistorsR
1andR
2insteadofR
B.R
Eis
connectedbetweenemitterandground.

57. •Inputsideoftheabovecircuitisredrawnbelow,

58. •V
THistheopencircuitvoltagebetweenpointsA&Bin
fig(a)givenby,

59. •R
THis the resistance between A & B with V
CCreplaced
by short circuit.
•Applying KVL to the input loop,
Substituting and rearranging, we get

60. •Applying KVL to the output loop, we get
Rearranging, we get
Also,
Where, V
Cis voltage from collector to ground.

61. •Also,
Where, V
Eis voltage from emitter to ground.
Sinceβ>>1,wehave(β+1)≈β.IfβR
E>>R
TH,then
equationofI
Breducesto,
Now,

62. •SinceequationforI
Cdoesnotcontainβ,wesaythatI
Cis
independentoftemperaturevariationandtransistor
replacement.
Advantagesofvoltagedividerbias
.Q-pointisstableagainstvariationintemperatureand
replacementoftransistor.
Disadvantagesofvoltagedividerbias
•Analysisanddesignarecomplex
•Morecircuitcomponentsrequired

63. Problem
1.DrawtheDCloadlineandmarktheQ-pointonfixedbiascct.
AssumeBetaDC=100andneglectBase–Emittervoltage.
(V
cc=30V,R
B=1.5Mohm,R
C=5Kohm).
Ans:V
CE,max=30v,V
CEQ=20v,I
CQ=2mA
2.Inafixedbiascct.Findthebasecurrentrequiredtoestablish
V
CE=6v,alsofindR
B&I
E,(V
BE=0.7v,BetaDC=120,
V
CC=12v,R
C=2.2.Kohm).
Ans:I
B=22.75uA,R
B=497Kohm
3.Determinetheregioninwhichthetransistoroperates.
(V
BE=0.2v,R
B=120kohm,R
C=1kohm,V
CC=15v,BetaDC=120).

64. End of BJTs: Junction
Transistor characteristics