Bipolar Junction Transistors
(BJTs)
Lecture # 6
1
BJT at a Glance
The most important
p
use of BJT is under severe environment
condition for reliability.
Also, in analogue applications still BJT is the best in business
which is true for high frequency applications, radio frequency for
wireless systems.
Emitter Coupled logic is also based on BJT.
BJT has been combined with MOSFET to have some innovative
circuits, the resulting technology is called BiMOS or BiCMOS.
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2
1
npn Transistor
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3
pnp Transistor
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4
2
BJT Modes
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse
Reverse Active
Reverse
Forward
Saturation
Forward
Forward
If a transistor is to be used a an amplifier it has to be operated in active
mode which is also called forward active mode
mode..
Switching applications (logic circuits) use cutoff and saturation mode
mode..
The reverse active also called inverse active is also used but in very
limited applications
applications..
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5
Active Mode
Current in BJT is consists of two components Electrons (injected from emitter to base) &
Holes (from base to emitter) that is why it is called BJT.
Base is normally very thin and less doped (lightly doped) as compared to emitter which is
very heavily doped. The emitter current is due to these two carriers but as electrons are high
in numbers due to doping it will be dominated by electron components.
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6
3
Concentration of Carriers in Base
iC  I S e v BE / vT where I S 
AE qD n n i2
N AW
iC  i B 
iB 
iC

I 
i E  i B  iC and i E   S  e v BE / vT
 
iC    i E

 1

 
1
 
n p ( 0 )  n p 0 e v BE / vT
α is called Common Base Current Gain and β is called Common Emitter Current Gain.
Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in
the active mode: vBE  0 and vCB  0.
7
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Large Signal Equivalent Circuit Models
Collector current is independent of
collector voltage as long as vCB ≥0.
Therefore, in active mode it behaves as
an ideal current source which is
controlled by vBE .
iE 
IS

 F iE  I S e v
BE
/ vT
e v BE / vT
Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.
It is a non-linear voltage controlled current source model. It can be converted into a Current Controlled
current source model as in (b) still it is non-linear due to the diode.
The current in collector is controlled by vBE (exponential term, non-linearity).
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4
Exercise 5.2, 5.3, 5.4 & 5.5
5.1 Consider an npn transistor with vBE = 0.7 V at iC = 1mA. Find vBE at iC = 0.1 mA and 10 mA.
Ans. 0.64 V; 0.76 V
 F i SE   R I SC ,
F
R
 , R 
1R
R
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9
Transistor Structure
It is not a symmetrical device, as collector surrounds the emitter area so it is not possible
for electrons to escape which are injected into the base so all of them are collected, that is
why beta is large and alpha is close to unity.
Therefore, if emitter and collector are interchanged and it is operated in the reverse active
mode the value of beta and alpha will be different from active mode values.
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5
Reverse Active Circuit Model
 F I SE   R I SC  I S
 F is close to unity and  F is large.
 R is in the range 0.01 - 0.5 ,  R is in the range 0.01 - 1.
Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased).
11
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Ebers--Moll Circuit Model
Ebers
iC   i DC   F i DE
i B  (1   F ) i DE  (1   R ) i DC
i E  i DE   R i DC
where
i DE  I SE ( e v BE / vT  1)
i DC  I SC ( e v BC / vT  1)
iC  I S e v BE
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VT
 1

 I S 
 1 
R

 I
i B   S
 F
 v BE
 e

 I
i E   S
F
 v BE
 e

VT
VT
 1
1 

 I S 

 F R 

1
 I S  1 
F




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6
iC – vCB Characteristics
To be operated in the active mode,
mode VCB has to be greater or equal to zero to ensure that CBJ is reversed
biased, we know that a pn junction cannot be forward biased until the voltage is 0.5 volts, so this means
that CBJ will remain reversed biased until -0.4 volts. After that it will enter into saturation and the current
will change.
The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters
the saturation mode of operation for vCB  –0.4 V, and the collector current diminishes.
13
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Electron Concentration in Saturation Mode
iC  I S e v BE / vT 
IS
R
e v BC / vT
We have shown earlier that concentration of electrons in the base is almost zero at the collector end of the base, infact things
changes
h
d
dramatically
i ll in
i saturation
i that
h if the
h collector
ll
bbase junction
j
i is
i forward
f
d biased
bi d (VCB negative)
i ) then
h the
h collector
ll
current
reduces.
When BJT goes to saturation the second term in the above expression becomes larger so more amount is subtracted so IC reduces,
the fact is that when the collector base junction is forward biased, the concentration of electrons is no more zero near the base
collectore junction so the slope changes (less) so causing reduction in the collector current.
Saturation mode of BJT is completely different from the MOSFET, for BJT it is equal to the triode region of the characteristics,
where as the saturation region of MOSFET corresponds to the active region of BJT.
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7
Exercise 5.6
 1

 I S 
 1 
R

 v BE VT
 1
1
 e
 I S 


 F R
 v BE VT

1 

 I S  1 
 e
 F 


iC  I S e v BE
 I
i B   S
 F
 I
i E   S
F
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VT



15
pnp Transistor
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16
8
pnp Transistor Circuit Model
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17
npn & pnp Transistor Symbols
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9
Exercise 5.8 & 5.9
IC  I E
I C  I S e v BE
VT
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Voltage Polarities in the Active Mode
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10
Summary: Current Voltage Relationships
i C  I S e v BE
 I
  S
 
iC
 I

  S

 
iB 
iE
/VT
iC

 v BE
 e

 v BE
e

/VT
/VT
VT is thermal voltage, which is

KT
 0.25 at room temperature.
q
iC   i E
i B  (1   ) i E 
iE
 1
iE  (   1)iB
 

1 
 
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
 1
21
Example 5.1
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11
Exercise 5.10
IC  I E
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Exercise 5.11
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12
iC –vBE Characteristic npn Transistor.
iC  I S e vBE / VT
For small value of base emitter voltage the current is small up to 0.5 V, over most of the
normal current range base emitter voltage changes between 0.6 to 0.8 V, the constant of
the exponent is quite high (1/VT =40) so the current rise sharply.
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Temperature Effect
For
rise
across emitter
base
junction
F 1 degree
d
i in
i temperature the
h voltage
l
i
b
j
i
decreases approximately by 2 mV.
Which is shown for 3 different temperatures.
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13
Common Base Characteristics
Base held Constant
So common
terminal for input
& output
The curves deviates in the active region in two ways, curves are not horizontal showing that iC is slightly
dependent on vCB. Secondly, the collector current shows a rapid increase (basically breakdown
phenomenon) at a large value of vCB. One can determine the value of alpha (incremental or small signal
alpha) by measuring change in collector current obtained as a result of change in emitter current.
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Exercise 5.12 & 5.13
W know
We
k
that
h for
f specific
ifi value
l off current base
b
emitter
i
voltage
l
change
h
b -22 mV/C
by
V/C0 .
vBE 
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14
Dependence of IC on Collector Voltage
iC  I S e vBE / vT
For a given base emitter voltage which can be adjusted we obtain IC – VCE characteristics at each point by
i the
th collector
ll t emitter
itt voltage.
lt
B i ll when
h we increase
i
th VCE for
f a given
i
th reverse bias
bi
varying
Basically,
the
VBE the
at collector base increases which in turn increases the width of the depletion region, and we know that
saturation current is inversely proportional to the width.
Therefore, saturation current increases which is directly proportional to collector current so that increases
and we see this slope increases, it is called early effect discovered by J. M. Early on his name. We extend
the slope we see that it meets at a point VA which is a parameter for the transistor in the range 50 – 100.
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Equivalent Circuit Models
The slight slope in the characteristics shows that the output resistance looking into the
collector is not infinite,, rather it is finite,, so we can include that in the circuit model,, it
should be noted that dependence of IC on VCE is rarely considered in dc bias design analysis,
however, the finite resistance can have significant effect on the gain of a transistor
amplifier.
The above models differ in a sense that (a) is a voltage controlled current source where as
(b) is a current controlled current source as IB is controlling things.
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15
Exercise 5.14 & 5.15
r0 
VA
IC
VC
 r0
I C
I C 
VC
r0
I C  I C  I C
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Common Emitter Characteristics
 DC 
 AC
I CQ
I BQ
i
 C
iB
vCE  cons tan t
Another alternative for common emitter configuration is to use base current rather
then the base emitter voltage parameter. The magnitude of beta for ac and dc
differ by 10% to 20%.
The characteristics are the same however, here breakdown occurs.
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16
β Dependence on IC
Typical dependence of  on IC and on temperature in a modern integrated-circuit
npn silicon transistor intended for operation around 1 mA.
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Expanded View
An expanded view of the common-emitter characteristics in the saturation region.
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17
Discussion
The steep slope indicate that the saturated transistor shows
low collector to emitter resistance.
VCEsat  VCEoff  I Csat RCEsat
R
Range
0 1 – 0.3
0.1
03V
(a) An npn transistor operated in saturation mode with a constant base current IB. (b) The iC–vCE characteristic curve corresponding to iB = IB. The curve can
be approximated by a straight line of slope 1/RCEsat. (c) Equivalent-circuit representation of the saturated transistor. (d) A simplified equivalent-circuit
model of the saturated transistor.
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Transistor Breakdown
The maximum voltages that can be applied to EBJ and CBJ are limited by the avalanche multiplication
mechanism discussed earlier.
For the common base configuration, BVCB0 is breakdown voltage at IE = 0 (Emitter open circuit), when IE is
greater than zero breakdown will occur at lower voltage typically, it is greater than 50 V.
For common emitter configuration it is BVCE0 , it is also known as sustaining voltage LVCE0 given by
manufacturer.
The Breakdown of CBJ either in common base or common emitter configuration is not destructive as long as
the power dissipation is kept in the safe limit, this is however, not the case for EBJ. The EBJ breaks down at a
voltage BVEB0 much smaller than BVCB0 , BVEB0 is in that range 6 – 8 V. The breakdown is destructive in the
sense that beta of the transistor permanently destroyed.
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18
Exercise 5.16, 5.17 & 5.18
Slope 
I C
1
, RCEsat 
VCE
Slope
VCEsat  VCEoff  I Csat RCEsat
5.18 What is the output voltage of the following circuit, if the transistor VBC0 = 70 V ?
Ans. -60 V
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