Course Outline
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1. Chapter 1: Signals and Amplifiers
2. Chapter 3: Semiconductors
3. Chapter 4: Diodes
4. Chapter 5: MOS Field Effect Transistors (MOSFET)
5. Chapter 6: Bipolar Junction Transistors (BJT)
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Chapter 6:
BJTs
Part I
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Learning Objectives
1) The physical structure of the bipolar transistor and how it
works.
2) How the voltage between two terminals of the transistor
controls the current that flows through the third terminal,
and the equations that describe these current-voltage
relationships.
3) How to analyze and design circuits that contain bipolar
transistors, resistors, and dc sources.
4) How the transistor can be used to make an amplifier
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Introduction
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 BJT (Bipolar Junction Transistor)
 bipolar junction transistor
 BJT was invented in 1948 at Bell Telephone Laboratories
 Ushered in a new era of solid-state circuits
 It was replaced by MOSFET as predominant transistor
used in modern electronics
 Still used today in Emitter-Coupled Logic (ECL): very fast
logic integrated circuits
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Comparison: MOSFET and BJT
 BJTs are more robust: can
operate in severe
environmental conditions
 Popular in discrete design
 Good performance at very high
frequencies (GHz range*)
 High current driving capability
 BJTs are not symmetrical: EBJ
(emitter to base junction) is
fixed and can only be operated
in specific way
 BJTs have lower 1/f (flicker
noise) but higher thermal noise
* These devices are called Heterojunction Bipolar Transistors (HBT); they have high
 MOSFETs have to be tailored
for harsh environments: not
common
 Popular in IC design
 Significant parasitic effect at
very high frequencies
 Low current driving capability
 MOSFETs are symmetrical:
source and drain are
interchangeable
 MOSFETs have higher 1/f
(flicker noise) but lower
thermal noise
gain and low noise at frequencies in the order of several hundred GHz
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6.1. Device Structure and Physical Operation
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 Figure 6.1. shows simplified structure of BJT
 BJT consists of three semiconductor regions:
 emitter region (n-type)
 base region (p-type)
 collector region (n-type)
 Operating mode depends on biasing
 active mode – used for amplification
 cutoff and saturation modes – used for switching
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6.1. Device Structure and Physical Operation
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NPN
PNP
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Mode of Operation
Amplifier
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse
Saturation
Forward
Forward
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6.1.2. Operation of the npn-Transistor in the Active Mode
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 Active mode is most
important
 Two external
voltage sources are
required for biasing
to achieve it
Figure 6.3: Current flow in an npn transistor biased to operate in the active mode.
(Reverse current components due to drift of thermally generated minority carriers
are not shown.)
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Current Flow
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 Forward bias on emitter-base junction will cause current to
flow
 This current has two components:
 1) electrons injected from emitter into base
 2) holes injected from base into emitter.
 First of the two (e flow from E to B) is desirable.
 This is achieved with heavy doping of emitter, light doping
of base.
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Current Flow
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 Emitter current (iE) – is current which flows across EBJ
 Flows “out” of emitter lead
 Minority carriers – in p-type region.
 These electrons will be injected from emitter into base.
 Opposite direction
 Because base is thin, concentration of excess minority carriers
within it will exhibit constant gradient
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6.1.3. Structure of Actual Transistors
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 Collector virtually surrounds entire emitter region.
 This makes it difficult for electrons injected into base to
escape collection.
 Device is not symmetrical.
 As such, emitter and collector cannot be interchanged.
 Device is uni-directional.
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np ( x )concentration of minority carriers a position x (where 0 represents EBJ boundary)np 0
np 0  thermal-equilibrium value of minority carrier (electron) concentration in base regionnp 0
vBE voltage applied across base-emitter junctionnp 0
VT thermal voltage (constant)np 0

(eq6.1) np  0   np 0 evBE / VT
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Straight line
represents
constant
gradient.
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Current Flow
 Concentration of minority
carrier np at boundary EBJ is
defined by (6.1).
 Concentration of minority
carriers np at boundary of CBJ
is zero.
 Positive vCB causes these
electrons to be swept
across junction
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np ( x )concentration of minority carriers a position
x (where 0 represents EBJ boundary)np 0
np 0  thermal-equilibrium value of minority carrier
(electron) concentration in base regionnp 0
vBE voltage applied across base-emitter junctionnp 0
VT thermal voltage (constant)np 0

(eq6.1) np  0   np 0 evBE / VT
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Current Flow
 Tapered minority-carrier
concentration profile exists.
 It causes electrons injected
into base to diffuse through
base toward collector.
 As such, electron diffusion
current (In) exists.
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AE cross-sectiona area of the base-emitter junction
q  magnitude of the electron charge
Dn  electron diffusivity in base
W  width of base


dnp  x 
(eq6.2) In  AE qDn
dx
 dnp  0  
(eq6.2) In  AE qDn 

W

this simplification
may be made if
gradient assumed
to be straight line
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Current Flow
 Some diffusing electrons will combine with holes (majority
carriers in base): since Base is thin, however, and
recombination is minimal
 Recombination does, however, cause gradient to take slightly
curved shape
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The Collector Current
 It is observed that most
diffusing electrons will reach
boundary of collector-base
(CBJ) depletion region.
 Because collector is more
positive than base, these
electrons are swept into
collector (even though
concentration profile reduces)
 Collector current (iC) is
approximately equal to In.
 iC = In
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(eq6.3) iC  IS evBE / VT

AE qDn np 0
saturation current: IS 
W

AE qDn ni2
(eq6.4) IS 
W NA

ni  intrinsic carrier density
NA doping concentration of base
AE = Cross sectional area of EBJ
q = electron charge
Dn = electron diffusivity into base
W = width of base
ni = intrinsic carrier density
NA = doping concentration in the base
= number of acceptor atoms=number of holes
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The Collector Current
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 Magnitude of iC is independent of vCB
 As long as collector is positive, with respect to base
 Saturation current (IS) – is inversely proportional to W
and directly proportional to area of EBJ
 Typically between 10-12 and 10-18A
 Also referred to as scale current
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The Base Current
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 Base current (iB) – composed of two
components:
  transistor parameter



 iB1 – due to holes injected from base region
i
into emitter.
(eq6.5) iB  C

 iB2 – due to holes that have to be supplied by
external circuit to replace those recombined.            
 Common-emitter current gain () – is
IS v / V
(eq6.6) iB  e
influenced by two factors:

 width of base region (W)
 relative doping of base emitter regions
(NA/ND)
 High Value of 
 thin base (small W in nano-meters)
 lightly doped base / heavily doped emitter
(small NA/ND)
BE
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The Emitter Current
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this expression is generated through combination of (6.5) and (6.7)

 1
 1
(eq6.8/6.9) iE 
iC 
IS evBE / VT 



 
 All current which
enters transistor
must leave.
 iE = iC + iB
iC

(eq6.10) iC   iE

this parameter is reffered to
as common-base current gain



(eq6.11)  

 1
, (eq6.13)  

1 

I
(eq6.12) iE  S evBE / VT

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Large Signal Models: Active Mode
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 Positive vBE causes iC (exponentially
relation ) to flow in the
Collector for vCB > 0
 In active mode, collector behaves
as a constant-current source
 Base current, iB = iC /β
 Emitter current, iE = iC /α ;
α = β/ (β + 1 ) ≈ 1
 Collector current
iC = Is ev /V
BE
T
 Note: α = common-base current
gain and β = common-emitter
current gain
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6.1.5. The pnp Transistor
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pnp: Large Signal Models in Active mode
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6.2.1. Circuit Symbols and Conventions
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Active Mode
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Table 6.2
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6.2.2. Graphical Representation of Transistor Characteristics
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Figure 6.15/16: (left) The iC-vBE characteristic for an npn transistor. (right) Effect
of temperature on the iC-vBE characteristic. Voltage polarities and current flow in
transistors biased in the active mode.
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6.2.3. Dependence of iC on Collector Voltage – The Early Effect
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 When operated in active
region, practical BJT’s
show some dependence
of collector current on
collector voltage.
 As such, iC-vCB
characteristic is not
“straight”.
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Large Signal Models with the Early Effect
Figure 6.18: Large-signal equivalent-circuit models of an npn BJT operating in
the active mode in the common-emitter configuration with the output resistance ro
included.
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Figure 6.19: Common-emitter characteristics. (a) Basic CE circuit; note that in (b) the
horizontal scale is expanded around the origin to show the saturation region in some detail. A
much greater expansion of the saturation region is shown in (c).
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6.1.4. Operation in Saturation Mode
 For BJT to operate in active mode, CBJ must be reverse biased:
active mode operation of npn-transistor may be maintained for vCB
down to approximately - 0.4V
 CBJ will conduct once vCB < - 0.4 V: Saturation
 Saturation Mode: iC reduces, β reduces, iB increases
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6.1.4. Operation in Saturation Mode
ISC 




collector current
(eq6.14)
: iC  IS evBE / VT  ISC evBC / VT



in saturation region
this terms
plays bigger
role as vBC
exceeds 0.4V

base current
IS vBE / VT
(eq6.15)
: iB  e
 ISC evBC / VT
in saturation region


Tunable current ratio
iC
(eq6.16) forced  :  forced 

iB saturation

As vBC is increased, the value of  is forced lower and lower.
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6.1.4. Operation in Saturation Mode
Two tests
⑴Is CBJ forward biased by more than 0.4 V (reverse biased by less
than -0.4 V)?
⑵Is the ratio iC / iB less than β?
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6.1.4. Operation in Saturation Mode
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 Two questions must be asked to determine whether BJT is
in saturation mode, or not:
 Is the CBJ forward-biased by more than 0.4V?
 Is the ratio iC/iB less than ?
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Saturation Mode: simplified equivalent-circuit model
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Exercises 6.1, 6.3, 6.11, D6.12
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