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Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
1
Chapter #6: Bipolar Junction Transistors
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
2
Introduction
• IN THIS CHAPTER YOU WILL LEARN• The physical structure of the bipolar transistor and how it works.
• 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.
• How to analyze and design circuits that contain bipolar transistors, resistors, and dc sources.
• How the transistor can be used to make an amplifier.
• How to obtain linear amplification from the fundamentally nonlinear BJT.
• The three basic ways for connecting a BJT to be able to construct amplifiers with different properties.
• Practical circuits for bipolar-transistor amplifiers that can be constructed by using discrete components.
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Introduction
• This chapter examines another three-terminal device.
• bipolar junction transistor
• Presentation of this material mirrors chapter 5.
• Three-terminal device• Multitude of applications
• Signal amplification/Digital logic/Memory circuit/Switch
• Voltage between two terminals to control the current flowing in third terminal
• 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.
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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4.1. Device Structure and Physical Operation
• Figure 4.1. shows simplified structure of BJT
• Consists of three semiconductor regions:
• emitter region (n-type)
• base region (p-type)
• collector region (n-type)
• Type described above is referred to as npn
• However, pnp types do exist
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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4.1.1. Simplified Structure and Modes of Operation
• Transistor consists of two pn-junctions:
• emitter-base junction (EBJ)
• collector-base junction (CBJ)
• Operating mode depends on biasing
• active mode – used for amplification
• cutoff and saturation modes – used for switching
• Bipolar(electron and hole) participate in conduction
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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4.1.2. Operation of the npn-Transistor in the Active Mode
• Active mode is “most important”
• Two external voltage sources are required for biasing to achieve it
• Refer to Figure 4.3
Figure 4.3: 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.)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Current Flow
• Forward bias on emitter-base junction will cause current to flow
• This current has two components:
• electrons injected from emitter into base
• holes injected from base into emitter
• It will be shown that first (of the two above) is desirable
• This is achieved with heavy doping of emitter, light doping of base
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Current Flow
• 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
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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0
0
( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier (elect
/
ron) concentration in base
0
reg
(eq6.1) 0 BE
p
p
Tv
nn
Vp
x
n e
pn
pn
0
0
0
ionvoltage applied across base-emitter junction
thermal voltage (constant)
p
pBE
pT
nn
nv
V
Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)
Straight line represents constant gradient
Figure 4.4
(eq4.1)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Current Flow
• Concentration of minority carrier np at boundary EBJ is defined by (4.1)
• Concentration of minority carriers np at boundary of CBJ is zero• Positive vCB causes these electrons to be swept across junction
0
0
( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier
/
(electron) concentration in ase
0
b
(eq6 0.1)
p
p
BE T
x
n
v V
n
pn e
pn
pn
0
0
0
regionvoltage applied across base-emitter junction
thermal voltage (constant)
p
pE
p
B
T
nn
nv
V
(eq4.1)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Current Flow
• Tapered minority-carrierconcentration profile exists
• It causes electrons injected into base to diffusethrough base toward collector
• As such, electron diffusion current (In) exists.
cross-sectiona area of the base-emitter junction magnitude of the electr
this simplificationmay be made if
gradient assumedto be straight line
(e
(eq6.2)
q6.2)
0
E
p
n E n
p
E n
Aq
n
dn xI A qD
dx
dnAI qD
W
on charge electron diffusivity in base
width of basenD
W
(eq4.2)
(eq4.2)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Current Flow
• Some “diffusing” electrons will combine with holes(majority carriers in base)
• Since base is very thin and lightly doped, recombination is minimal
• Recombination does, however, cause gradient to take slightly curved shape
• The straight line is assumed
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
13
0
0
( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier (elect
/
ron) concentration in base
0
reg
(eq6.1) 0 BE
p
p
Tv
nn
Vp
x
n e
pn
pn
0
0
0
ionvoltage applied across base-emitter junction
thermal voltage (constant)
p
pBE
pT
nn
nv
V
Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)
Recombination causes actual gradient to be curved, not straight.
Figure 4.4
(eq4.1)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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The Collector Current
• It is observed that most diffusing electrons will reach boundary of collector-basedepletion region
• Because collector is more positive than base, these electrons are swept into collector• collector current (iC) is
approximately equal to In
• iC = In intrinsic carrier density doping concentration of base
/
0
2
(eq6.3)
saturation current:
(eq6.4)
BE Tv VC S
E
n
n p
S
E n iS
A
iNA
i I
A qD nI
W
A qD nI
W N
e(eq4.3)
(eq4.4)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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The Collector Current
• 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
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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The Base Current
• base current (iB) – composed of two components:
• ib1 – due to holes injected from base region into emitter
• ib2 – due to holes that have to be supplied by external circuit to replace those recombined
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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The Base Current
• common-emitter current gain(b.) – is influenced by two factors:
• width of base region (W)
• relative doping of base emitter regions (NA/ND)
• High Value of b 50~200, >1000
• thin base (small W in nano-meters)
• lightly doped base / heavily doped emitter (small NA/ND)
transistor parameter
/
(eq6.5)
(eq6.6) BE T
CB
v VSB
ii
Ii
b
b
b
e
(eq4.5)
(eq4.6)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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The Emitter Current
• All current which enters transistor must leave
• iE = iC + iB• Equations (4.7)
through (4.13) expand upon this idea
• α : common-base current gain (less than but very close to unity)
this expression is generated through combination of (6.5) and (6.7)
/(eq6.8/6.9)
(eq6.10)
(eq6.11
1
)
1
BE T
C
v VE C S
C E
i
i i I
i i
b b
b b
b
b
e
this parameter is reffered
/
toas
(eq6.13)
(eq6.1
1 1
2)
,
BE Tv VSE
Ii
b
common-base current gain
e
(eq4.8/4.9)
(eq4.10)
(eq4.11)
(eq4.12)
(4.5) and (4.7)
(eq4.13)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Recapitulation and Equivalent-Circuit Models
• Previous slides present first-order BJT model.
• Assumes npn transistor in active mode
• Basic relationship is collector current (iC) is related exponentially to forward-bias voltage (vBE)
• It remains independent of vCB as long as this junction remains reverse biased
• vCB > 0
• iB is much smaller than iC• Nonlinear voltage-controlled current source
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Figure 4.5: Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.
Common base model
Common emitter model