Lecture 17
Bipolar Junction Transistors (BJT): Part 1
Qualitative Understanding - How do they work?
Reading:
Pierret 10.1-10.6, 11.1
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ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Looks sort of
like two diodes
back to back
pnp mnemonic:
“Pouring ‘N’ Pot”
npn mnemonic:
“Not Pouring ‘N’ ”
Voltage Nomenclature Standard V+Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Georgia Tech
Collector “collects” electrons
emitted by the emitter
Narrow Base controls number
of electrons emitted
Emitter “emits” electrons
Collector “collects” holes
emitted by the emitter
Narrow Base controls
number of holes emitted
Emitter “emits” holes
Bipolar Junction Transistor Fundamentals
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
I E = I B + IC
V EB + VBC + VCE = 0
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ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Both the
input and
output
share the
base “in
common”
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Both the
input and
output
share the
emitter “in
common”
Both the
input and
output
share the
Collector
“in
common”
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
•Active: Is useful for amplifiers. Most common mode
•Saturation: Equivalent to an on state when transistor is used
as a switch
•Cutoff : Equivalent to an off state when transistor is used as
a switch
•Inverted: Rarely if ever used.
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Saturation
Equilibrium
Base
Base
Collector
Emitter
Collector
Emitter
Forward
Biased
Cutoff
Forward
Biased
Active (or Forward Active)
Emitter
Base
Collector
Accelerated
by the
Electric
Field
Base
Forward
Biased
Emitter
Reverse
Biased
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Reverse
Biased
Collector
Reversed
Biased
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
•Operational modes can be
defined based on base-emitter
voltages and base-collector
voltages
•When there is no base current,
almost no collector current
flows
•When base current flows, a
collector current can flow
•The device is then a current
controlled current device
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals: Electrostatics in Equilibrium
Emitter Doping >Base Doping>Collector Doping
Emitter is heavily doped
W=width of the base
quasi-neutral region
WB=Total Base width
WEB=Base-Emitter
depletion width
WCB=Base-Collector
depletion width
WEB< WCB
Base-Emitter
built in voltage
Base-Collector
built in voltage
Note: This slide refers to a pnp transistor
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Equilibrium
Many Holes injected into the base
Narrow Base
required to
minimize
recombination
Forward Biased
Few electrons injected into the emitter
Reverse Biased
Active Mode
Injected Holes diffuse through the
base and are collected by the huge
electric field at the collector
Note: This slide refers to a pnp transistor
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Note: Subscript indicates
emitter/collector current (E/C) and
hole/electron contribution (p/n)
I E = I Ep + I En
Neglecting
recombinationgeneration means
ICp~=IEp
I C = I Cp + I Cn
Since emitter is more heavily doped than the base, IEn<<IEp
Since the base-collector junction is reverse biased, ICn<<Icp
IC ~=IE and (IB= IE - IC ) is small compared to IC and IE
Note: This slide refers to a pnp transistor
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals
Consider a pnp Transistor: A small electron base current (flowing
into the emitter from the base) controls a larger hole current
flowing from emitter to collector. Effectively, we can have the
collector-emitter current controlled by the base-emitter current.
Note: This slide refers to a pnp transistor
Georgia Tech
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals:
Performance Parameters
(1) γ
=
(2) α T
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I Ep
IE
=
=
I Cp
I Ep
I Ep
I Ep + I En
Emitter Efficiency: Characterizes how
effective the large hole current is controlled
by the small electron current. Unity is best,
zero is worst.
Base Transport Factor: Characterizes how
much of the injected hole current is lost to
recombination in the base. Unity is best, zero
is worst.
Note: This
slide3040
refers
to aAlan
pnp transistor
ECE
- Dr.
Doolittle
Bipolar Junction Transistor Fundamentals:
Performance Parameters
Active Mode, Common Base Characteristics
Forward
Biased
Reverse
Biased
ICBo is defined as the
collector current when the
emitter is open circuited.
It is the Collector-base
ICBo
junction saturation
current.
Reverse
Biased
IC=fraction of emitter current making it across the base + leakage current
(3)
I C = α dc I E + I CBo
where αdc is the common base DC current gain
Combining (1) and (2),
I Cp = α T I Ep = γα T I E
I C = I Cp + I Cn = α T I Ep + I Cn = γα T I E + I Cn
Thus comparing this to (3),
α dc = γα T
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and
I CBo = I Cn
Note: This slide refers to a pnp transistor
ECE 3040 - Dr. Alan Doolittle
Bipolar Junction Transistor Fundamentals:
Performance Parameters
Reverse Active Mode, Common Emitter Characteristics
Biased
ICEo is defined as the
collector current when the
base is open circuited.
Forward
Biased
IC=multiple of the base current making it across the base + leakage current
(4)
I C = β dc I B + I CEo where βdc is the common emitter DC current gain
But using IE=IC+IB in (2), (5) I C = α dc (I C + I B ) + I CBo
and solving for I C
(6)
IC =
I
α dc
I B + CBo
1 − α dc
1 − α dc
comparing (4) and (6)
β dc =
Note: This slide refers to a pnp transistor
Georgia Tech
α dc
1 − α dc
and
I CEo =
I CBo
1 − α dc
and
β dc =
IC
IB
ECE 3040 - Dr. Alan Doolittle