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EE141© 2010, Associate Professor PhD. T.Vasileva
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Bipolar Junction Transistor
Semiconductor
Elements
EE141© 2010, Associate Professor PhD. T.Vasileva
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Transistor Invention – 1947
William Shockley,
Walter Brittain, and John Bardeen
Winners of the 1956 Nobel Prize in Physics
First Point Contact Transistor
AT&T Bell Laboratories
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Advantages
Bipolar transistors effectively replace the huge vacuum tubes.
Their advantages are:
solid body and small size;
low heat generation;
relatively small power requirements.
This makes the miniaturization of complex circuitry possible.
EE141© 2010, Associate Professor PhD. T.Vasileva
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Basic Feature
It is an active semiconductor device - element that supplies the rest of the
circuit with energy. It allows a small signal to control a much larger, high
powered one.
The Bipolar Junction Transistor (BJT) is a solid-state device for amplifying,
controlling, and generating electrical signals.
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Transistor Structure
The bipolar transistor has three regions: the emitter, the base, and
the collector;
The emitter is heavily doped and emits free charge carriers.
The base controls the flow of charges. It is very thin.
The collector collects the charge carriers from the base.
Emitter
Base
Collector
Emitter junction Collector junction
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Types and Schematic Symbols
Two types of bipolar transistors exist - NPN and PNP. Both transistor
types operate the same way but have opposite voltage polarities and
currents directions.
The figure shows the schematic symbols for a transistor and the relation
between the symbol electrodes and the transistor's structure. The arrow
indicates that this electrode is the emitter.
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Transistor's Mode of OperationA transistor's mode of operation depends on the various conditions possible at
both pn junctions. They are caused by the polarity of the applied voltage at
these junctions. Four transistor’s regimes are possible:
Forward-active mode
emmiter junction forward biased
collector junction reverse biased
Cut off mode
emmiter junction reverse biased
collector junction reverse biased
Saturation mode
emmiter junction forward biased
collector junction forward biased
Inverse-active mode
emmiter junction reverse biased
collector junction forward biased
E C
B B
Emmiter
Junction
Voltage
Collector
Junction
Voltage
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There are three transistor configurations depending on which electrode is common
to both the input and output circuits.
Common-base
connection
Common-emitter
connectionCommon-collector
connection
Input Output
Input
Output
Input
Output
The left side of the circuit is called the input circuit, and the right side is called
the output circuit.
The circuit arrangement in the first figure is known as a common-base configuration
because the base is common to both the emitter and collector circuits.
Transistor Connections
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Biased Transistor – CB Connection
The emitter-base junction is forward biased and the collector-base junction
is reverse biased. This mode of operation is called the active mode.
For a PNP transistor, all voltage polarities are the opposite.
The bipolar junction transistor is a normally off device. It needs dc voltages
applied to both pn junctions to start conducting.
EE141© 2010, Associate Professor PhD. T.Vasileva
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Principle of Operation
The principle of operation of the bipolar transistors is based on processes
occurring in two closely spaced and influenced each other pn junctions.
The name "transistor" is derived from "trans resistor", meaning that it can
transfer its internal resistance from low R in the forward biased emitter-base
circuit to a much higher R in the reverse biased collector-base circuit.
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Physical Processes in Emitter
If VEE is greater than the barrier potential, the emitter emits or injects electrons into the
base. Since the emitter is doped more heavily than the base the forward current at the
emitter-base junction is carried primarily by electrons.
IE
InE
Injection coefficient1E
nE
I
I
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Physical Processes in Base&Collector
Electrons entering the P base are the minority carriers in this region. Since the
base is lightly doped and very thin, a very small number of electrons recombine
with holes in the base. Most of the free electrons move on to the collector junction.
They appear as extra minority carriers there and are extracted into the collector by the
reverse collector voltage. As a result, almost all electrons supplied by the emitter flow
in the collector circuit.
InC
1nE
nC
I
I Transfer coefficient
InE
nCC MII
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Transistor Currents
There are three different currents in a transistor:
The emitter current is the largest current because it is the source of free charges.
EnEnCnCC IIMIMMII M
The collector current approximately equals the emitter current but less than it.
The base current is the smallest one. IB is measured in microamperes.
EC II
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Currents Relations in CB Connections
0CBEC III BCE III
10
E
C
E
CBC
I
I
I
II
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Common Emitter Connection
000 )( CBBCCBBCCBEC IIIIIIIII
0CBEC III
BCE III
Principle of operation of BJT does
not depend of it’s circuit connection.
0)1( CBBC III 0
)1(
1
)1(CBBC III
1 0)1( CBBC III
000 )( CBBCCBBCCBEC IIIIIIIII
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Currents Relations in CE Connection
0CBEC III BCE III
0)1( CBBC III 00 )1( CBCE II If IB=0, IC = ICE0
0CEBC III 10
B
C
B
CBC
I
I
I
II
The ratio of the collector current to the base current is called the current gain,
symbolized as βdc or hFE.
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Examples
β = ? IC = ? IE = ?
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The Currect Gain
In response to a small change in the base current a large change in collector
current could appear. This is called current amplification, a very useful
circuit property. Common-emitter configuration provides both current and
voltage amplification and hence has high power amplification factor.
The CE current gain has a high
value since the collector current
is much larger than the base
current.
For low-power transistor, the βdc
is typically 100 to 300.
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The Currect Gain Variation
The current gain has a wide variation with collector current, temperature
change and transistor replacement. Because of manufacturing tolerances,
the current gain of a transistor may vary over as much as 3:1 range with
transistor replacement of the same transistor type.
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Transistor Curents and VoltagesIB
IC
UBE
UEC
BBBEBB RIUE
CCCCCE RIEU
C
CECCC
R
UEI
B
BEBBB
R
UEI
BCE III
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ExamplesIB
IB = ? UCE = ?RC, RB = ?
so that UCE = 7.5V
UCE = ? PC =?PC =?
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VA Characteristics
Uin Uout
Iin Iout
Output
Characteristics
Input
Characteristics
Transfer
Characteristics
Characteristics of
voltage feedback
Iout
Uout
Iin = constUout = const
Iin = constUout = const
Iin
Uin
There are internal relationship
between input and output
voltages and currents in BJT.
Figure shows four families of VA
characteristics. The most important
are output and input characteristics.
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CB – Output Characteristics
Uin = UEBUout = UCB
Iout = IcIin = IEForward-active mode
Saturation
Cut-off (IE = 0, IC = ICB0)
IE = const
IC = f (UCB)IE= const
C
CBCBEC
r
UIII 0 α=f (IE)
C
CBC
dI
dUr
IE = const
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CB – Input Characteristics
Uin = UEBUout = UCB
Iout = IcIin = IE
IE = f (UEB)UCB=const
E
EB
E
EBin
I
U
dI
dUr
UCB = const
ΔIE
ΔUEB
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EE141© 2010, Associate Professor PhD. T.Vasileva
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CE – Output Characteristics
Uout = UCE
Uin = UBE
Forward-active mode
Cut-off (IB = 0, IC = ICE0)
Saturation
IC = f (UCE)IB = const
*0
C
CECEBC
r
UIII β = f (IC)
1
* C
C
CEC
r
dI
dUr
IB
IC
ΔUCE
ΔIC
EE141© 2010, Associate Professor PhD. T.Vasileva
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CE – Input Characteristics
Uout = UCE
Uin = UBE
IB = f (UBE)UCE = const
UCE= 0VUCE= 5V
ΔIB
ΔUBE
B
BE
B
BEin
I
U
dI
dUr
IB
IC
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Influence of Temperature
BJT in CE connection is more temperature depended than in CB connection
since ICE0 and β increases faster with temperature than ICB0 and α.
ICE0 = (1+β)ICB0β = f(T)
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Rathings
Maximum ratings are the limits on the transistor currents, voltages, powers
and other quantities.
These parameters normally represent a destructive level that a designer
usually avoids under all operating conditions. For reliable device operation
maximum ratings should not even be approached. Otherwise, the device may
cease to function properly or its useful lifetime may be greatly shortened.
Maximum ratings for either type of transistor are given in data sheets.
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Max Power Max junction temperature TCmax < Ti, where n=p=ni
Max collector power PCmax
th
aCC
R
TTP
max
max
th
aC
R
TTP
P = UCIC Power dissipated in
collector pn junction
Power conducted
into the ambient
surroundings
th
aC
R
TTUI
The power dissipation must always be
less than the max power PCmax. Otherwise
the transistor will be destroyed.
When power is dissipated within a device, its junction
temperature TC tends to rise. The higher the power, the
higher the junction temperature.
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Bipolar Transistor Packages
Large power transistors have the collector connected to
the metal case to remove heat as easy as possible.
The power rating can be increased if the internal heat
is dissipated faster.
Bipolar transistors are available with plastic or metal cases.
When the power increases a metal tab provides a path out
of the transistor for heat.
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Heat Removal — Thermal Resistance
jcca thth RR cajc ththth RRR
hachjc thththth RRRR
Thermal energy can be easily reduced through conduction and radiation from
the device's case.
Heat sink
Thermal resistance Rth indicates efficiency in removing heat from the transistor
in units oK/W.
The less thermal resistance the higher power rating.
th
aCC
R
TTP
max
max
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Max Current
th
aCCCCE
R
TTPIU
max
maxmax
The maximum collector current of a bipolar transistor ICmax gives the
maximum current a bipolar transistor can handle without exceeding its
power rating PCmax.
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Reverse Breakdown Ratings
0CEBRU
0CBBRU
represents the voltage from collector to emitter with the base
open. This is the breakdown voltage in CE connection.
represents the voltage from collector to base with the emitter
open. This is the breakdown voltage in CB connection.
For normal transistor operation voltages should always be less than the
breakdown values.
000 EBCECB BRBRBR UUU
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Safty Operation Area
ICmax
PCmax
UCmax< UBR
SOA
If the BJT works in a SOA there
is a guarantee that it never will
exceed transistor max rathings.
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Transistor Testing
The emitter-base junction for a normal transistor should have a low resistance in one
direction and a high resistance in the other. The collector junction is tested the same way.
An ohmmeter can be used to check a PN junction either for an open circuit or a
short circuit. A good diode has very high ratio of reverse to forward resistance. For
testing bipolar transistors, the same diode test can be used for each PN junction.