Download Chapter 3 Bipolar Junction Transistor

DMT 121 Electronic I
Chapter 3
Bipolar Junction Transistor
(BJT)
Transistor Structure
• BJT (bipolar junction transistor) constructed with three layer
semiconductor device consisting either TWO n- and ONE p-type layer
(npn transistor) OR TWO-p and ONE n-type layer of material (pnp
transistor)
• BJT is constructed with 3 doped semiconductor regions separated by 2 pnjunctions.
• 3 regions are called emitter, base and collector.
• Emitter –most heavily doped region.
• Base –thin and lightly doped region.
• Collector –largest and moderately doped region.
Symbol
npn transistor
Not pointing in
pnp transistor
Pointing in
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BASIC TRANSISTOR OPERATION
Fig. 3.3
Forward-biased junction of a pnp transistor.
Fig. 3.4
Reverse-biased junction of a pnp transistor.
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BASIC TRANSISTOR OPERATION
Fig. 3.5
Majority and minority carrier flow of a pnp transistor.
IE = IC + IB
IC = IC
majority
+ ICO
(minority)
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ICO
(minority)
is called leakage current
BASIC TRANSISTOR OPERATION
npn transistor operation:
o
The forward bias from base to emitter narrow the BE depletion region, and the
reverse bias from base to collector widens the BC depletion region.
o
The heavily doped n-type emitter region is teeming with conduction-band (free )
electrons that easily diffuse through BE junction into the p-type base region where
they become minority carriers.
o
The base region is lightly doped and very thin so that it has a very limited number
of holes.
o
Thus only a small percentage of all the electrons flowing through the BE junction
can combine with the available holes in the base.
o
A few recombined electrons flow out of the base lead as valence electrons,
forming the small base electron current.
o
Most of electrons from the emitter diffuse into the BC depletion region.
o
Once in this region they are pulled through the reverse-biased BC junction by the
electric field set up by the force of attraction between the positive and negative
ions.
o
The electron now move through the collector region out through the collector lead
into the positive terminal of the collector voltage source.
o
The operation of pnp transistor is the same as for the npn except that the roles of
electrons and holes, the bias voltage polarities and the current directions are all
reversed.
BASIC TRANSISTOR OPERATION
C
Illustration of BJT action:
B
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E
BASIC TRANSISTOR OPERATION
Look at this one circuit as two separate circuits, the baseemitter(left side) circuit and the collector-emitter(right
side) circuit. Note that the emitter leg serves as a conductor
for both circuits.The amount of current flow in the baseemitter circuit controls the amount of current that flows
in the collector circuit. Small changes in base-emitter
current yields a large change in collector-current.
BJTs transistor are
known as current
controlled
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TRANSISTOR CURRENTS
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TRANSISTOR CHARACTERISTICS AND PARAMETERS
As previously discussed, baseemitter current changes yields large
changes in collector-emitter current.
The factor of this change is called
beta().
DC= IC/IB
is dc current gain of transistor
DC is usually equivalent hybrid (h) parameters hFE on transistor
datasheets.
hFE = DC
DC is ratio of collector current (IC) to the emitter current (IE). Less used
parameter than beta in transistor circuits.
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DC = IC/IE
Beta ()
Relationship between amplification factors  and 
α
β
β1
β
α
α 1
Relationship Between Currents
I C  βI B
I E  (β  1)I B
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
There are three key dc voltages and three key dc currents to
be considered. Note that these measurements are important
for troubleshooting.
IB: dc base current
IE: dc emitter current
IC: dc collector current
VBE: dc voltage across
base-emitter junction
VCB: dc voltage across
collector-base junction
VCE: dc voltage from
collector to emitter
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
For proper operation the base-emitter junction is forward
biased by VBB and conducts just like a diode.
The collector-base junction is reverse biased by VCC and
blocks current flow through it’s junction just like a diode.
Remember current
flow through the
base-emitter
junction will help
establish the path
for current flow from
the collector to
emitter.
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Analysis of this transistor circuit to predict the dc voltages and
currents requires use of Ohm’s law, Kirchhoff’s voltage law and
the beta for the transistor.
Application of these laws begins with the base circuit to determine
the amount of base current. Using Kirchhoff’s voltage law, subtract
the 0.7 VBE and the remaining voltage is dropped across RB.
Determining the current for the base with this information is a
matter of applying of Ohm’s law.
VRB/RB = IB
The collector current is
determined by multiplying
the base current by beta.
DC = IC/IB
0.7 VBE will be used in most analysis examples.
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Base-Emitter (Forward Bias)
Collector – Base (Reverse Bias)
VBB  IBRB  VBE  0
VBB  VBE
IB 
RB
VBB  IBRB  VBE  0
VCB  VCE  VBE
Collector - Emitter
VCC  ICRC  VCE  0
VCE  VCC  ICRC
IC   DCIB
0.7 VBE will be used in most analysis
15 examples.
TRANSISTOR CHARACTERISTICS AND
PARAMETERS
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
•What we ultimately
determine by use of
Kirchhoff’s voltage law for
series circuits is that in the
base circuit VBB is
distributed across the baseemitter junction and RB in
the base circuit.
•In the collector circuit we
determine that VCC is
distributed proportionally
across RC and the
transistor(VCE).
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Common-base
Common- Emitter
Common- Collector
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TRANSISTOR CHARACTERISTICS
AND PARAMETERS
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Fig. 3.8 Output or collector characteristics for a common-base transistor amplifier.
TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Collector characteristic
curves gives a graphical
illustration of the relationship
of collector current and VCE
with specified amounts of
base current. With greater
increases of VCC , VCE
continues to increase until it
reaches breakdown, but the
current remains about the
same in the linear region
from 0.7V to the breakdown
voltage.
Collector characteristics for
Common emitter = common
collector
Fig. 3.14 Characteristics of a silicon transistor20
in
the common-emitter configuration: (a) collector
characteristics;
TRANSISTOR CHARACTERISTICS AND
PARAMETERS
With no IB the transistor is in the cutoff region and just
as the name implies there is practically no current
flow in the collector part of the circuit. With the
transistor in a cutoff state the the full VCC can be
measured across the collector and emitter(VCE)
VCC  VCE
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Current flow in the collector part of the circuit
is, as stated previously, determined by IB
multiplied by . However, there is a limit to
how much current can flow in the
collector circuit regardless of additional
increases in IB.
Continue….at next page.
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
Once this maximum is reached, the transistor is said to
be in saturation. Note that saturation can be
determined by application of Ohm’s law. IC(sat)=VCC/RC
The measured voltage across this now seemingly
“shorted” collector and emitter is 0V (VCE = 0V).
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
The dc load line graphically illustrates IC(sat) and Cutoff for a
transistor.
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TRANSISTOR CHARACTERISTICS AND
PARAMETERS
The beta for a transistor is not always constant.
Temperature and collector current both affect beta,
not to mention the normal inconsistencies during the
manufacture of the transistor.
There are also maximum power ratings to consider.
The data sheet provides information on these
characteristics.
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TRANSISTOR AMPLIFIER
Amplification of a relatively
small ac voltage can be had
by placing the ac signal
source in the base circuit.
Recall that small changes in
the base current circuit
causes large changes in
collector current circuit.
The small ac voltage causes
the base current to increase
and decrease accordingly and
with this small change in
current the collector current
will mimic the input only with
greater amplitude.
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TRANSISTOR SWITCH
A transistor when used as a switch is simply being biased so
that it is in cutoff (switched off) or saturation (switched
on). Remember that the VCE in cutoff is VCC and 0V in
saturation.
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Transistor Specification Sheet
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TROUBLESHOOTING
Troubleshooting a live transistor circuit
requires us to be familiar with known good
voltages, but some general rules do apply.
Certainly a solid fundamental understanding
of Ohm’s law and Kirchhoff’s voltage and
current laws is imperative (important). With
live circuits it is most practical to
troubleshoot with voltage measurements.
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TROUBLESHOOTING
Opens in the external resistors or connections of the base or the
collector circuit would cause current to cease (to stop) in the
collector and the voltage measurements would indicate this.
Internal opens within the transistor itself
could also cause transistor operation to
cease.
Erroneous voltage measurements that
are typically low are a result of point that
is not “solidly connected”. This called a
floating point. This is typically indicative
of an open.
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TROUBLESHOOTING
Testing a transistor can be viewed more simply if you view it
as testing two diode junctions. Forward bias having low
resistance and reverse bias having infinite resistance.
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TROUBLESHOOTING
The diode test function of a multimeter is more reliable than
using an ohmmeter. Make sure to note whether it is an npn or
pnp and polarize the test leads accordingly.
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TROUBLESHOOTING
In addition to the traditional DMMs there are also
transistor testers. Some of these have the ability
to test other parameters of the transistor, such as
leakage and gain. Curve tracers give us even more
detailed information about a transistors
characteristics.
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Summary
BJT is constructed of three regions: base, collector, and
emitter.
 BJT has two pn junctions : base-emitter junction and
base-collector junction.
 The two types of transistors are : pnp and npn.
 For the BJT to operate as an amplifier, the base-emitter
junction is forward biased and the collector-base junction is
reverse biased.
 Of the three currents IB is very small in comparison to IE
and IC.
 Beta is the current gain of a transistor. This the ratio of
IC/IB.
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Summary
 A transistor can be operated as an electronics switch.
 When the transistor is off it is in cutoff condition (no
current).
 When the transistor is on, it is in saturation condition
(maximum current).
 Beta can vary with temperature and also varies from
transistor to transistor.
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