History
 Invented in 1947 by Shockley, Bardeen, and Brattain at
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Bell Laboratories
Awarded with the Nobel Prize
Transistors were originally manufactured using
Germanium
Today’s Silicon-based transistors were adopted
because Germanium breaks down at 180 degrees F
It’s a three terminal device.
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Intro-Transistor
 The name transistor is derived from the term transfer
resistance. Transistor – Trans-Resistor
 The current (voltage) through two of the terminals is
controlled by the current (voltage) through another
pair of terminals.
 Transistor can act as a switch.
 Transistor has the ability to amplify a signal between
one pair of terminals by using an input signal at
another pair of terminals. Thus, a transistor can also
act as an amplifier
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symbol
Construction
 The BJT is a three terminal device and has two junctions
 The N and P regions are different both geometrically and in
terms of the doping concentration of the regions
 Emitter region - this is usually a heavily doped region (P/N).
The emitter ‘emits’ the carriers into the base.
 Base region - this is a lightly doped (N/P) region. The base
region is also physically thin so that carriers can pass through
with minimal recombination.
 Collector region - this is a (P/N) type region. The collector
region has a larger width that the other two regions since
charge is accumulated here from the base.
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Construction
 The doping concentrations in the collector, base and
emitter are not the same
• Collector doping is usually ~ 106
• Base doping is slightly higher ~ 107 – 108
• Emitter doping is much higher ~ 1015
 Therefore the behavior of the device is not electrically
and geometrically symmetric and the terminals cannot
be cannot be interchanged
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Types – PNP and NPN Transistors
Symbols
Note: NPN type is most commonly used Transistor. The majority charge carriers in
an NPN transistor are electrons and the majority carriers in a PNP transistor are holes.
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The electrons have better mobility than holes and helps in better conduction.
Working Principle
 Emitter Base Junction  Forward Biased
 Collector Base Junction  Reverse Biased
 Forward bias at emitter base junction causes electrons
to move towards base leading to emitter current
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Working Principle
 As electrons flow towards p-type base, recombination
will happen with the holes
 Since the base is lightly doped, only few recombination
would happen within the base
 Small recombination results in small base current
 Remainder electrons cross the base and constitute the
collector current because the electrons are attracted
towards the collector due to high reverse bias
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Operation of Transistor
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BJT Configurations
A Transistor has 3 terminals, the emitter, the base and the collector.
Using these 3 terminals the transistor can be connected in a circuit with
one terminal common to both input and output in a 3 different possible configurations
 Common-Base (CB) :
 Input
= VEB & IE
 Output = VCB & IC
 Common-Emitter (CE):
 Input
= VBE & IB
 Output = VCE & IC
 Common-Collector (CC):
 Input
= VBC & IB
 Output = VEC & IE
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BJT Configurations-Comparison
AMPLIFIER TYPE
COMMON BASE
COMMON EMITTER
COMMON
COLLECTOR
INPUT/OUTPUT
PHASE RELATIONSHIP
0°
180°
0°
VOLTAGE GAIN
HIGH
MEDIUM
LOW
CURRENT GAIN
LOW
MEDIUM
HIGH
POWER GAIN
LOW
HIGH
MEDIUM
INPUT RESISTANCE
LOW
MEDIUM
HIGH
OUTPUT RESISTANCE
HIGH
MEDIUM
LOW
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Modes of Operation
 Active
 Most important mode of operation
 Central to amplifier operation
 The region where current curves are practically flat
 Saturation
 Barrier potential of the junctions cancel each other out
causing a virtual short- closed switch
 Cut-Off
 Current reduced to zero
 Ideal transistor behaves like an open switch
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Common Emitter Configuration
 In CE Configuration, the Emitter terminal of the transistor will be
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connected common between the output and the input terminals.
Input voltage VBE is applied between base and emitter terminals
and output voltage VCE is taken across emitter and collector.
The output current IC is taken across the emitter and collector
terminals.
Input side is forward biased and the output side is reverse biased.
Input current IB is measured in µA because the base region is very
lightly doped
The CE configuration is the most widely used
configuration.
Common emitter transistors provides Moderate
current gain, Moderate voltage gain and High
power gain.
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CE Configuration-Input Characteristics.
 Input
characteristics
are
the
relationship between the input
current and the input voltage
keeping output voltage constant.
 Input current is the base current
IB, input voltage is base emitter
voltage VBE and the output voltage
is collector emitter voltage VCE.
 when the input voltage VBE is
increased initially there is no
current produced, further when it is
increased beyond 0.7V the input
current IB increases steeply
 If the output voltage VCE is further
increased the curve shifts right
side.
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CE Configuration-Output Characteristics.
 Output characteristics is the relationship
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between the output current and the
output voltage keeping input current
constant.
Output current IC and the output voltage
VCE is noted keeping input current IB
constant
Active region when the output voltage
is increased there is very slight change
in the input current
Cut off region is the region where the
input current is below zero.
When both the junctions are forward
biased, it is in saturation region.
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CE Configuration
 Cutoff region
 both BE and BC reverse biased
 Active region
 BE Forward biased
 BC Reverse biased
 Saturation region
both BE and BC forward biased
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Amplification factors
 The ratio of change in collector current with respect to base
current is known as the base amplification factor ꞵ
 The ratio of change in collector current with respect to emitter
current is known as the current amplification factor α
𝛼=
𝛽
𝛽+1
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Problem
Given: IB = 50  A , IC = 1 mA
Find: IE , ꞵ , and 
Solution:
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
ꞵ = IC / IB = 1 mA / 0.05 mA = 20
 = IC / IE = 1 mA / 1.05 mA = 0.95238
 could also be calculated using the
value of ꞵ
=
ꞵ
= 20 = 0.95238
ꞵ+1
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Transistor as Switch
 Solid state switches are one of the
main applications for the use of
transistor to switch a DC output “ON”
or “OFF”.
 When transistor can be operated
between Saturation Region and the
Cut-off Region, it can be used as a
Switch
 Transistor used as a switch is driven
back and forth between its “fullyOFF” (cut-off) and “fully-ON”
(saturation) by changing the biasing
suitably
 When transistor is operated in active
region it works as an Amplifier.
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Transistor as Switch-Cutoff region
 Zero input base current ( IB ), zero output collector
current ( IC ) and maximum collector voltage ( VCE )
which results in a large depletion layer and no current
flowing thru the transistor.
 Transistor is said to be switched “Fully-OFF”.
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Transistor as Switch-Saturation region
 Maximum base current is applied, resulting in maximum
collector current resulting in the minimum collector emitter
voltage drop which results in the depletion layer being as
small as possible and maximum current flowing thru the
transistor.
 Transistor is said to be switched “Fully-ON”
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Transistor as a Switch
 Transistor operates as a “single-pole single-throw”
(SPST) solid state switch.
 With a zero signal applied to the Base of the transistor
it turns “OFF” acting like an open switch and zero
collector current flows.
 With a positive signal applied to the Base of the
transistor it turns “ON” acting like a closed switch and
maximum collector current flows
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Transistor as a switch- Analogy
Analogy
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Thank you
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