Bipolar Junction Transistors

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Bipolar Junction Transistors

• CEC

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Contents

• Introduction.
• BJT (Transistor) Types.
• Operating Regions.
• Early Effect.
• Transistor Configurations.
• Transistor Characteristics.
• Comparison.
• Transistor Testing.

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Transistor Invention
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Transistor Invention

• First transistor called a point-contact
  transistor on December 23, 1947.
• Walter H. Brattain and John Bardeen demonstrated
  the amplifying action of first transistor at Bell
  Telephone Laboratories.
• Was the predecessor to the junction transistor
  invented by Shockley in 1948.
• Shared Nobel Prize in Physics in 1956.

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Bipolar Junction Transistor

• Three terminal device Emitter, Base and
  Collector.
• Two p-n junctions Emitter-Base junction and
  Collector-Base junction.
• PNP and NPN transistors.
• Current controlled base current controls the
  device.
• Can be used in amplifiers or as switches.
• Four regions of operation.
• Three Configurations Common Emitter, Common
  Base and Common Collector.

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Bipolar Junction Transistor

• Three doped regions.
• Base narrow region sandwiched between the larger
  collector and emitter regions.
• Emitter region heavily doped.
• Emitter injects current carriers into the base.
• Base region very thin and lightly doped.

Electrons are the majority carriers in a n type
material.

Holes are the majority carriers in a p type
material.
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Bipolar Junction Transistor

• Most of the current carriers injected into the
  base from the emitter pass on to the collector,
  do not flow out of the base lead.
• Collector region moderately doped, largest of all
  three regions.
• Collector region attracts carriers injected into
  the base region.
• Collector region the largest region, must
  dissipate more heat than emitter or base regions.

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Doping in Transistor Structures

Thin
Largest
Emitter Heavily Doped Base Lightly Doped Collector Moderately Doped
c
e
b
Two types of carriers holes and electrons,
hence bipolar.
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Depletion Regions(Unbiased)

• Diffusion of electrons from both n regions into
  the p -type base causes a barrier potential for
  both p-n junctions.
• e-b depletion region narrower than the c-b
  depletion region due to difference in doping
  levels.

Barrier Potential 0.7 V
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Depletion Regions(Unbiased)

• Heavy doping in the emitter region.
• Penetration into the n material minimal due to
  the availability of many free electrons.
• Fewer free electrons due to moderate doping level
  in the collector region.
• Depletion layer penetrate deeper into the
  collector region.

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Bipolar Junction Transistors
Can two back to back connected pn junction diodes
function as a npn transistor?

Recombination in the base?
Base width higher?
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Bipolar Junction Transistors

• Two back-to-back pn diodes can't function as a
  single BJT.
• Transistor operation not achieved with standalone
  pn diodes which conduct a negligible currents
  under reverse bias.
• Excess electrons from the p side of the forward
  biased diode can not be swept to the p side of
  the reverse biased diode through the metal wire
  in a "BJT like diode configuration".

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Bipolar Junction Transistors

• Excess electrons from the p side of the forward
  biased diode are swept to the power supply
  (stronger pull) providing a voltage bias to the
  common terminal of the diodes.
• For transistor functionality, semiconductor only
  thin lightly doped Base region required.
• With a metal introduced in the path (two
  back-to-back diodes), no BJT functionality
  possible.

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Transistor Operating Regions
Operating Region Emitter Base Junction Collector Base Junction Applications
Active Region Forward Biased Reverse Biased Amplifiers
Saturation Region Forward Biased Forward Biased Switches (on/off)
Cut off Region Reverse Biased Reverse Biased Switches (on/off)
Inverse Active Region Reverse Biased Forward Biased -

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Transistor Currents

• For a transistor amplifier, emitter-base junction
  forward-biased, and collector-base junction
  reverse-biased.
• Common connection for voltage sources at the base
  lead.
• Electrons in the n -type emitter repelled into
  the base by the negative terminal of the emitter
  supply voltage.

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Transistor Currents

• Base is very thin and lightly doped.
• Only a few electrons combine with holes in the
  base.
• Small current flowing out of the base lead called
  recombination current.
• Free electrons injected into the base must fall
  into a hole before they can flow out the base
  lead.

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Transistor Currents

• Most of the emitter-injected electrons pass
  through the base region to the collector region.
• A small voltage creates a strong electric field
  in the collector-base junction to attract almost
  all free electrons injected into the base.
• Positive collector-base voltage attracts free
  electrons in the p - type base to the collector
  side before they recombine with holes in the base.

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Early Effect/Base Width Modulation

• As reverse bias across the collector-base
  junction increases, depletion region penetrates
  more into the base.
• Thin base lightly doped, penetration into the
  base region larger than into the collector,
  effective base width decreases/ modulated.
• Punch through large reverse bias, depletion
  region penetrates the entire base, transistor
  action lost, large current flow damage.

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Transistor Configurations

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Common Base Configuration

• Collector current nearly identical to the emitter
  current.

RC
RE
Base is Common
IC IE RC, RE for Current Limiting.
Common Base Configuration
RC
RE
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Common Base Current Gain

• dc alpha (adc) describes how closely the emitter
  and collector currents are in a common base
  circuit.
• In most cases, the dc alpha 0.99 or greater.
• The thinner and more lightly doped the base, the
  closer the value of alpha to one.

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Circuit for Common Base Characteristics

Input Side
Output Side
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Common Base Characteristics

Input Characteristics
Output Characteristics
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Common Emitter Configuration

• Emitter common to
  input and output.

Phase Inversion.
_at_ 25oC
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Common Emitter Current Gain

• dc current gain of a transistor in common-emitter
  connection called dc beta.
• Again,
• Or,

DC Equivalent of a Transistor
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Circuit for Common Emitter Characteristics

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Common Emitter Input Characteristics
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Common Emitter Output Characteristics

Active Region
Cut Off Region
Saturation Region
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Common Emitter Amplifier
RC Coupled Amplifier
For Current Limiting
Voltage Divider Bias
Coupling Capacitor
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Transistor Switch

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CB and CE Parameters
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Common Collector Configuration
Collector common to input and output

Emitter Follower. No phase inversion for the
output. For Impedance Matching. Voltage Gain
1. Used in cascade amplifiers.
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Common Collector Current Gain

• Common Collector Current Gain
• Again
• and

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Circuit for Common Collector Characteristics

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Common Collector Characteristics

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Voltage Follower Circuit

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Brief Comparison
Characteristic CB CE CC
Input Resistance Low Medium High
Output Resistance Very High High Low
Voltage Gain High Medium Low
Current Gain Low Medium High

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Darlington Pair

ßd ß1.ß2
ß1
ß2
Current Gain Improves
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Cascode Configuration

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Transistor Testing

• Identify whether pnp or npn.
• Identify transistor leads.
• Use of Ohmmeter/Multimeter.
• Consider the transistor as two diodes.
• Emitter-base and collector-base junctions forward
  biased low resistance.
• Emitter-base and collector-base junctions reverse
  biased high resistance.
• Emitter to collector high resistance.

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Transistor Testing
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Thank You