The attached narrated power point presentation explains the construction, working and applications of bipolar junction transistors. The material will benefit KTU first year B Tech students who prepare for the subject EST 130, Part B, Basic Electronics Engineering.
4. 4
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.
6. 6
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.
7. 7
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.
8. 8
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.
9. 9
Doping in Transistor Structures
Emitter
Heavily
Doped
Base
Lightly
Doped
Collector
Moderately
Doped
e
b
c
Thin Largest
Two types of carriers – holes and electrons, hence bipolar.
10. 10
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
11. 11
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.
12. 12
Bipolar Junction Transistors
Can two back to back connected pn
junction diodes function as a npn
transistor?
Base width higher?
Recombination
in the base?
13. 13
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".
14. 14
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.
15. 15
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
Inverse Active
Region
Reverse
Biased
Forward
Biased
-
16. 16
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.
17. 17
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.
18. 18
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.
19. 19
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.
21. 21
Common Base Configuration
• Collector current nearly identical to the
emitter current.
IC ≈ IE
RC, RE for Current Limiting.
RC
RE
RC
RE
Common Base Configuration
Base is Common
22. 22
Common Base Current Gain
• dc alpha (αdc) 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.
26. 26
Common Emitter Current Gain
• dc current gain of a transistor in common-
emitter connection called dc beta.
• Again,
• Or,
DC Equivalent of a Transistor
33. 33
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.
34. 34
Common Collector Current Gain
• Common Collector Current Gain:
• Again
and
E
dc
B
I
I
1
dc dc
1
1
dc
dc
38. 38
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
42. 42
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.