Physics of a BJT

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Physics of Bipolar Junction
Transistor
Section 4.1-4.3
Schedule
9
2/11
Tuesday
Physics of a BJT
L
2/11
Tuesday
Measure Beta of a transistor
10
2/13
Thursday Small signal model, PNP [homework: small
eq. circuit, (PNP)]
4.4-4.6
11
2/18
Tuesday
4.5
L
2/18
Tuesday
12
2/20
BJT in saturation mode
4.1-4.3
BJT in saturation/BJT implementation of an
NAND gate
Thursday Transistor bias , [homework assigned]
5.1-5.2
Overview
Outline
• Schematic Symbol
• Review
– Forward Bias Diode
– Reverse Bias Diode
• Energy Band Diagram
– Intrinsic Semiconductor under an Applied
Voltage
– Forward Biased Diode
– Reverse Biased Diode
• Apply KCL to a BJT
Voltage and Current Polarities of
NPN and PNP transistors
A “fat” voltage
between collector
and emitter voltage
places a transistor in
the active region!
A “skinny” voltage between collector
and emitter voltage places a transistor
in the active region!
Structure and Circuit Symbol
(collects charge carriers)
(controls the carriers
that make the journey
from E to C)
(emits charge carriers)
Two PN junctions (BE and BC)
C and E cannot be swapped.
A Bipolar Transistor in the
Active Region
VBE=0.8 V
VBC=-0.2 V
BE Junction: Forward Bias
BC Junction: Reverse Bias
Over Simplified View of BJT
In the Active Region
(not necessarily the right way
to understand circuit)
Review: Forward Bias Diode
Depletion region shrinks due to charges from the battery.
The electric field is weaker.
Majority carrier can cross via diffusion;
Greater diffusion current.
Current flows from P side to N side
Review: Forward Bias Diode
Equilibrium
Forward Biased Diode
Majority carriers cross the junction via diffusion.
Minority carriers increased on both sides of the junction.
Energy Band Diagram of a
Semiconductor Under an
Applied Voltage
Electrons roll downhill like stones.
Holes float up like bubles!
Forward Biased Diode
Graphical Illustration
Review: PN Junction under
Reverse Bias
Reverse: Connect
the + terminal to the
n side.
Depletion region widens.
Therefore, stronger E.
E
Minority carriers cross
the PN junction easily
through diffusion.
Current is composed
mostly of drift current contributed
by minority carriers.
np to the left and pn to the right.
Current from n side to p side,
the current is negative.
Energy Band Diagram of a
Reverse Biased PN Junction
n
p
pn
Stronger E field in the
depletion region
pn
np
Injection of Electrons into
Depletion Region
Into depletion region on the p side.
Outcome: The electron is swept
to the n side by E.
An Overview
Electrons are injected
into the B; holes to the E.
Electrons
are injected
into the BC junction
Electrons
are swept across
the reversed biased BC
Thin Base Region
The base region is made thin in order to reduce recombination
as electrons travel from BE junction to BC junction.
Highly Doped Emitter
In order to emphasize the current contribution
due to the electrons (which can cross the BC junction),
the emitter is heavily doped by N type materials.
Electrons in the Base
Electrons injected into
the base; high electron
density at x1.
Electrons are swept
Into the collector;
low electron density at x2
The electron gradient allows electrons to travel through diffusion.
Structures of BJT Transistors
(NPN transistor)
(PNP transistor)
BJT Current
Assumption:
BEJ: Forward Biased
BCJ: Reverse Biased
Base Current
The proportional of hole current and electron current
is determined by dopants (ND and NA).
Even though the presence of holes are minimized, a small
number holes still must enter through the base.
Recombination
Recombination
Base must supply holes that will enter the emitter and
for recombination with the electrons.
KCL
Optional Slides
Emitter Area
Determine the Output
Voltage
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