PPT - Introduction to Transistors Electrical Engineering (EE) Notes | EduRev

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Electrical Engineering (EE) : PPT - Introduction to Transistors Electrical Engineering (EE) Notes | EduRev

 Page 1


Introduction to Transistors
 A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the
order pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer (The Art of Electronics, Horowitz
and Hill, 2
nd
Ed.)
(Introductory Electronics, Simpson, 2
nd
Ed.)
Page 2


Introduction to Transistors
 A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the
order pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer (The Art of Electronics, Horowitz
and Hill, 2
nd
Ed.)
(Introductory Electronics, Simpson, 2
nd
Ed.)
Introduction to Transistors
 Thus when a transistor is
off it behaves like a
two–diode circuit
 A transistor operates (or
turns on) when the base–emitter junction is forward
biased and the base–collector junction is reversed
biased (“biasing”)
(Electronic Devices and
Circuits, Bogart, 1986)
(The Art of Electronics,
Horowitz and Hill, 2
nd
Ed.)
(Lab 4–1)
Page 3


Introduction to Transistors
 A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the
order pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer (The Art of Electronics, Horowitz
and Hill, 2
nd
Ed.)
(Introductory Electronics, Simpson, 2
nd
Ed.)
Introduction to Transistors
 Thus when a transistor is
off it behaves like a
two–diode circuit
 A transistor operates (or
turns on) when the base–emitter junction is forward
biased and the base–collector junction is reversed
biased (“biasing”)
(Electronic Devices and
Circuits, Bogart, 1986)
(The Art of Electronics,
Horowitz and Hill, 2
nd
Ed.)
(Lab 4–1)
Transistor Biasing (npn Transistor)
 Electrons are constantly
supplied to the emitter by
the battery with voltage V
EE
 These electrons can:
1. Recombine with holes in
the base, giving rise to I
B
2. Diffuse across base and be swept (by electric field at
base–emitter junction) into collector, then diffuse around
and eventually recombine with holes injected into
collector, giving rise to I
C
 Since the base region is designed so thin, process 2
dominates (no time for #1 to occur as often)
– In an actual npn transistor, 98 or 99% of the electrons that
diffuse into the base will be swept into the collector
Page 4


Introduction to Transistors
 A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the
order pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer (The Art of Electronics, Horowitz
and Hill, 2
nd
Ed.)
(Introductory Electronics, Simpson, 2
nd
Ed.)
Introduction to Transistors
 Thus when a transistor is
off it behaves like a
two–diode circuit
 A transistor operates (or
turns on) when the base–emitter junction is forward
biased and the base–collector junction is reversed
biased (“biasing”)
(Electronic Devices and
Circuits, Bogart, 1986)
(The Art of Electronics,
Horowitz and Hill, 2
nd
Ed.)
(Lab 4–1)
Transistor Biasing (npn Transistor)
 Electrons are constantly
supplied to the emitter by
the battery with voltage V
EE
 These electrons can:
1. Recombine with holes in
the base, giving rise to I
B
2. Diffuse across base and be swept (by electric field at
base–emitter junction) into collector, then diffuse around
and eventually recombine with holes injected into
collector, giving rise to I
C
 Since the base region is designed so thin, process 2
dominates (no time for #1 to occur as often)
– In an actual npn transistor, 98 or 99% of the electrons that
diffuse into the base will be swept into the collector
Current Flow Inside a Transistor
 Current flow for an npn transistor (reverse for pnp):
– From conservation of current
(I
E
= I
B
+ I
C
) we can obtain the following
expressions relating the currents:
where b ˜ 20 – 200
(depends on emitter current)
 b increases as I
E
increases (for very small I
E
) since there is less
chance that recombination will occur in the base
 b decreases slightly (10–20%) as I
E
increases beyond several mA
due to increased base conductivity resulting from larger number of
charge carriers in the base
 Thus b is not a constant for a given transistor!
 An average value of 100 is typically used
(Electronic Devices and Circuits,
Bogart, 1986)
B C
I I b = ( )
B E
I I 1 + = b
(and thus I
C
˜ I
E
)
(Lab 4–5)
Page 5


Introduction to Transistors
 A transistor is a device with three separate layers of
semiconductor material stacked together
– The layers are made of n–type or p–type material in the
order pnp or npn
– The layers change abruptly to form the pn or np junctions
– A terminal is attached to each layer (The Art of Electronics, Horowitz
and Hill, 2
nd
Ed.)
(Introductory Electronics, Simpson, 2
nd
Ed.)
Introduction to Transistors
 Thus when a transistor is
off it behaves like a
two–diode circuit
 A transistor operates (or
turns on) when the base–emitter junction is forward
biased and the base–collector junction is reversed
biased (“biasing”)
(Electronic Devices and
Circuits, Bogart, 1986)
(The Art of Electronics,
Horowitz and Hill, 2
nd
Ed.)
(Lab 4–1)
Transistor Biasing (npn Transistor)
 Electrons are constantly
supplied to the emitter by
the battery with voltage V
EE
 These electrons can:
1. Recombine with holes in
the base, giving rise to I
B
2. Diffuse across base and be swept (by electric field at
base–emitter junction) into collector, then diffuse around
and eventually recombine with holes injected into
collector, giving rise to I
C
 Since the base region is designed so thin, process 2
dominates (no time for #1 to occur as often)
– In an actual npn transistor, 98 or 99% of the electrons that
diffuse into the base will be swept into the collector
Current Flow Inside a Transistor
 Current flow for an npn transistor (reverse for pnp):
– From conservation of current
(I
E
= I
B
+ I
C
) we can obtain the following
expressions relating the currents:
where b ˜ 20 – 200
(depends on emitter current)
 b increases as I
E
increases (for very small I
E
) since there is less
chance that recombination will occur in the base
 b decreases slightly (10–20%) as I
E
increases beyond several mA
due to increased base conductivity resulting from larger number of
charge carriers in the base
 Thus b is not a constant for a given transistor!
 An average value of 100 is typically used
(Electronic Devices and Circuits,
Bogart, 1986)
B C
I I b = ( )
B E
I I 1 + = b
(and thus I
C
˜ I
E
)
(Lab 4–5)
Transistor Current Amplification
 If the “input” current is I
B
and the “output” current is
I
C
, then we have a current amplification or gain
– Happens because base–emitter junction is forward-biased
– Forward bias ensures that the base–emitter junction
conducts (transistor is turned on)
– Reverse bias ensures that most of the large increase in
the base–emitter current shows up as collector current
(Student Manual for The Art
of Electronics, Hayes and
Horowitz, 2
nd
Ed.)
Thus small gains in I
B
result in large gains in I
E
and hence I
C
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