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CHAPTER 18
TRANSISTORS AND
THYRISTORS
1
BIPOLAR JUNCTION
TRANSISTORS (BJTs)
• BJT is constructed with three doped
semiconductor regions separated by two pn
junction
• There are three regions :
– Emitter
– Base
– Collector
• There are two type of BJT:
– npn
– pnp
2
Basic construction of BJT
3
Transistor Biasing
• The BE junction is forward-biased
• The BC junction is reverse-biased
4
Transistor Operation
5
Transistor Currents
6
Alpha
and Bata
• The collector current is equal to
the emitter current
times
has a value between
0.950 and 0.995
• The collector current is equal to the base
current multiplied by
has a value between
20 and 200
7
Transistor Voltages
VC  VCC  I C RC
VB VE VBE
8
VOLTAGE-DIVIDER BIAS
• Use only a single dc
source to provide
forward-reverse bias
to the transistor
• Resistor R1 and R2
form a voltage
divider that provides
the base bias
voltage
9
Input Resistance at the Base
VB
RIN 
IB
VB VE I E RE
I E βDC I B
RIN
βDC I B RE

βDC RE
IB
10
Base Voltage
 R2 RIN
VB 
 R1R2 RIN
RIN R2

VCC


 R2 
VCC
VB 
 R1R2 
VE VB 07 V
11
THE BIPOLAR JUNCTION
TRANSISTOR AS AN AMPLIFIER
12
THE BIPOLAR JUNCTION
TRANSISTOR AS AN AMPLIFIER
• When both junction are forward-biased, the
transistor is in the saturation region of its
operation
• When VCE exceeds 0.7 V, the base-collector
junction becomes reverse-biased and the
transistor goes into the active or linear region
• When IB=0 the transistor is
I in the cutoff region.
CEO
– Collector leakage current,
– B-E and B-C are reverse-biased
13
Load Line Operation
14
Quiescent or Q-Point
15
Quiescent or Q-Point
16
Signal (ac) Operation of an
Amplifier
• The circuit which produces an output
signal with the same waveform as input
signal but with a greater amplitude is
called amplification
RC
Av 
RE
17
Signal Operation on the Load Line
18
THE BJT AS A SWITCH
Conditions in cutoff
VCE ( cutoff )  VCC
Conditions in saturation
I C ( sat)
VCC

RC
I B (min) 
IC sat 
 DC
19
BJT PARAMETERS AND RATINGS
IF the temperature goes up,  DC goes up, and
vice versa.
20
BJT PARAMETERS AND RATINGS
• Maximum Transistor Ratings
IC 
PD max 
VCE
PD(max) = maximum power dissipation
21
THE JUNCTION FIELD-EFFECT
TRANSISTOR (JFET)
• The JFET is a type of FET that operates
with a reverse-biased junction to control
current in the channel
22
Categories
FET
JUNCTION
Depletion
P channel N channel
METAL-OXIDE
semiconductor
Depletion
P channel N channel
Enhancement
P channel
N channel
23
JFET Basic Operation
• The JFET is always operated with the
gate-to-source pn junction reverse-biased
24
JFET Symbols
25
JFET CHARACTERISTICS
IDSS (Drain to Source current with gate shorted) is
maximum drain current occurring for VGS =0 V,
and the value of VDS which ID becomes constant is the
pinch-off voltage
26
Cutoff Voltage
• The value of VGS that makes ID
approximately zero is the cutoff voltage
VGS(off)
27
VGS controls ID.
•JFET must be operated between VGS =0
and VGS(off)
28
Comparison of Pinch-Off and
Cutoff
• Vp is the value of VDS at which the drain
current becomes constant and is always
measured at VGS =0 V
• VGS(off) and Vp are always equal in
magnitude but opposite in sign
29
THE METAL-OXIDE
SEMICONDUCTOR FET(MOSFET)
• Depletion MOSFET (D-MOSFET)
– D-MOSFET can be operated either the
depletion mode or the enhancement mode
30
Depletion Mode (D-MOSFET)
31
Enhancement Mode (D-MOSFET)
32
D-MOSFET Symbols
33
Enhancement MOSFET(E-MOSFET)
34
E-MOSFET Symbols
35
FET BIASING
• Self-Biasing a JFET
VGS VG VS 0V I D RS
VGS I D RS nchannel
VGS I D RS  pchannel
VD VDD I D RD
VDS VD VS
VDS VDD I D RD RS 
36
D-MOSFET Bias
VGS 0V ,I D I DSS
VDS VDD I DSS RD
37
• Example
38
E-MOSFET Bias
Drain-feedback bias
negligible gate current ; VGS  VDS
Voltage-Divider bias
 R2 
VDD
VGS 
 R1R2 
VDS VDD I D RD
(a) Drain-feedback bias
(b) Voltage-Divider bias
39
The Other Types of MOSFET
LDMOSFET
TMOS
VMOS
Dual-gate n-channel40
UNIJUNCTION TRANSISTORS
(UJTs)
41
UJT operation
• In normal UJT operation, base 2 (B2) and the
emitter are biased positive with respect to base
1 (B1), and total resistance between B1 and B2 is
RBB
• The ratio RB1/RBB is designated η and is defined
as the intrinsic standoff ratio
• It take an emitter voltage of VB + η *VBB to turn the
UJT on, called the peak voltage
• When the emitter voltage decreases, it reaches
a value called the valley voltage, and the UJT
turns off
42
UJT operation
43
UJT Applications
44
THYRISTORS
• Thyristors are devices constructed of four
layers of semiconductive material such as
the silicon-controlled rectifier (SCR), the
diac, and the triac
• When turned on (triggered), they become
low-resistance current paths and remain,
although the trigger is removed, until the
current is reduced to a certain level or until
they are turned off
45
Silicon-Controlled Rectifier (SCRs)
• The SCR has three terminals
46
SCR application
Phase control circuit
47
Triacs
• The triac is also known as a bidirectional
triode thyristor
48
Diacs
• The diac us a bidirectional device that
does not have a gate
• It conducts current in either direction when
a sufficient voltage, called the breakover
potential
49
Transistor packages
50
FIGURE 4-26
Metal cases for general-purpose/small-signal transistors.
Thomas L. Floyd
Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
51
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 4-27
Typical multiple-transistor packages.
Thomas L. Floyd
Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
52
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 4-28
Typical power transistors.
Thomas L. Floyd
Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
53
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
FIGURE 4-29
Examples of RF transistors.
Thomas L. Floyd
Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
54
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.