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Electronics II
Lecture 5(b): Metal-Oxide Si FET MOSFET
A/Lectr. Khalid Shakir
Dept. Of Electrical Engineering
College of Engineering
Maysan University
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
Page 1-21
st Semester
2008 Pearson
Education, Inc.
Electronics II- Lecture 5(b)/1©
013/014
All rights reserved
Summary
The MOSFET
The metal oxide semiconductor FET uses an insulated gate
to isolate the gate from the channel. Two types are the
enhancement mode (E-MOSFET) and the depletion mode
(D-MOSFET).
E-MOSFET
An E-MOSFET has no
channel until it is induced
by a voltage applied to the
gate, so it operates only in
enhancement mode. An nchannel type is illustrated
here; a positive gate voltage
induces the channel.
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
RD
Drain
ID
Induced
SiO
n
2
Gate
p substrate
n
n
channel
+
+
–
+
+
–
–
+
–
+
– VDD
n
VGG –
Source
Page 2-21
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013/014
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Summary
The MOSFET
The D-MOSFET has a channel that can is controlled by the
gate voltage. For an n-channel type, a negative voltage
depletes the channel; and a positive voltage enhances the
channel.
D-MOSFET
RD
A D-MOSFET can
operate in either
mode, depending on
the gate voltage.
RD
n
–
–
–
–
–
–
+
+
+
+
+
+
p
n
+
–
+
+
+
+
+
+
V DD
–
VGG
+
p
+
–
V
DD
+
n
operating in D-mode
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
–
–
–
–
–
–
Page 3-21
VGG
–
n
operating in E-mode
st Semester
2008 Pearson
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Electronics II- Lecture 5(b)/1©
013/014
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Summary
The MOSFET
MOSFET symbols are shown. Notice the broken line
representing the E-MOSFET that has an induced channel.
The n channel has an inward pointing arrow.
E-MOSFETs
D
D
G
G
G
S
n channel
D-MOSFETs
D
D
S
S
p channel
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
G
n channel
Page 4-21
S
p channel
st Semester
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013/014
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Summary
The MOSFET
The transfer curve for a MOSFET is has the same parabolic
shape as the JFET but the position is shifted along the x-axis.
The transfer curve for an n-channel E-MOSFET is entirely in
the first quadrant as shown.
I
D
The curve starts at VGS(th), which is a
nonzero voltage that is required to have
channel conduction. The equation for
the drain current is
ID
K VGS VGS(th)
0
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
Page 5-21
VGS(th)
+VGS
st Semester
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013/014
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Summary
The MOSFET
Recall that the D-MOSFET can be operated in either mode.
For the n-channel device illustrated, operation to the left of
the y-axis means it is in depletion mode; operation to the
right means is in enhancement mode.
I
D
As with the JFET, ID is zero at VGS(off).
When VGS is 0, the drain current is
IDSS, which for this device is not the
maximum current. The equation for
drain current is
ID
I DSS
VGS(off)
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
I DSS
–VGS
VGS(off)
Page 6-21
0
st Semester
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Summary
MOSFET Biasing
E-MOSFETs can be biased using bias methods like the BJT
methods studied earlier. Voltage-divider bias and drainfeedback bias are illustrated for n-channel devices.
+VDD
+VDD
RD
R1
R
RD
G
R2
Voltage-divider bias
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
Page 7-21
Drain-feedback bias
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Summary
MOSFET Biasing
The simplest way to bias a D-MOSFET is with zero bias. This
works because the device can operate in either depletion or
enhancement mode, so the gate can go above or below 0 V.
+VDD
+V DD
RD
VG = 0 V
VGS = 0
RD
IDSS
ac
C
input
RG
RG
Zero bias, which can only be used for the D-MOSFET
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
Page 8-21
st Semester
2008 Pearson
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Electronics II- Lecture 5(b)/1©
013/014
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Selected Key Terms
JFET
Junction field-effect transistor; one of two
major types of field-effect transistors.
Drain One of three terminals of a FET analogous to
the collector of a BJT.
Source One of three terminals of a FET analogous to
the emitter of a BJT.
Gate One of three terminals of a FET analogous to
the base of a BJT.
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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Selected Key Terms
Transconductance The ratio of a change in drain current to a
(gm ) change in gate-to-source voltage in a FET.
MOSFET Metal oxide semiconductor field effect
transistor; one of two major types of FETs;
sometimes called IGFET.
Depeletion In a MOSFET, the process of removing or
depleting the channel of charge carriers and
thus decreasing the channel conductivity.
Enhancement In a MOSFET, the process of creating a
channel or increasing the conductivity of the
channel by the addition of charge carriers.
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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013/014
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Quiz
1. If an n-channel JFET has a positive drain voltage and the
gate-source voltage is zero, the drain current will be
a. zero
b. IDSS
c. IGSS
d. none of the above
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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Quiz
2. For a JFET, two voltages with the same magnitude but
opposite signs are
a. VD and Vp
b. VD and VS
c. VGS(th) and Vcutoff
d. Vp and VGS(off)
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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Quiz
3. A set of characteristic curves for a JFET are shown. The
blue lines represent different values of
a. VDS
ID
b. VGS
c. VS
d. Vth
VDS
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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Quiz
4. Transconductance can be expressed as
a. gm
g
b.
m
g
c.
VDS
I
D
m
g
d.
ID
VGS
IG
m
VIDS
G
GS
V
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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Quiz
5. JFETs cannot be biased using
a. self bias
b. voltage-divider bias
c. zero bias
d. current-source bias
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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2008 Pearson
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013/014
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Quiz
6. The JFET operating point in the circuit shown is
a. at the origin
b. at Isat
VCC
+15 V
c. at VCC
C1
d. undefined
Page 16-21
R
3.9C k
Vout
Q1
2N3904 C2
1.0 µF
Vs =
400 mV pp
1.0 kHz
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
R1
56 k
R2
39 k
10 µF
R
E
6.2 k
Q2
2N5458
R3
100 k ff
VGG
st Semester
2008 Pearson
Education, Inc.
Electronics II- Lecture 5(b)/1©
013/014
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Quiz
7. The JFET in this circuit acts like a(n)
a. voltage source
b. amplifier
VCC
+15 V
c. capacitor
C1
d. resistor
Page 17-21
RC
3.9 k
Vout
Q1
2N3904 C2
1.0 µF
Vs =
400 mV pp
1.0 kHz
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
R1
56 k
R2
39 k
10 µF
RE
6.2 k
Q2
2N5458
R3
100 k ff
VGG
st Semester
2008 Pearson
Education, Inc.
Electronics II- Lecture 5(b)/1©
013/014
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Quiz
8. The symbol for a p-channel E-MOSFET is
D
G
D
D
G
G
D
G
S
S
S
S
a.
b.
c.
d.
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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013/014
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Quiz
9. The transfer curve shown is for an n-channel
a. E-MOSFET
ID
b. D-MOSFET
c. JFET
d. all of the above
0
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
Page 19-21
VGS(th)
+VGS
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Quiz
10. A type of FET that can use the same bias method as a
BJT is a(n)
a. E-MOSFET
b. D-MOSFET
c. JFET
d. all of the above
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
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013/014
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Quiz
Answers:
Copyright @2013 by Dept. of Electrical Engineering,
College of Engineering, Maysan University
1. b
6. a
2. d
7. d
3. b
8. b
4. a
9. a
5. c
10. a
Page 21-21
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2008 Pearson
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