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In this chapter, we will:
Microelectronics
Circuit Analysis and Design
Study and understand the operation and characteristics of
the various types of MOSFETs.
Donald A. Neamen
Understand and become familiar with the dc analysis and
design techniques of MOSFET circuits.
Chapter 3
Examine three applications of MOSFET circuits.
Investigate current source biasing of MOSFET circuits, such
as those used in integrated circuits.
The Field Effect Transistor
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-1
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-2
MOS Capacitor Under Bias:
Electric Field and Charge
Basic Structure of MOS Capacitor
Parallel plate capacitor
Negative gate bias:
Holes attracted to gate
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-3
Neamen
Microelectronics, 4e
McGraw-Hill
Positive gate bias:
Electrons attracted to gate
Chapter 3-4
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Basic Transistor Operation
Schematic of n-Channel
Enhancement Mode MOSFET
After electron
inversion layer is
formed
Before electron
inversion layer is
formed
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-5
Basic Transistor Operation
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-7
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-6
Current Versus Voltage Characteristics:
Enhancement-Mode nMOSFET
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Microelectronics, 4e
McGraw-Hill
Chapter 3-8
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Family of iD Versus vDS Curves:
Enhancement-Mode nMOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-9
p-Channel Enhancement-Mode
MOSFET
Neamen
Symbols for n-Channel
Enhancement-Mode MOSFET
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-11
Microelectronics, 4e
McGraw-Hill
Chapter 3-10
Symbols for p-Channel
Enhancement-Mode MOSFET
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Microelectronics, 4e
McGraw-Hill
Chapter 3-12
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n-Channel Depletion-Mode MOSFET
Family of iD Versus vDS Curves:
Depletion-Mode nMOSFET
Symbols
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Microelectronics, 4e
McGraw-Hill
Chapter 3-13
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-14
Cross-Section of nMOSFET and pMOSFET
p-Channel DepletionMode MOSFET
Both transistors are used in the fabrication of CMOS circuitry.
Symbols
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Microelectronics, 4e
McGraw-Hill
Chapter 3-15
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-16
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Summary of I-V Relationships
Region
NMOS
PMOS
Nonsaturation
vDS<vDS(sat)
vSD<vSD(sat)
Conduction Parameters
2
SD
2
iD = Kn[2(vGS −VTN)vDS −vDS
] iD = Kp[2(vSG +VTP)vSD −v ]
Saturation
vDS>vDS(sat)
i D = K n [vGS − VTN ]
NMOSFET
Kn =
PMOSFET
Kp =
vSD>vSD(sat)
2
iD = K p [vSG + VTP ]2
Transition Pt.
vDS(sat) = vGS - VTN
vSD(sat) = vSG + VTP
Enhancement
Mode
VTN > 0V
VTP < 0V
Depletion
Mode
VTN < 0V
VTP > 0V
where:
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-17
Neamen
Channel Length Modulation:
Early Voltage
Wµ n Cox
W
= k n'
2L
2L
Wµ p Cox
2L
= k p'
W
2L
Cox = ε o tox
Microelectronics, 4e
McGraw-Hill
Chapter 3-18
Body Effect
• Causes an increase in the required threshold voltage
• This is a small effect for a single device and will be ignored
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Microelectronics, 4e
McGraw-Hill
Chapter 3-19
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-20
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Subthreshold Condition
• Small current flows for voltages threshold voltage.
• This is a small effect for a single device and will be ignored.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-21
Problem-Solving Technique:
NMOSFET DC Analysis
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Microelectronics, 4e
McGraw-Hill
Chapter 3-22
NMOS Common-Source Circuit
1. Assume the transistor is in saturation.
a. VGS > VTN, ID > 0, & VDS ≥ VDS(sat)
2. Analyze circuit using saturation I-V relations.
3. Evaluate resulting bias condition of transistor.
a. If VGS < VTN, transistor is likely in cutoff
b. If VDS < VDS(sat), transistor is likely in
nonsaturation region
4. If initial assumption is proven incorrect, make
new assumption and repeat Steps 2 and 3.
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-23
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-24
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Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-25
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-26
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-28
PMOS Common-Source Circuit
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Microelectronics, 4e
McGraw-Hill
Chapter 3-27
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Use Non Sat. EQN.
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Microelectronics, 4e
McGraw-Hill
Chapter 3-29
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-30
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-32
Load Line and Modes of Operation:
NMOS Common-Source Circuit
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Microelectronics, 4e
McGraw-Hill
Chapter 3-31
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Enhancement Load Device
2-Input NMOS NOR Logic Gate
V1 (V)
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Microelectronics, 4e
McGraw-Hill
0
0
High
VTN = 1V
5
0
Low
0
5
Low
5
5
Low
Chapter 3-33
MOS Small-Signal Amplifier
Neamen
Microelectronics, 4e
McGraw-Hill
V2 (V) VO (V)
Kn = 1mA/V2
Chapter 3-35
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-34
Cross Section of n-Channel Junction
Field Effect Transistor (JFET)
Neamen
Microelectronics, 4e
McGraw-Hill
Chapter 3-36
9