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# Field-Effect Transistors (2) Notes | EduRev

## : Field-Effect Transistors (2) Notes | EduRev

``` Page 1

1
Chapter 5
Field-Effect Transistors
Chapter Goals
• Describe operation of MOSFETs and JFETs.
• Define MOSFET characteristics in operation regions of cutoff,
triode and saturation.
• Discuss mathematical models for i-v characteristics of MOSFETs
and JFETs.
• Introduce graphical representations for output and transfer
characteristic descriptions of electronic devices.
• Define and contrast characteristics of enhancement-mode and
depletion-mode MOFETs.
• Define symbols to represent MOSFETs in circuit schematics.
• Investigate circuits that bias transistors into different operating
regions.
• MOSFET and JFET DC circuit analysis
• Explore MOSFET modeling in SPICE
Types of Field-Effect Transistors
• MOSFET (Metal-Oxide Semiconductor Field-Effect
Transistor)
– Primary component in high-density VLSI chips such
as memories and microprocessors
• JFET (Junction Field-Effect Transistor)
– Finds application especially in analog and RF circuit
design
Page 2

1
Chapter 5
Field-Effect Transistors
Chapter Goals
• Describe operation of MOSFETs and JFETs.
• Define MOSFET characteristics in operation regions of cutoff,
triode and saturation.
• Discuss mathematical models for i-v characteristics of MOSFETs
and JFETs.
• Introduce graphical representations for output and transfer
characteristic descriptions of electronic devices.
• Define and contrast characteristics of enhancement-mode and
depletion-mode MOFETs.
• Define symbols to represent MOSFETs in circuit schematics.
• Investigate circuits that bias transistors into different operating
regions.
• MOSFET and JFET DC circuit analysis
• Explore MOSFET modeling in SPICE
Types of Field-Effect Transistors
• MOSFET (Metal-Oxide Semiconductor Field-Effect
Transistor)
– Primary component in high-density VLSI chips such
as memories and microprocessors
• JFET (Junction Field-Effect Transistor)
– Finds application especially in analog and RF circuit
design
2
The MOS Transistor
Polysilicon
Aluminum
The NMOS Transistor Cross Section
n areas have been doped with donor ions
(arsenic) of concentration N
D
- electrons
are the majority carriers
p areas have been doped with acceptor
ions (boron) of concentration N
A
- holes
are the majority carriers
Gate oxide
n+
Source Drain
p substrate
Bulk (Body)
p+ stopper
Field-Oxide
(SiO
2
) n+
Polysilicon
Gate
L
W
MOS Capacitor Structure
• First electrode - Gate :
Consists of  low-resistivity
material such as highly-doped
polycrystalline silicon,
aluminum or tungsten
• Second electrode -
Substrate or Body: n-or p-
type semiconductor
•Dielectric - Silicon dioxide:
stable high-quality electrical
insulator between gate and
substrate.
Substrate Conditions for Different
Biases
Accumulation
V
G
<< V
TN
Depletion
V
G
< V
TN
Inversion
V
G
> V
TN
Page 3

1
Chapter 5
Field-Effect Transistors
Chapter Goals
• Describe operation of MOSFETs and JFETs.
• Define MOSFET characteristics in operation regions of cutoff,
triode and saturation.
• Discuss mathematical models for i-v characteristics of MOSFETs
and JFETs.
• Introduce graphical representations for output and transfer
characteristic descriptions of electronic devices.
• Define and contrast characteristics of enhancement-mode and
depletion-mode MOFETs.
• Define symbols to represent MOSFETs in circuit schematics.
• Investigate circuits that bias transistors into different operating
regions.
• MOSFET and JFET DC circuit analysis
• Explore MOSFET modeling in SPICE
Types of Field-Effect Transistors
• MOSFET (Metal-Oxide Semiconductor Field-Effect
Transistor)
– Primary component in high-density VLSI chips such
as memories and microprocessors
• JFET (Junction Field-Effect Transistor)
– Finds application especially in analog and RF circuit
design
2
The MOS Transistor
Polysilicon
Aluminum
The NMOS Transistor Cross Section
n areas have been doped with donor ions
(arsenic) of concentration N
D
- electrons
are the majority carriers
p areas have been doped with acceptor
ions (boron) of concentration N
A
- holes
are the majority carriers
Gate oxide
n+
Source Drain
p substrate
Bulk (Body)
p+ stopper
Field-Oxide
(SiO
2
) n+
Polysilicon
Gate
L
W
MOS Capacitor Structure
• First electrode - Gate :
Consists of  low-resistivity
material such as highly-doped
polycrystalline silicon,
aluminum or tungsten
• Second electrode -
Substrate or Body: n-or p-
type semiconductor
•Dielectric - Silicon dioxide:
stable high-quality electrical
insulator between gate and
substrate.
Substrate Conditions for Different
Biases
Accumulation
V
G
<< V
TN
Depletion
V
G
< V
TN
Inversion
V
G
> V
TN
3
Low-frequency C-V Characteristics for MOS Capacitor
on P-type Substrate
• MOS capacitance is non-
linear function of voltage.
• Total capacitance in any
region dictated by the
separation between
capacitor plates.
• Total capacitance modeled
as series combination of
fixed oxide capacitance
and voltage-dependent
depletion layer
capacitance.
NMOS Transistor: Structure
• 4 device terminals:
Gate(G), Drain(D),
Source(S) and Body(B).
• Source and drain regions
form pn junctions with
substrate.
• v
SB
, v
DS
and v
GS
always
positive during normal
operation.
• v
SB
must always reverse
bias the pn junctions
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