Short Notes: Field Effect Transistor (FET) | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


Field Effect Tr ansistor (FET) Notes
Introduction
The Field Effect Tr ansistor (FET) is a three-terminal semiconductor device used
for amplification and switching in electronic circuits. It controls current flow
through an electric field, offering high input impedance and low power con-
sumption compared to bipolar junction tr ansistors (BJT s).
K ey Concepts
• Structure : FET s have three terminals: Sour ce (S), Dr ain (D), and Gate (G).
The gate control s the current between source and dr ain.
• Types :
– Junction FET (JFET) : Uses a pn -junction to control the channel.
– Metal-Oxide-Semiconductor FET (MOSFET) : Uses an insulated gate (ox-
ide la yer) to control the channel. Subtypes include enhancement and
depletion modes.
• Oper ation : The gate voltage modulates the conductivity of a channel be-
tween source an d dr ain, controlling the dr ain currentI
D
.
Oper ating Regions
The FET oper ates in three regions based on the gate-source voltageV
GS
and dr ain-
source voltageV
DS
:
• Cut-off Region :V
GS
<V
th
(threshold voltage), no channel forms, andI
D
˜ 0 .
• Linear/Triode Region : V
GS
>V
th
andV
DS
<V
GS
-V
th
, the channel acts as a
resistor , and current is proportional toV
DS
.
• Satur ation Region : V
GS
> V
th
and V
DS
= V
GS
-V
th
, the channel is fully
formed, andI
D
is nearly constant.
K ey E quations
• JFET Dr ain Curren t (Satur ation) :
I
D
=I
DSS
(
1-
V
GS
V
P
)
2
whereI
DSS
is the maximum dr ain current, andV
P
is the pinc h-off voltage.
• MOSFET Dr a in C urrent (Linear Region) :
I
D
=µ
n
C
ox
W
L
[
(V
GS
-V
th
)V
DS
-
V
2
DS
2
]
1
Page 2


Field Effect Tr ansistor (FET) Notes
Introduction
The Field Effect Tr ansistor (FET) is a three-terminal semiconductor device used
for amplification and switching in electronic circuits. It controls current flow
through an electric field, offering high input impedance and low power con-
sumption compared to bipolar junction tr ansistors (BJT s).
K ey Concepts
• Structure : FET s have three terminals: Sour ce (S), Dr ain (D), and Gate (G).
The gate control s the current between source and dr ain.
• Types :
– Junction FET (JFET) : Uses a pn -junction to control the channel.
– Metal-Oxide-Semiconductor FET (MOSFET) : Uses an insulated gate (ox-
ide la yer) to control the channel. Subtypes include enhancement and
depletion modes.
• Oper ation : The gate voltage modulates the conductivity of a channel be-
tween source an d dr ain, controlling the dr ain currentI
D
.
Oper ating Regions
The FET oper ates in three regions based on the gate-source voltageV
GS
and dr ain-
source voltageV
DS
:
• Cut-off Region :V
GS
<V
th
(threshold voltage), no channel forms, andI
D
˜ 0 .
• Linear/Triode Region : V
GS
>V
th
andV
DS
<V
GS
-V
th
, the channel acts as a
resistor , and current is proportional toV
DS
.
• Satur ation Region : V
GS
> V
th
and V
DS
= V
GS
-V
th
, the channel is fully
formed, andI
D
is nearly constant.
K ey E quations
• JFET Dr ain Curren t (Satur ation) :
I
D
=I
DSS
(
1-
V
GS
V
P
)
2
whereI
DSS
is the maximum dr ain current, andV
P
is the pinc h-off voltage.
• MOSFET Dr a in C urrent (Linear Region) :
I
D
=µ
n
C
ox
W
L
[
(V
GS
-V
th
)V
DS
-
V
2
DS
2
]
1
whereµ
n
is electron mobility ,C
ox
is oxide capacitance, and
W
L
is the width-
to-length r a tio o f the channel.
• MOSFET Dr ain C urrent (Satur ation Region) :
I
D
=
1
2
µ
n
C
ox
W
L
(V
GS
-V
th
)
2
Applications
• Amplifiers (e.g., in audio and RF circuits).
• S witching circuits (e.g., in power management and digital logic).
• V oltage-controlled resistors and analog switches.
2