Formula Sheet: Field Effect Transistor (FET)

 Page 1


Field Effect T ransistor (FET) F orm ula Sheet for
GA TE (Electronics)
FET Basics
• T yp es of FET s : Junction FET (JFET), Metal-Oxide-Semiconductor FET (MOS-
FET).
• JFET : V oltage-con trolled device with high input imp edance, op erates in depletion
mo de.
• MOSFET : T yp es include Enhancemen t and Depletion mo des, N-c hannel and P-
c hannel.
JFET Characteristics
• Drain Curren t (I
D
) in saturation region:
I
D
=I
DSS
(
1-
V
GS
V
P
)
2
where I
DSS
is the drain-source saturation curren t (A ), V
GS
is gate-source v oltage (V ),
V
P
is pinc h-off v oltage ( V ).
• Pinc h-Off V oltage ( V
P
) : V oltage at whic h c hannel is fully depleted.
• T ransconductance (g
m
) :
g
m
=
?I
D
?V
GS
=
2I
DSS
|V
P
|
(
1-
V
GS
V
P
)
• Op erating Regions : Cut-off ( V
GS
<V
P
), Ohmic (T rio de), Saturation.
MOSFET Characteristics
• Enhancemen t MOSFET (N-c hannel) Drain Curren t :
– Ohmic Region :
I
D
=µ
n
C
ox
W
L
[
(V
GS
-V
TH
)V
DS
-
V
2
DS
2
]
– Saturation Region :
I
D
=
1
2
µ
n
C
ox
W
L
(V
GS
-V
TH
)
2
whereµ
n
is electron mobilit y (m
2
V
-1
s
-1
),C
ox
is o xide capacitance p er unit area
(Fm
-2
),W/L is c hannel width-to-length ratio,V
TH
is threshold v oltage (V ),V
DS
is drain-source v oltage (V ).
1
Page 2


Field Effect T ransistor (FET) F orm ula Sheet for
GA TE (Electronics)
FET Basics
• T yp es of FET s : Junction FET (JFET), Metal-Oxide-Semiconductor FET (MOS-
FET).
• JFET : V oltage-con trolled device with high input imp edance, op erates in depletion
mo de.
• MOSFET : T yp es include Enhancemen t and Depletion mo des, N-c hannel and P-
c hannel.
JFET Characteristics
• Drain Curren t (I
D
) in saturation region:
I
D
=I
DSS
(
1-
V
GS
V
P
)
2
where I
DSS
is the drain-source saturation curren t (A ), V
GS
is gate-source v oltage (V ),
V
P
is pinc h-off v oltage ( V ).
• Pinc h-Off V oltage ( V
P
) : V oltage at whic h c hannel is fully depleted.
• T ransconductance (g
m
) :
g
m
=
?I
D
?V
GS
=
2I
DSS
|V
P
|
(
1-
V
GS
V
P
)
• Op erating Regions : Cut-off ( V
GS
<V
P
), Ohmic (T rio de), Saturation.
MOSFET Characteristics
• Enhancemen t MOSFET (N-c hannel) Drain Curren t :
– Ohmic Region :
I
D
=µ
n
C
ox
W
L
[
(V
GS
-V
TH
)V
DS
-
V
2
DS
2
]
– Saturation Region :
I
D
=
1
2
µ
n
C
ox
W
L
(V
GS
-V
TH
)
2
whereµ
n
is electron mobilit y (m
2
V
-1
s
-1
),C
ox
is o xide capacitance p er unit area
(Fm
-2
),W/L is c hannel width-to-length ratio,V
TH
is threshold v oltage (V ),V
DS
is drain-source v oltage (V ).
1
• T ransconductance (g
m
) in saturation:
g
m
=µ
n
C
ox
W
L
(V
GS
-V
TH
)
• Op erating Regions : Cut-off ( V
GS
< V
TH
), Ohmic (T rio de), Saturation (V
DS
=
V
GS
-V
TH
).
• Bo dy Effect : Threshold v oltage v ariation due to substrate bias:
V
TH
=V
TH0
+?
(
v
|2?
F
+V
SB
|-
v
|2?
F
|
)
where V
TH0
is zero-bias threshold v oltage, ? is b o dy-effect parameter, ?
F
is F ermi
p oten tial, V
SB
is source-bulk v oltage.
Small-Signal P arameters
• Output Resistance (r
o
) :
r
o
=
1
?I
D
where ? is the c hannel-length mo dulation parameter (V
-1
).
• Small-Signal V oltage Gain (A
v
) (common-source amplifier):
A
v
=-g
m
R
D
where R
D
is drain resi stance.
Key Notes
• JFET is alw a ys depletion mo de; MOSFET can b e depletion or enhancemen t.
• Use SI units for calculations (e.g., V , A , F ).
• F or GA TE, fo cus on saturation region equations and small-signal mo dels.
• T ypical v alues: V
TH
˜0.7V to 2V for MOSFET s, ?˜0.01V
-1
to 0.05V
-1
.
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