Junction Field Effect Transistor

1. JFET
Fundamentals
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
5/19/2021 1
Arpan Deyasi, RCCIIT, India

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What is meant by Field-Effect Transistor?
controlling electric field is perpendicular
to direction of current flow
controlling
E. field
Current

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Arpan Deyasi, RCCIIT, India
Formation
of
JFET
n-type
S D
G
G
p-type
p-type

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Practical Structure of channel
Ideal Structure of channel
n
n
p
p
p
p
n
n

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Symbol of JFET

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Features of JFET
Voltage controlled device
Operation is majority-carrier controlled, i.e., unipolar device
Higher input impedance and lower output impedance
Negative temperature coefficient, i.e., less probability of
thermal breakdown
Higher switching speed and cut-off frequency
Less noisy

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Schematic Structure of JFET
a
L
h(z)
w(z)
z

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Pinch-off Voltage
1/2
2
( )
D A
j A
D A
N N
W V V
q N N
ε
 
 
+
−
 
 
 
 
Depletion width for p-n junction
For n-channel, depletion region primarily consists
of acceptor types
A D
N N
>>

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Pinch-off Voltage
1/2
2
( )
A
j A
D A
N
W V V
q N N
ε
 
 
−
 
 
 
 
1/2
2 1
( )
j A
D
W V V
q N
ε
 
 
= −
 
 
 
 

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Pinch-off Voltage
A j GS
V V V
= −
Applied voltage
1/2
2 1
( )
GS
D
W V
q N
ε
 
 
=  
 
 
 

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Pinch-off Voltage
1/2
2 1
( )
P
D
a V
q N
ε
 
 
=  
 
 
 
( )
w L a
=
Boundary conditions
GS P
V V
=

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Pinch-off Voltage
2
2
D
P
qN a
V
ε
=

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Electrical
Characteristics:
Static
VDS
ID
VGS1
VGS2
VGS3
Active/
Ohmic
region
Saturation region
Breakdown
region
pinch-off voltage

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Electrical Characteristics: Transfer
VGS
ID

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Small-signal parameters
output resistance
GS
DS
d
D V
V
r
I
∂
=
∂
transconductance
DS
D
m
GS V
I
g
V
∂
=
∂
amplification factor
D
DS
GS I
V
V
µ
∂
=
∂

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Relation between small-signal parameters
( , )
D DS GS
I f V V
=
. .
GS DS
D D
D DS GS
DS GS
V V
I I
I V V
V V
∂ ∂
∆
= ∆ + ∆
∂ ∂

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Relation between small-signal parameters
GS DS
DS
D D D
GS DS GS GS
V V
V
I I I
V V V V
 
∆
∆ ∂ ∂
+
 
∆ ∂ ∆ ∂
 
For constant drain current
0
D
I
∆ =

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Relation between small-signal parameters
0
GS DS
D
DS
D D
DS GS GS
V V
I
V
I I
V V V
 
∆
∂ ∂
+ =
 
∂ ∆ ∂
 
1
( ) 0
m
d
g
r
µ
− + =
.
d m
r g
µ =

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VGS
ID
Expression of transconductance
VGS(off) VP/2
IDSS

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Expression of transconductance
2
1 GS
D DSS
P
V
I I
V
 
= −
 
 
1
2 1 GS
D
DSS
GS P P
V
I
I
V V V
  
∂
= × − −
  
∂   

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Expression of transconductance
2
1
DSS GS
m
P P
I V
g
V V
 
=
− × −
 
 
1 GS D
P DSS
V I
V I
 
− =
 
 
From current equation

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Expression of transconductance
2 DSS D
m
P DSS
I I
g
V I
=
− ×
2
m D DSS
P
g I I
V
= −

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Expression of transconductance
at 0
GS
V =
0
2 DSS
m m
P
I
g g
V
= = −
0 1 GS
m m
P
V
g g
V
 
= × −
 
 

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Expression of transconductance
2
m D DSS
P
g I I
V
= −
at
D DSS
I I
=
2
m DSS
P
g I
V
= −

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/ 2
DSS
m
P
I
g
V
= −
Expression of transconductance
Tangent cuts the VGS axis at VP/2

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Expression of transconductance
P GS off
V V
=
2
1 GS
D DSS
GS off
V
I I
V
 
 
= −
 
 

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2
m D DSS
GS off
g I I
V
= −
Expression of transconductance
0 1 GS
m m
GS off
V
g g
V
 
 
= × −
 
 

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Application of JFET
Used in RF amplifiers due to lower noise level
Used as buffer in measuring instruments and receivers due to
higher input impedance and lower output impedance
Used in voltage variable resistor in OPAMP as it is
voltage-controlled device
Used in oscillator circuit due to lower frequency drift
Used in digital circuits and memory devices due to
lower cross-section