Electrical characteristics of Varactor diode

1. Course: Electronic Devices
paper code: EC301
Course Coordinator: Arpan Deyasi
Department of Electronics and Communication Engineering
RCC Institute of Information Technology
Kolkata, India
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Topic: Varactor Diode

2. Varactor = Variable Reactor
9/3/2020 2Arpan Deyasi, RCCIIT, India
Doping profile: p+n or pn+
p
p+n+
n
z z
n
n

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Electrical Characteristics of Varactor Diode
Doping profile of pn+ device
( ) m
DN z Bz=
B: background concentration
m: doping index
z: actual depth of the region

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Electrical Characteristics of Varactor Diode
Poisson’s equation:
2
2
d
dz
φ ρ
ε
= −
2
2
( )D
d q
N z
dz
φ
ε
= −

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Electrical Characteristics of Varactor Diode
2
2
md q
Bz
dz
φ
ε
= −
( 1)
1
( 1)
m
d qB z
C
dz m
φ
ε
+
=− +
+

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Electrical Characteristics of Varactor Diode
( 2)
1 2
( 1)( 2)
m
qB z
C z C
m m
φ
ε
+
=− + +
+ +
Boundary conditions
at z = 0, φ = 0
at z = W, dφ/dz = 0
at z = W, φ = VA+Vbi

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Electrical Characteristics of Varactor Diode
at z = 0, φ = 0 2 0C =
at z = W, dφ/dz = 0
( 1)
1
( 1)
m
qB W
C
mε
+
=
+

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( 2) ( 1)
( 1)( 2) ( 1)
m m
qB z qB W
z
m m m
φ
ε ε
+ +
=− +
+ + +
Electrical Characteristics of Varactor Diode
at z = W, φ = VA+Vbi
( 2)
( 2)
m
A bi
qB W
V V
mε
+
+ =
+

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( )1
2
( 2)( )
m
A biW m V V
qB
ε +
 
= + + 
 
Electrical Characteristics of Varactor Diode

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Electrical Characteristics of Varactor Diode
Total charge stored in depletion region
0
( )
W
D DQ qA N z dz= ∫
0
W
m
DQ qAB z dz= ∫

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Electrical Characteristics of Varactor Diode
( 1)
( 1)
m
D
qAB
Q W
m
+
=
+
( )1
2
( 2)( )
( 1)
m
m
D A bi
qAB
Q m V V
m qB
ε
+
+
 
= + + 
+  

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( )
D
j
A bi
dQ
C
d V V
=
+
Electrical Characteristics of Varactor Diode
Junction capacitance
( )1
( 2)
( 1)
( 2)( )
m
j
A bi
m qB
C A
m V V
ε +
 +
=  
+ + 

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Electrical Characteristics of Varactor Diode
at m = 0, ND(z) = B
at m = 1, ND(z) = Bz
at m = -1, ND(z) = B/z
at m = -3/2, ND(z) = Bz-1.5
( ) m
DN z Bz=

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