Brief analysis of shockley Diode and SCR

1. Shockley Diode & SCR
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
12/27/2020 1Arpan Deyasi, RCCIIT

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p n p n
Shockley Diode
P-N-P-N diode
named after William Shockley
Four layer device
Three junctions

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Shockley Diode
p n p n
J1 J2 J3
J1, J3: forward biased
J2: reverse biased

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p
n
p
n
p
n
p
n
p
n
Equivalent Structure

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n
p
n
p
n
p
Q1
Q1
Q2
Q2
Equivalent Circuit

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Q1
Q2
IA
IEP
IBP
ICP
IBN
IA
ICN
IEN
IA: Anode current
or total current
Current components in Equivalent Circuit

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For any BJT
C E COI I Iα= +
CN N EN CONI I Iα= +
CP P EP COPI I Iα= +Q1:
Q2:
Analysis

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IA
IEP
IBP
ICP
IBN
Q1
Q2
IA
ICN
IEN
CP P EP COPI I Iα= +
Analysis
EP BP P EP COPI I I Iα− = +
EP P EP COP BPI I I Iα= + +

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A EPI I=
IA
IEP
IBP
ICP
IBN
Q1
Q2
IA
ICN
IEN
Analysis
EP P EP COP BPI I I Iα= + +
A P A COP BPI I I Iα= + +
(1 )A P COP BPI I Iα− = +

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Analysis
IA
IEP
IBP
ICP
IBN
Q1
Q2
IA
ICN
IEN
BP CNI I=
(1 )A P COP BPI I Iα− = +
(1 )A P COP CNI I Iα− = +
(1 )A P COP N EN CONI I I Iα α− = + +

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Analysis
EN AI I=
IA
IEP
IBP
ICP
IBN
Q1
Q2
IA
ICN
IEN
(1 )A P COP
N EN CON
I I
I I
α
α
− = +
+
(1 )A P COP N A CONI I I Iα α− = + +

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Analysis IA
IEP
IBP
ICP
IBN
Q1
Q2
IA
ICN
IEN
(1 )A P N
COP CON
I
I I
α α− −
= +
(1 )A P N COI Iα α− − =
(1 )
CO
A
P N
I
I
α α
=
− −

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Analysis
(1 )
CO
A
P N
I
I
α α
=
− −
low supply condition ( ) 1P Nα α+ <<
Forward blocking state / OFF state
A COI I≈

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Analysis
(1 )
CO
A
P N
I
I
α α
=
− −
low supply condition ( ) 1P Nα α+ <<
Forward blocking state / OFF state
A COI I≈

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Analysis
(1 )
CO
A
P N
I
I
α α
=
− −
high supply condition ( ) 1P Nα α+ 
Forward breakdown state / ON state
AI ∞

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I-V characteristics of Shockley Diode
VAKFVAKR
IAF
IAR
Diode region
Unstable region
Reverse
bias region
VBR
ICO
0.7 V
IL
IH

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Drawbacks of Shockley Diode
Electrical length of unstable region is very long
Q: How to reduce it?
VBR should be lowered
Solution: Shockley diode with additional bias

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I-V characteristics of Shockley Diode
VAKFVAKR
IAF
IAR
Unstable region
VBR
0.7 V
(VJ1+VJ3-VJ2) < 2.1 V

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Modified Shockley Diode
p n p n
J1 J2 J3
Silicon
Controlled
Rectifier

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I-V characteristics of SCR
VAKFVAKR
IAF
IAR
Unstable region
VBR
0.7 V
VBR1
VBR2VBR3

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1P Nα α+ =
1
1 1
NP
P N
ββ
β β
+ =
+ +
1P Nβ β =
Characteristic Equation of SCR